Crustacea

CRUSTACEA

THERE is probably no class among the Invertebratawhich offers so many striking family and individual peculiarities as are to be met with among the Crustacea. Having a special type of structure, and possessed of numerous characteristics in common, they nevertheless put on such diverse appearances both in the young and adult stages of their existence as frequently to have baffled the most able investigators, whilst many of the vagrant members of the class still challenge further research.

The masterly and exhaustive labours of Charles Darwin on the Cirripedia have rescued that aberrant group from obscurity, and many of the parasitic forms have been relegated to the various orders of which they are in reality only degenerated members, their organs having suffered atrophy, and frequently their entire structure having undergone modification, in consequence of the peculiar existences which they lead. But our advance towards a more correct knowledge of the class, as a whole, has been mainly derived from the accumulated store of embryological and developmental studies which—commenced in 1823 by Vaughan Thompson—have in these later years been so successfully prosecuted by Audouin and Milne-Edwards, Darwin, Spence Bate, Van Beneden, Claus, Anton Dohrn, A. S. Packard, O. F. Müller, Fritz Müller, and very many other naturalists. Much, however, still remains to be accomplished.

The Crustacea belong to the sub-kingdom Annulosa, and to the division Arthropoda, in which are also included the Insecta, the Myriapoda, and the Arachnida ; they are known also as Articulata, from the body being composed of a series of distinct rings or segments, each segment usually possessing a pair of jointed appendages or limbs articulated to it. They may be defined to be those Articulata which, whenever respiratory organs ⅛re specially developed, possess branchiae and not tracheae. By this definition they are at once separated from all insects and myriapods, which invariably possess tracheae. But it remains a difficulty, if it be not altogether impossible in the present state of science, to frame any definition which shall similarly include all Crustacea and exclude all Arachnida. In both classes, in fact, there are forms which possess no special respiratory organs; and if in these cases we resort to other characters, none which are of universal application have as yet been discovered.

It may be said, however, that as a rule these exceptional Crustacea possess more than four pairs of locomotive appendages, have two pairs of antennary organs, and possess a simple alimentary canal; while the Arachnida generally have not more than four pairs of locomotive appendages, possess at most one pair of antennary organs, and have their alimentary canal produced into caeca (Huxley).

External Structure. —The skin of the Crustacea is more or less completely hardened by a horny deposit called “chitine,” with or without the addition of lime, ] thus forming a defensive covering to the softer tissues of the animal. This hardened envelope serves also as an external skeleton, giving rigidity and support to the internal organs; and to its inflections, projections, and rugosities the muscles and membranes of the body and appendages are attached.

Body-Segments. —The crustacean exo-skeleton consists of a series of rings, usually a repetition of each other, and differing only in modification according to the necessity of the various portions of the animal. Each of these divisions is called a somite (Huxley).

The normal number of somites, or segments, is twenty-one; but instances occur among the extinct Trilobita and the recent Phyllopoda and Branchiopoda, in which a larger number of segments than twenty-one· are to be met with. On the other hand, many recent and fossil forms offer examples of Crustacea in which one or more segments are never developed; but this apparent absence is generally due to their coalescence, and we shall not unfr§quently find indications of this if we bear in mind the theory of Oken, that each pair of appendages indicates a separate segment. A knowledge, too, of the earlier (larval) stages of some of these forms has revealed the presence of the normal number of free segments in the young individual, which in later life are permanently soldered together.

Although the segments of tho Crustacea are greatly modified in the different orders, yet they can nevertheless be shown to be all composed of two lateral moieties resembling each other; we can also distinguish two arcs, the one superior, the other inferior, as shown in tho annexed figure (fig. 2).

The superior central pair united constitute the tergum (í, t), and the lateral are called the epimera (ep, ep).

The inferior arc is composed of the same number of pieces. The two median pieces unite to form the sternum (s, s), and the latero-inferiorpair are called the episternum (es, es). They are always united at the sternum; but there generally exists, between the inferior arc and the epimera situated above, a wide space destined for the articulation of the corresponding appendage (Milne-Edwards).

Mr C. Spence Bate, in his “Report on the British Edriophthalmia” (Brit. Assoc. Repts., 1855), differs from Milne-Edwards and other previous writers who had considered the series of scale-like plates at the sides of the body-segments of the Amphipoda as representing the “epimeral pieces” of each somite; on the contrary, he considers them to be the dilated coxal joint (or protopodite) of each limb. This view he adheres to in his chief work (1863, p. 3) on the sessile-eyed Crustacea, and reiterates in his Report (1875, p. 47), where he writes the epimera as sectional pieces in a theoretical construction of a somite cannot exist; they are really portions of the integument of the appendages. That they are present in the Amphipoda attached to the coxal joint of seven pairs of the limbs is readily seen; but in the Decapoda, Stomaρoda, and Isopoda it cannot by any means be so easily demonstrated. “We know of no example of a ring in which we are able to distinguish all the pieces that we desire to enumerate. In some there is an absence of certain pieces from the places they should occupy; sometimes they are very intimately soldered together, so that we cannot detect even a trace of separation; but in studying each of them separately, where it is most distinct, we shall be able to form a clear idea, and recognize its character in spite of its union with its neighbouring pieces. Moreover, although this analysis of the ring may not be always practicable, it is none the less true that it facilitates much the study of the exterior skeleton of articulated animals, and that it will permit us often to establish analogies where the greatest difference would at first sight appear to exist” (Milne-Edwards).

Professor Huxley in his lectures has somewhat simplified Milne-Edwards’s view of the crustacean segment. Taking for his type one of the body rings, or somites, of the common lobster (fig. 3), he points out that it consists of a convex upper part, called the tergum (∕), and a flatter inferior side, the sternum (s), the angle of junction on each side of the tergum and sternum being produced downwards, forming what is termed the pleuron (or epimer α) (y>Z.) For ordinary studies this view of a somite will be found sufficiently clear and simple.

From the inner surface of these tegumentary rings in the Crustacea various plates are given off which project into the internal cavity, constituting in some instances cells and canals.

According to Milne-Edwards, these processes are always developed at the points of union of two rings or of two neighbouring pieces of the same segment; and this disposition has obtained for them the name of apodema (fig. 4). They are the result of a fold of the integumentary membrane which penetrates more or less deeply between the organs, and which is strengthened with calcareous matter like the rest of the external structure, and always formed of two thin plates soldered together. They serve the office fulfilled by the bones in the Vertebrata, viz., that of affording solid surfaces to which the muscles are attached.

Divisions of the Body in the Crustacea.— By general consent and usage the body of a crustacean is divided into three regions, namely, head (or cephalon), thorax, and abdomen (see figs. 1 and 6), to each of which divisions seven out of the twenty-one segments are attributed. These terms are used as a matter of convenience, and are not assumed to be homologous with the grand divisions of head, thorax·, and abdomen, in the Vertebrata. All writers agree in considering the last seven segments to be abdominal. The only difference of opinion is as to the division of the first fourteen,—how many are cephalic and how many thoracic.

Dr Dana is of opinion that Crustacea have no distinct head, but only a cephalothorax composed of the fourteen anterior segments, yet he admits that in the Edriophthalmia the seven most anterior pairs of appendages are devoted to the mouth and organs of sensation, and in the extinct Pterygotus and in the Trilobita a distinct head certainly exists.

Professor Huxley considers that the division in the Podophthalmia is indicated by the cervical fold and by the sudder change in the character of the appendages between the sixth and seventh somites, and an equally marked similarity between the latter and those of the eighth and ninth pair. So that according to this view we should have six cephalic, eight thoracic, and six abdominal somites (for Professor Huxley regards the telson as not being a true somite but a median appendage, like the post-oral plate in Pterygotus). But, as Dana well observes, the mouth organs may become legs, or the legs may become mouth organs by slight variations, therefore this line of division can hardly be maintained on the character of the appendages alone, seeing it does not hold good for other groups equally entitled to consideration, as is the Podophthalmia.

The subjoined table may serve to show these several views more clearly :—

Arrangement and Nomenclature of the Somites or Bodyrings of the Crustacea, with their Appendages. [table]

The Appendages. —Just as we find a typical number of twenty-one body srgments to prevail among the Crustacea, so also in the appendages the type number of joints is seven, any departure from which is disguised by fusion of one or more joints together, the obsolete condition of others, or the depauperization of the limb into numerous articuli (see fig. õ). At the coxal joint, or protopodite, each limb usually bifurcates; firstly, there arises the external limb proper, or exopodite, which is normally seven-jointed; and secondly, the internal branch, or endopodite, which may serve as a mouth organ, a branchial appendix, a swimming organ, or a protection for the ova or the young. The appendages belonging to the three divisions of the body differ from each other in a greater or less degree in proportion to the higher or lower grade which we examine. Thus among the Decapoda (crabs and lobsters) the cephalic, thoracic, and abdominal somites all possess appendages with well marked characters, each series being highly differentiated for the functions to be performed by it. Among the Entomostraca, the appendages of the anterior cephalic somites alone are highly specialized, the others being either mere vegetative repetitions of one another, or else altogether wanting.

First pair.— The first cephalic somite supports the eyes, the most constant of all the organs, and probably the only pair of appendages which are never diverted from their normal use, though sometimes atrophied. In the more highly cephalized forms, the Decapoda-Brachyura (crabs), the eyes are placed on the outer s i de of the two pairs of antennae, but the anatomical evidence shows that the most anterior pair of nerves, in this as in all other orders of Crustacea, is that which is connected with the ophthalmic organs.

Second pair.— The second somite bears the first pair of antennae (or ardennules), called the inner pair in the higher forms, and the upper pair among the lower Crustacea. Ordinarily they are slender, tapering, freely-moving, multi-articulate organs. In the lobster both the endopodite and exopodite are equally developed from a common basal-joint or protopodite (see 2 in fig. 6).

Third pair.— The third somite supports the second or posterior pair of antennae, sometimes called the outer pair in the higher, and the inferior pair in the lower forms of Crustacea. In the lobster these are represented by a basal-joint or protopodite, and a long filamentary, multi-segmented endopodite, to the outer base of which a small scale is attached representing the exopodite. In all the higher Crustacea the three most anterior somites are always praeoral, being in front of the buccal orifice, and in most genera, moreover, they are specially set apart as bearing the organs of sense. Generally they are so closely blended together, both in the earlier stages of development and also in the adult forms, as to defy separation, the presence of three somites being only demonstrable by dissection, and by a knowledge of the fact that each pair of appendages, wherever it exists, presupposes a segment or ring to which it belongs. The genus Squilla affords, perhaps, the best evidence of the separate existence of these cephalic rings. In it the first, or ophthalmic somite, is quite distinct from the second and third antennary, which are also separable from one another, although the latter blends with the next somite, and the succeeding ones can only be distinguished by dissection. Usually the antennae or feelers are constant appendages; but still, the number and disposition of these organs varies extremely. Some of the lowest forms are wholly without antennae, or are furnished with them in a merely rudimentary state. Some species have only a single pair, the normal number, however, as we have already pointed out, is two pairs. In position they are inserted on the superior or inferior surface of the head, according to the development of the somites composing the cephalon. Ordinarily they are slender flexible multiarticulate appendages, but even among the higher forms they are subject to extraordinary modifications; thus in the Scyllaridae, the external pair are developed into broad flat organs of natation, and probably also for burrowing. In Arcturus, an Isopod, they are the nursery for the young; in the Entomostraca they are usually natatory organs; in Pterygotus and Limulus they are chelate, serving, as in the Copepoda, as clasping organs for the male. In the last free condition of the Cirripedia and Rhizocephala they serve as the organs for attachment,—being converted into cement-ducts in the former, and into root-like organs of nutrition in the latter. The second pair of antennae and sometimes even the first pair become mouth organs in the Merostomata.

Fourth to Ninth pairs. Epistomial or Mouth Organs.— In the Decapoda the six succeeding pairs may be called mouth-organs or epistomial appendages, being all engaged in duties subservient to nutrition. The fourth somite bears the actual jaws or mandibles proper with their palpi, outside which lie two pairs of maxillae, followed by three pairs of maxillipeds or jaw-feet. In each successive somite, these organs become less highly specialized mouthorgans, and betray the fact that they are after all only simple feet modified. Thus in Squilla (a Stomapod) the eighth somite (first thoracic) bears a pair of robust claws, the terminal joint of which is furnished with long and sharp teeth, these forming the.principal organs of prehension; whilst the ninth somite bears a pair of ordinary feet like the two following pairs (see fig. 71).

In most of the Edriophthalmia the mouth-organs extend only to the seventh somite, the eighth and ninth being included with the ambulatory members.

In the Decapoda we can detect the more or less rudimentary endopodite and exopodite in the fifth pair of appendages, and in each succeeding pair to the ninth; the eighth and ninth pairs also bear a third organ called an epipodite, and a gill or branchial organ.

Tenth to Fourteenth Pairs.— In the higher forms the five somites which follow (and which might be termed the postoral somites) bear the true walking limbs (perciopoda, Spence Bate), the first pair of which in the Decapoda are usually developed into powerful chelae, and serve as the chief organs of prehension.

These podites are usually seven-jointed, and each bears a gill on its basal-joint. They are formed by the endopodite, the exopodite being present only in the larval-limb of the Decapoda; but in the adult Mysis (Stomapoda), eight pairs of limbs (that is to say, the five pairs of pereiopodites or “walking-feet,” and the three pairs of maxillipeds or “jaw-feet,”) are all furnished with two branches, one the endopodite, the other the exopodite, as in the larval Decapod.

Fifteenth to Twentieth pairs.— The next six somites bear each a pair of swimming-feet (or pleopodites). In the Decapoda-Brachyura these remain (like the segments on which they are borne) as extremely rudimentary organs furnished with hairs which serve for the protection of the eggs after extrusion. In the Macroura they assist in swimming, and are composed of a simple exopodite and endopodite. The sixth pair are developed into broad plates, forming the lateral lobes to the tail.

The Twenty first or caudal segment is destitute of appendages, and has therefore been considered by Prof. Huxley and others as a' “median appendage,” and not as a somite. It varies in form, being sometimes a broad flat plate in the Decapoda, or a minute terminal one in the Amphipoda, or greatly developed as a roof to the branchiae in some Isopoda, or forming a long terminal spine in the Xiphosura.

Internal Structure.—Nervous System. —The typical form of the nervous system in the young of all the Articulata is a chain of ganglions placed along the ventral surface of the body, and traversed in front by the gullet. This typical arrangement, however, undergoes great modifications in the several orders of Crustacea. Taking the Edriophthalmia, for example (fig. 7), we find the nervous system to consist of two parallel chords traversing the length of the body, each having its own ganglionic enlargement in each somite in juxtaposition, but not confluent; so that there is a distinct pair of ganglions for each segment. These are again united by transverse commissures, and each ganglionic knot gives off nervous filaments to the limbs of its particular somite.

In the lobster (Decapoda-Macrura), the nervous system consists of a longitudinally disposed series of differentsized ganglions connected together by commissural cords (fig. 8).

Primitively there is a pair of ganglions to each somite, but the three first pairs are fused together in the adult so as to form a large cerebral ganglion placed in front of the mouth, and called the supra-aesophageal ganglion. From this a nervous chord passes back on each side of the gullet to the large postoral ganglion, which is made up of six pairs of primitive ganglions fused together. Then follow five pairs of thoracic and six abdominal ganglions all distinct, but connected one with another by a nervous band formed of the primitive commissural chords which have coalesced along the middle line.

No solid internal skeleton separates this nervous axis from the alimentary system, though reflections of the external integument (apodemata) pass inwards, and more or less protect it. From this nervous axis all the nerves are given off, but none arise by two distinct roots like the spinal nerves of man.

In the prawn (Palaemon) and spiny lobster (Palinurus) the thoracic ganglia coalesce to form a long, elliptical, perforated nervous mass. In the hermit-crab (Pagurus) the cephalic ganglion presents a transversely quadrate form, sending off the usual nerves to the eyes, antennae, &c. The lateral oesophageal chords, after supplying the digestive system with the stomato gastric nerves, unite below to form the ganglion which distributes nerves to the maxillary apparatus and pharynx. This is succeeded by a large oblong ganglion situated at the base of the great nippers, and of the second pair of feet, both of which pairs it supplies. The lateral chords, diverging for the passage of the artery, re-unite to form a third thoracic ganglion, smaller than the second, supplying the third pair of thoracic feet, and sending off three pairs of nerves posteriorly. Of these the lateral pair goes to the fourth diminutive pair of feet; the median pair supplies the fifth pair of feet; the two remaining dorsal nerves, which are of minute size, form the continuations of the abdominal chords, and pass along the under or concave side of the soft, membranous, and highly sensitive abdomen to the anus, anterior to which the last small ganglion is situated; this supplies the nerves to the muscles of the caudal plates, here converted into claspers for enabling the animal to adhere to the columella of the spiral shell which it may have selected to protect the portion of its body undefended by the usual dense and insensible crustaceous covering (Owen).

The general progress of the development of the nervous system in the Crustacea has been, as we have seen, attended with increased size and diminished numbers of its central or ganglionic masses. The divisions of each pair of ganglions first coalesce by transverse approximation; distinct pairs of ganglions approximate longitudinally, conjoining as usual from behind forwards; confluent groups of ganglions are next found in definite parts of the body, as on the thorax of those species which have special developments and uses for particular legs. In the crab, in which the general form of the body attains most compactness, the ventral nervous trunks are concentrated into one large oval ganglion, from which the nerves radiate to all parts of the trunk, the legs, and the short tail.

A corresponding structure of the nervous system is also well displayed in Maia (fig. 9.) An analogous concentration, but not an homologous one, obtains iu Limulus. Here the nervous substance is chiefly massed round the oesophagus, the fore part of the ring expanding into a pair of ganglions, from which the nerves are sent off to the small median ocelli and the large lateral eyes; the nerves to the latter are of great length, wind round the anterior apodemata, and bend back to their termination, breaking up into a fasciculus of minute filaments before penetrating the large compound eye. Two stomato-gastric nerves arise from the upper and fore part of the ring. From the under surface of the fore part of the ring, a small pair of nerves pass to the first short pair of forcipated antennules; five large nerves proceed from each side of the ring to the five succeeding jaw-feet. A pair of slender nerves pass to the spiny-edged lamelliform appendage. The posterior part of the nervous ring is prolonged backwards in the form of a chord having four ganglionic enlargements on its ventral surface, and terminates opposite the penultimate post-abdominal plate in a fifth slight ganglionic enlargement which bifurcates; each division sends off a few nerves as it proceeds to the caudal appendage, on entering which it is resolved into a plexus or kind of cauda equina. Besides the principal nerves above mentioned many smaller nerves are given off to other parts of the body. The sides of the great oesophageal ring are united by two transverse commissural bands; but the most remarkable feature of the nervous axis of this crustacean is its envelopment by an arterial trunk. A pair of aorta from the fore part of the heart arch over each side of the stomach, and seem to terminate by intimately blending with the sides of the oesophageal nervous ring. They, in fact, expand upon, and seem to form its neurilemma; a fine injection thrown into them coats the whole central mass of the nervous system with its red colour (Owen, Lectures—Invertebrata, 1855).

Tracing the nervous system in the Crustacea from its simplest type, a vermiform cord with a series of independent nerve centres, we see these becoming successively conjoined in a greater and greater degree, as if in obedience to home law of attraction, until in the crab the maximum Centralization of the class is attained. But in whatever form it exists in this section of the Arthropoda we may bear in mind the conclusion that

“the nervous system of the Crustacea consists uniformly of medullary nuclei (ganglions) the normal number of which is the same as that of the members or rings of the body, and that all the modifications encountered, whether at different periods of the incubation, or in different species of the series, depend especially on the approximation, more or less complete, of these nuclei (an approximation Which takes place from the sides towards the median line, as well is in the longitudinal direction), and to an arrest of development occurring in a variable number of the nuclei.”—(Milne-Edwards.)

Divisions of the Nerves in the Crustacea. —Three principal divisions have been recognized in the nervous system of the Crustacea :—

(1.) All those nervous filaments which take their rise in, and are exclusively connected with, the supraaesophageal nerve-centre, forming the true sensori-volitional system. (2. ) Other ganglions superadded to the abdominal columns with their nervous filaments, serving for the automatic reception and reflection of stimuli, forming the motor system. (3.) The stomato-gastric nerves, connected partly with the brain and partly with the oesophageal columns (analogous to the great sympathetic or organic nerves of the Vertebrata), forming a third group, the ganglionic system.

Seat of the Senses in the Crustacea. —Although, as regards the relative size of the several ganglions in the nervous chain of the Crustacea generally, there is little difference between the anterior and posterior masses, and often a disparity between the supra-as compared with the infra-aesophageal ganglions, yet, nevertheless, it is generally admitted that in these animals there is an evident tendency observable towards a centralization of the nervous functions in the anterior portion of the ganglionic chain, viz., the supra-aesophageal ganglion. But still there is a wide interval between this first indication and the concentration of the faculties of perception and of will in a single organ—the brain—of which every other portion of the nervous system then becomes a mere dependency.

Organs of Feeling. —As regards the development of the individual senses, one may reasonably conjecture that the sense of touch can be but feebly exercised by the common integument of the Crustacea, if indeed it can be said to exist at all, except in those parts of the body which remain soft and undefended by a calcareous crust, such as the under side of the abdomen or the soft body of the hermit-crabs. The hairs with which many of the Crustacea are indued may to some extent compensate for this low endowment of the tactile sense.

There can be no doubt, however, that the sense of touch is mainly concentrated in the two pairs of long, manyjointed, and highly flexible antennae with which numbers of this class are provided. These special organs of touch are directly connected with the cephalic ganglion, and are well adapted, both by actual exploration and as media for conveying vibratory sensations, to furnish to the brain most correct and rapid ideas of surrounding objects.

The smaller but similarly-formed flabelliform appendages attached to the maxillae and maxillipeds doubtless perform a similar office in the testing of all objects brought near to the mouth; these latter, however, are not directly connected with the supra-aesophageal, but with the mandibular ganglion.

Organs of Sight. —The eyes of Crustacea present a greater variety in their gradation than is to be found in any other class of the Arthropoda.

Commencing with only a median fixed (bifid X) eye-spot in the larval and simpler forms, we see these organs advance progressively, through all the stages of sessile-eyed development in the Merostomata and Edriophthalmia, to the highest condition in the Podophthalmia, that of two distinct compound eyes, endowed with all the essentia^ optical apparatus, and placed on movable peduncles.

It has been doubted by some naturalists whether the eyes are organs of so much importance in the economy of the Crustacea as are the antennae or organs of touch; but experiments performed on the eye of the living lobster, when out of water, or even in a shallow aquarium, can hardly be deemed as either a conclusive or a satisfactory test of the sensitiveness of the cornea in an eye accustomed to convey impressions of surrounding objects to the optic nerve when at a depth of several fathoms beneath the water.

If the open-air experiments as to the sensitiveness of the lobster’s eye had Fig. 10.—Structure of Eye of Lobster (Homarus vulgaris). After Newton.

lα. Longitudinal and horizontal section of a right eye seen by reflected light (X 4). a, cornea; δ, first hand of plument, beneath which are the crystalline cones; c, a broad band of radiating fibres free from pigment; d, second black band composed of the pigmented spindle-shaped bodies, the lower ends of these bodies are covered with an opaque white pigment which forms e, the first white band; Λ, bundles of radiating fibres; ι, enlarged end of the optic nerve, x. The muscles and connective tissue which naturally fill the cavity have been omitted in this figure.

16. A group of elements showing the relation of the pigment to the cones. The cornea is not present, a, substance intermediate between cornea and cone; ⅛ crystalline cone; c, nerve red; p, pigment.

lc. A partly diagrammatic view of one of the elements of the eye from the cornea to the optic ganglion, α, coinea; δ, substance between cornea and cone; b'. lower end of cιystalline cone; c, nerve rod, around which is seen the investing membrane with its nuclei; ⅛ spindle-shaped or transversely striated body; ∕, perforated membrane at surface of optic ganglion; p, pigment.

Id. A portion of cornea as seen by reflected light, showing the cross and central spot. le. Perpendicular section from the middle of the cornea, showing the smooth outer and slightly convex inner surfaces.

been made instead on a living shore or land crab, of the genus Grapsus, Gelasimus, Gecarcinus, or Ocypoda, it would speedily have been found that in point of rapidity of perception and movement, guided by sight, these shore and land crabs are quite equal to the most sharp-sighted insect, or the most agile of lizards.

As already stated, the eyes are the most constant and persistent organs possessed by the Crustacea as a class; indeed, if we except certain parasitic Isopodous forms and the Cirripedia and Rhizocephala, we shall find that the faculty of sight is possessed by the whole class. Even in those exceptional cases in which the eyes are aborted, we find that in the earlier and larval stages of their existence the parasitic and sedentary forms possessed eyes, and it is only as an effect of a kind of retrograde metamorphosis which the animal undergoes that the organs of vision disappear in the adult. 2 Two forms of visual organs are met with in the Crustacea, namely, smooth or simple eyes (ocelli or stemmata) and compound eyes; but though there are some few forms in which (as in Λpus and Limulus) both ocelli and compound eyes are present, the latter form of eyes is that most generally met with in the class.

The structure of the simple eye does not differ greatly from that observed among higher animals. There is, firstly, a transparent cornea, smooth and rounded, which is, in fact, only a part of the general tegumentary covering modified. Immediately behind the cornea is the crystalline lens, generally of a spherical form; this is again followed by a gelatinous mass analogous to the vitreous humour, and this mass is, in its turn, in contact with the extremity of the optic nerve. A layer of pigment of a deep colour envelops the whole of these parts, lining the internal wall of the globe of the eye up to the point at which the cornea begins to be formed by the thinning of the tegumentary envelope rendering it transparent. The number of these simple eyes does not exceed two or three.

In the Branchiopoda (Nebalia, Branfhipus, Daphnia) behind a simple cornea, undivided externally, we find a variable number of distinct crystalline lenses and vitreous humours, each included in a pigmentary cell, and terminating by contact with the optic nerve. These are, no doubt, an aggregation of stemmata under a common cornea.

In some of the Edriophthalmia a still further advance to a true compound eye is met with. In these the cornea appears to consist of two transparent laminae, the external layer being smooth and the internal one facetted, each facet being a distinct cornea resting on a separate crystalline lens of its own. In the compound eye, properly so called, the two membranes, external and internal, constituting the cornea, are both divided into facets, each facet seemingly being equivalent to a distinct ocellus, furnished with its own crystalline cone (or lens) and nerve rod; each invested with its own pigment coating, which, being darker at intervals, gives to a section through the compound eye as a whole the appearance of pigment-bands repeated at various depths beneath the cornea, and in front of the expanded termination of the optic nerve (retina). Although these facets are always hexagonal in the eye of an insect, they are variable in form among the Crustacea. Thus in Homarus, Ástacus, Penaeus, Galathaea, and Scyllarus, the facets are square; whilst in Pagurus, Squilla, Gébia, Callianassa, and the crabs, they are hexagonal. In Limulus and the Trilobites the lenses are round, not being in actual close contact with each other. Milne-Edwards mentions that in Idotea each facet has a kind of supplemental lens of a circular shape set within the cornea in front of each proper crystalline lens, and equal in size to the corneal facet, and apparently evolved in the substance of the cornea itself, but under favourable circumstances capable of being detached from it. In Phacops caudatus the small circular lenses of the external compound eyes drop out, leaving a corresponding concavity beneath.

Emmerich long since proposed to use the external characters of the eyes of Trilobites as a means of classification, dividing them into “hyaline eyed and “facetted’’-eyed groups; but he does not seem to have been aware of the perfect analogy which the structure of the eyes of the modern Edriophthalmia afford in illustration of this ancient and extinct group.

Stemmata or ocelli are always immovable and sessile; the compound eyes with smooth corneae, although usually sessile, are, however, occasionally supported on pedicles, as in Nebalia and Branaehipus.

The compound facetted eyes are subject to the same variations,—genera being found with hyaline and facetted corneae in the same order. In some of the compound sessile eyes the facets are round; but in all the pedunculated compound eyes they are either square or hexagonal.

The peduncles supporting the eyes in the Stomapoda and Decapoda vary greatly in length, but every consideration tends to the conclusion that these movable eye-stalks are really the pair of appendages of the first cephalic ring. Indeed, in Sqιιilla one is actually able to separate the eye-stalks with the segment upon which they are borne from the cephalic shield.

In Macrophthalmus and some other crabs the eye-stalk is of very considerable length (see fig. 65), extending even to the outer angle of the front of the carapace, which is furnished with a long groove or furrow into which the eye can be folded down, and so placed out of reach of injury when not in active use. This furrow is called the orbital fossa. Organ of Hearing. —Milne-Edwards, Owen, Bell, and others consider the external organ connected with the sense of hearing to be situated on the first joint of the outer and larger antennae in the lobster and other Macroura, and to consist of a conical process beneath which is a cavity having a round orifice closed by a membrane. Behind the process, and connected with the cavity, is a large sac filled with a clear liquor; a nerve arising in common with the external antennal nerve is spread upon the delicate walls of the supposed acoustic sac.

In Maia and other crabs the membrane is replaced by a movable calcareous disc pierced by a small oval opening, over which is stretched a thin elastic membrane (which might be termed the internal auditory membrane), near to which the auditory nerve appears to terminate.

The auditory apparatus of the Crustacea consequently consists essentially of a cavity full of fluid, over which a nerve adapted to perceive sonorous impulses is distributed, assisted by an elastic membrane, and placed near the base of the antennae which, like a rigid stem, assists in rendering certain vibrations perceptible.

In both the lobster and the crab a gland filled with a greenish substance is connected with the membranous sac. This structure and the absence of otolites has led Farre to suggest that the organ may be olfactory; but the chief parts of the structure bear a close correspondence with an auditive vesicle and a tympanic membrane. Organ of Smell. —Professor Owen refers the sense of smell to a small sac, fringed with fine hairs, opening externally by a narrow cleft in the basal joint of the first or median antennae. A branch of the antennal nerve terminates in a small prominence at the bottom of this sac. From the presence of some minute siliceous particles within the cavity (although it is admitted that these must have found their way in from the exterior fortuitously) Dr Farre has been led to suggest that the small antennae are acoustic organs, and that the grains of sand may act as otolites.

Milne-Edwards admits as indubitable the presence of well-developed organs of smell, but considers we are reduced to conjecture when we are required to point out the precise seat of those organs. Organs of Taste. —Like almost all other animals the Crustacea select their food, showing decided preference for particular kinds; this selection is doubtless actuated by two senses, «niemand taste. Whether we are correct in assigning to the inner pair of antennae the duties of the olfactory organ or not, it cannot be doubted that the sense of taste is distributed over that portion of the tegumentary membrane which lines the interior of the mouth and aesophagus, but there is no modification of these parts which needs to be specially noticed here.

Organs of Nutrition.— In the larval stages of the higher Crustacea, andalso among the adult lower andsimpler forms, fewer of the somites have their paired appendages differentiated to perform special offices. Thus in the larval Decapod the chief natatory organs are the maxillipeds; this is also the case in the Merostomata. In Limulus (fig. 12) all the locomotory organs are also subservient to the duties of nutrition, being organs of locomotion at their distal, and mandibles and maxillae at their proximal extremity.

In fact, as already stated, we have abundant evidence to prove that the maxillary organs of the Malacostraca are but modifications of entire limbs, translated from the locomotive series and set apart as special mouth-organs. By far the larger proportion of the Crustacea have a proper normal mouth furnished with suitable organs .of mastication, but among the parasitic Copepoda and certain aberrant parasitic Isopods, <fcc., they become merely organs of attachment, the mouth being suctorial; or (as in the Rhizocephala) it may be altogether wanting, and the limbs completely lost, and from the point of attachment root-like tubes may be developed, which, sinking deep into the body of the host, convey to the parasite its nutriment ready digested and prepared.

If instead of these latter we examine the Decapoda we shall find the mouth placed centrally near the front and upon the under side of the cephalon. It is provided with a small simple median piece, called a labrum, or upper lip, in front, and a bifid metastoma, or lower lip, behind; the paired appendages (mandibles, maxillae, &c.) being placed on either side of the buccal orifice.

The food, whether living or dead, being first seized by the forcipated thoracic feet, is brought near to the maxillipeds, and by the help of these external organs of prehension portions are separated and introduced by the maxillae to the trenchant and powerful mandibles, when having undergon e further subdivision they are swallowed.

No organ corresponding to a tongue exists in the Crustacea, the mouth being only the anterior and outward expansion of the aesophagus, which is short, rises vertically, and terminates directly in the stomach.

The wall of the stomach is composed of twomembranous layers, separated by one of muscular fibres, which increase in thickness at the openings leading from the aesophagus and into the intestine.

The stomach is globular in form and of great capacity, and maybe divided into an anterior or “cardiac” part, and a posterior or “pyloric” region. The food on reaching the “cardiac” region of the stomach is subjected to a further process of mastication, by means of a complex apparatus composed of several calcareous pieces, moved by appropriate muscles, inserted in the membranous wall of the stomach (fig. 13, la), armed with a smooth median plate and two lateral molar-like-organs, having a singular mimetic and superficial resemblance to the molar teeth of some small marsupial rodent. Two smaller points (bicuspid in the lobster, tricuspid in the crab) complete the calcareous apparatus; in the pylorus a series of fine hairs are placed, which, doubtless, act like a strainer, preventing the escape of the coarser particles of the food until they have repeatedly been subjected to the molar-like action of the gastric teeth. A long and straight intestine continues from the stomach backwards, and terminates beneath the telson.

Two caecal salivary glands of a greenish colour are situated on either side of the aesophagus.

The liver in the Decapoda is of large size, and bilaterally symmetrical: its structure is highly ramified, not solid like the human liver. The secreted fluid or bile is poured by two openings into the pylorus.

This organ, so large in the crab, undergoes great modifications in the various orders; in the Edriophthalmia only three pairs of biliary vessels, analogous to those of insects, remain. No vessels have as yet been detected by •which the chyle or nutritious fluid elaborated by the digestive processes is taken up, as it passes along the intestinal canal, and transferred to the circulatory system; we can only, therefore, conclude that it is transferred by absorption to the irregular venous receptacles which are in contact with the walls of the intestines.

Circulatory System. —In most of the Crustacea the circulation is of the same simple character as that observed in the aquatic larvae of insects, save that in the Crustacea the blood is conveyed to the gills for the purpose of oxygenation; but where no special respiratory organs are developed, the fine hairs and filamentous appendages attached to the feet doubtless subserve that office, or in some the entire surface of the body. The heart consists of an elongated contractile dorsal vessel, larger behind than before, connected anteriorly and posteriorly by several branches with the inferior or returning vessels, which running along the whole body receive the blood from the anterior extremity, and carry it into the posterior extremity of the dorsal vessel.

In the male Entoniscus Cancrorum the heart (fig. 14). is situated in the third abdominal segment. In the Cassidina the heart (fig. 15) is likewise short and furnished with two pairs of fissures, and is situated in the last segment of the thorax and the first of the abdomen. Lastly, in the young Anilocra the heart (fig. 16) extends through the whole length of the abdomen, and is furnished with four or five fissures which are not placed in pairs; but alternately to the right and left in successive segments (Fritz Müller, Für Darwin, pρ. 41-42).

In the Decapoda the heart is placed near the dorsal surface of the cephalic shield immediately beneath the integument above the intestinal tube, and is retained in its place by lateral pyramidal muscles. It consists of a single chamber or ventricle, suspended in a large sac, called the pericardium, but wholly distinct from the part so named in man. The structure of the heart is made up of the interlacement of numerous muscualr fibres fixed by their extremities to neighbouring parts, and passing for some distance over the aggregate at each end, the whole structure reminding one of a number of stars superposed on each other, the rays of which do not correspond (Milne - Edwards). The ventricle has three pairs of apertures so closed by valves as to readily allow the entrance of blood from the pericardium, but to hinder its regurgitation. It has three other pairs of openings, each of which is the commencement of an arterial trunk conveying blood all over the body. These arteries have valves at their origin, and ramify and end ultimately in capillaries, which open into what are called venous sinuses, because they are channels without any definito shape. The venous blood collects in a great sternal sinus, and thence passes up into the gills to be oxygenated, after which it proceeds to the pericardium to find its way into the ventricle (fig.

17). From the researches of MM. Audouin and Milne-Edwards, it had been considered as conclusively proved that in the Decapod Crustacean only aerated or arterial blood found its way into the heart, to be distributed by it over the general system. But Professor Owen has shown that in addition to the two great branchial trunks which pour their streams of aerated blood into the heart from the gills, four other valvular orifices, two connected with the series of caudal, and two with the series of lateral sinuses, communicate with the ventricle, and return a portion of carbonized or venous blood to the heart; the circulation is, therefore, to some extent mixed, and as both venous and arterial blood reach the ventricle, they are propelled thence through the system (see fig. 18). The returning blood is not redistributed through the liver, as in man, i.e., there is no portal circulation. There are no lymphatic vessels. The blood is a slightly dusky fluid, containing numerous nucleated corpuscles, which change their form with remarkable rapidity.

Respiratory Organs. —As the type upon which the Crustacean class is constructed is specially fitted for aquatic existence, branchial organs or gills in some form are essential for the aeration of the blood. The appendages which fulfil this office are attached either to the thoracic or abdominal members or to both. Where they are most highly developed, as in the crab and lobster, they assume the form of pyramidal bodies, each consisting of a central ascending stem, with numerous horizontal branches or plates folded close together through which the blood circulates. There are twenty-two such structures in the lobster, eleven on each side of the thorax, attached to the basal joints of the thoracic limbs, each pair of gills being furnished with its epipodits, or upper footlet, which serves to keep the gills apart from each other. In their most simple form they consist of a mere sac-like appendage held by a small neck pendant from the coxal joint, and exposed in the water without protection.

In the Decapoda they become more complex in structure and more voluminous, and would be extremely liable to injury if not protected by some means. But, as Spence Bate truly observes, being external to it, they could not be covered or protected by their own somite, as, if it had passed over them, the branchial appendages would have become internal, their character and constitution would therefore be changed; they would cease to be external; in fact, they would cease to be branchiae. These appendages, however, exist as branchiae, and are nevertheless securely covered and protected; not, indeed, by their own somite, but by the great development of the mandibular and posterior antennal somites incorporated together, forming the carapace so characteristic of the typical Crustacean.

The branchial appendages are thus external in relation of the body of the animal, but covered over and protected by the lateral walls of the carapace. To complete this so as effectually to prote:t these organs without pressing on them or interfering with their functions, a very considerable amount of lateral development has taken place, and a peculiar reflection so as to bring the margin of the carapace below the branchial appendages, and to protect them from rude contact with the limbs. Externally, the carapace covers and protects both the hepatic and branchial organs; but, internally, a calcareous wall of demarcation exists between the two. This wall, which Milne-Edwards terms the apodema, is continued into a thin membranous tissue that makes a distinct and well-defined separation between the branchial appendages and the internal system, so that the aqueous element so necessary for the aeration of the blood as it passes through the branchiae may have full power to play upon the gills, and yet leaving no passage that would admit it to the internal viscera so as to derange the general economy of the animal (Spence Bate). The gills are not ciliated, and thus they require that the water within the branchial caι ity in which they are placed should be incessantly renewed by other means. In the crabs two passages communicate with the branchial chamber, one for the entrance, the other for the exit of the water necessary to respiration. The efferent orifice always opens on each side in front of the mouth under the posterior maxilliped. The afferent opening varies greatly in position in the different groups.

In the Macroura (lobsters), and in some of the Anomoura (hermit-crabs), the margin of the carapace is not accurately fitted to the thorax along its lower lateral border; the branchial cavity is thus open along the whole extent of its inferior edge, and so the water finds its way readily into the respiratory chamber.

In the Brachyura (crabs), the afferent orifice is more circumscribed, but varies in a still greater degree. In nearly all it exists as a cleft of considerable breadth in front of the base of the first pair of ambulatory appendages between the carapace and the thorax.

In the Ocypoda, the third and fourth pair of feet are more closely approximated than the rest, and their margins bear a dense border of long silky peculiarly-formed hairs. Between the basal-joints of these feet, Fritz Müller has discovered a round orifice opening into the branchial cavity, and he finds this to be a true incunent orifice for the admission of air or water into the branchial chamber.

In the genus Banina, according to Milne-Edwards, the ordinary anterior entrant orifice to the gill-cavity is altogether wanting; it is placed instead at the origin of the abdomen.

In Grapsus, Fritz Müller has observed that, when under water, the respiratory in-current enters near the front in the usual manner, but when air is breathed, the anterior incurrent orifice being closed, and the hinder border of the carapace elevated, a wide fissure is opened upon each side above the last pair of feet leading directly into the branchial chamber.

In Leucosia the two apertures are close together, the incurrent opening being situated in front of the mouth, and the water passing in by a conduit parallel to the excurrent canal. The circulation of the medium within the respiratory atrium is brought about partly by the movements of the legs to which the branchiae are attached, and partly by the epipodites which ascend between the gills. Tlιe main agent, however, is the “scaphognathite,” a flabelliform appendage of the second pair of maxillipeds, which, rising and falling continually, occasions a rapid current from behind forwards in the water, filling the branchial chamber.

Branchiae such as we have described, enclosed beneath the overarching lateral walls of the carapace, are specially characteristic of the Decapoda (crabs and lobsters).

In the Amphipoda the head shield is small, and no longer covers the thoracic somites, as in the Decapoda. The branchiae, however, are still borne on the coxal joint of the thoracic legs, but they depend unprotected from each limb, and are bathed in the surrounding medium, which is made to pass rapidly over them by the action of the abdomina 1 flabellae.

In Squilla wí, find the appendages of the first five pairs of abdominal somites devoted to the office of aerating the blood; the branchiae, however, are not included in a cavity, but float freely in the water which bathes the entire surface of the animal.

In the Isopoda the abdominal appendages are all devoted to respiration, the anterior and outer pair in ldotea (fig. 19) being specially modified into a strong operculum (op), opening laterally and shutting over the five pairs of delicate branchial appendages within.

In Limulus five pairs of thoracic feet are modified into broad lamellae, to the inner and upper surfaces of which the gills are attached, whilst the most robust and anterior pair is modified into a broad operculum covering the succeeding five branchigerous pairs, and also the reproductive organs (see fig. 12).

In Apus (Branchiopoda), save the antennae and oral appendages of the head, all the other somites bear simple lamelliform gill-feet, of which there are, according to Baird, about sixty pairs, affording an excellent illustration of mere vegetative repetition of parts.

Although the act of respiration by gills seems a peculiarly aquatic method of aerating the blood, yet in both the Podophthalmia and the Edrιoρhthalmia we meet with numerous amphibian and terrestrial forms.

No Macrouran Decapod, so far as ascertained, voluntarily quits the water, although the common lobster, the river crayfish, and the spiny lobster, all display great tenacity of life when removed from their native element. Their inability to leave the water is, no doubt, due to the fact that the carapace is less acurately fitted to the thorax than in the crabs. Certain of the Anomoura, or hermit-crabs, however, find no difficulty in adapting themselves to terrestrial conditions. The writer has kept the Cenobita Diogenes from the Antilles, tenanting an Achatina shell, alive in a Wardian case for three months, during which period he displayed great activity and most remarkable powers as a climber. These West Indian crabs are not infrequently brought over alive to England with cargoes of guano and other natural products (fig. 20).

Darwin refers to the abundance of hermit-crabs on Keeling Island in the Indian Ocean (Voyage of the Beagle, p. 544), all living on the cocoa-nut, and each ensconced in some shell obtained from the neighbouring beach. On the same island is found another most remarkable and very large terrestrial Anomourous Crustacean, the Birgus latro, living in burrows at the base of the cocoa-nut trees, upon the fruit of which it subsists. This large hermit-crab seeks no artificial covering for its fleshy body, the integument of which is chiefly membranous, but has the tergal pieces of its abdominal somites calcified. It is said (by Darwin) to visit the sea nightly for the purpose of moistening its gills, and the young are hatched in the water, and pass there their earlier stages of existence.

Many of the shore-crabs, as Graρsus, and the freshwater crab, Thelphusa, are not only able to leave the water temporarily; the former habitually lives out of that element, whilst many sub-tropical forms, as Gecarcinus, Gelasimus, <fcc., frequently live at a distance from the sea, and certainly possess the power of breathing air. But in all land-crabs it seems essential that the gills should, if not immersed in water, at least have the air surrounding them saturated with moisture. Milne-Edwards found that Gecarcinus (fig. 21) has the membrane lining the walls of the respiratory cavity modified in a manner analogous to that observed in fishes of the order Acanthopterygiae. Sometimes this provision consisted of folds and lacunae serving as reservoirs for the water; sometimes, as in Birgits, of a spongy mass well calculated to store up the fluid necessary to keep the branchiae sufficiently moistened to enable them to perform their functions.

The swift-footed sand-crabs (Ocypoda) are exclusively terrestrial, and can scarcely live for a single day in water; in a much shorter period, a state of complete relaxation occurs, and all voluntary movements cease. In fact, these Lnd-dwelling crabs are as truly asphyxiated by immersion in water, as the aquatic species of Cancer become when taken from that element and left in the air.

Among the Amphipoda, Talitrus and Orchestia both live out of the sea, the former making a burrow for itself, the latter choosing moist places under sea-weed, or hiding in the damp sancL With us Orckestia lives within reach of the sea spray, but in the southern hemisphere species have been met with many miles inland, choosing terrestrial plants for their abode, sometimes at an elevation of 1500 feet above the sea.

Among the Isopods Sphaeroma is quite a littoral form, ranging from the equator to the colder temperate shores. The genus Ligia also lives above high-water mark, but never far away from the sea.

All the Oniscidae are terrestrial in their habits, livingunc er stones, moss, or decaying wood, and in similar damp situations; they breathe air (which, however, must be saturated with moisture) by the aid of a series of respiratory branchial plates on the under side of the abdominal somites, in the same manner as in Idotea (already noticed), and in addition to this by the inspiration of air by means of certain spiracular orifices on several of the basal pairs of these same appendages (6g. 22).

In a large number of the lower and simple forms, including also the parasitic Crustacea and the Cirripedia, no special organs of respiιa-tion exist, and we are led to conclude that this office is performed by the surface of the body and its appendages generally.

Muscular System. —All the muscles of the body, even those of the intestine, are composed of striated fibres.

Reproductive Organs. —The organs of generation are easily to be discerned in most of the Crustacea, but the analogy between these parts in the male and female is so great in many genera as to need the most careful examination in order to discriminate between the two. Generally, however, the males may be discerned by their having cue or more pairs of limbs especially modified to assist in the marital act.

With the exception of the Cirripedia the two sexes appear never to exist together in the same individual among the Crustacea.

The small size and great dissimilarity of the males of some of the parasitic genera caused them to remain long unknown, and led to the error of supposing the females to be hermaphrodite, as Darwin has shown to be really the case in the Cirripedes. But even in this division Darwin has found small males parasitic on the female which he has named “complemental males.” They are destitute of a mouth, aud appear to exist only for the performance of this one function of reproduction (Darwin, Cirripedia, Ray Soc. 1851).

Bilateral symmetry generally prevails among the members of this class, and as a consequence we find always a pair of these organs arranged one on either side of the body, perfectly distinct, and often wholly independent of each other. The male is provided with a paired gland or testes, and two excretory ducts, by which the spermatozoa are discharged on reaching the efferent openings, usually situated on either side in the basal joints of the seventh pair of thoracic appendages, or the first pair of abdominal limbs. In both the Crab and Lobster tho first pair of abdominal appendages of the male are specially modified to take part in the process of fecundating the female.

Milne-Edwards denies that these appendages have any claim to be considered as fulfilling the office of conveying the fecundating fluid to the body of the female, but Spence Bate has frequently taken Carcinus maenas with these styliform appendages deeply inserted within the vulvae of the female. He has also shown the existence of a vas deferens in these false feet (Ann. and Mag. Nat. Hist., 2nd series, vol vi., p. 109).

The ovaries in the crab resemble four cylindrical tubes placed longitudinally in the thorax, and divided into two symmetrical pairs, each opening into a distinct oviduct, yet communicating with each other by a transverse canal and by the intimate union of the two posterior tubes. The oviducts and ovaries are of a whitish colour, and become united to a kind of sac 1 on each side, the neck of which opens externally in the sternal pieces of the fifth thoracic somite, which bears the third pair of walking appendages.

In the Anomoura and Macroura there are no copulatory pouches, and the vulvae open on the basal joint· of the third pair of ambulatory legs. It is possible, therefore, that in these forms the fecundation of the ova does not take place until the eggs are actually extruded, which we know to be the case in Limulus, and probably in some other forms, and as is also the case in fishes.

If we except Gecarcinus, certain other land-crabs, and 1 The copulatory pouches of Milne-Edwards.

Limulus, the female does not abandon her eggs after their extrusion. Those of the Decapoda when extruded are coated with a viscous secretion which thickens into threads, and causes the eggs to adhere to each other and to the fine hairs with which the swimmerets of the abdomen of the lobster and the female crab are fringed (fig. 23). Fig. 24 shows the method of attachment of the eggs. Here they are retained securely until the period of hatching has arrived, when the brood in most cases is dispersed.

This is not, however, always the case, for whilst examining a female Dromia from Australia, the writer discovered more than a dozen young ones adhering to the false abdominal feet of the parent,—the young, except in size, agreeing perfectly with the parent.

In Mysis the two endopodites of the hinder pair of thoracic feet in the female are developed into a broad plate on either side, and bent under the sternum, thus forming together an incubatory pouch or marsupium, in which the eggs are first deposited, and within which the young are secluded during their minority. In Thysanopoda the eggs and young are contained in a pair of oval sacs dependent from the posterior feet, forcibly reminding one of the ovarian sacs in Cyclops.

In the Amphipoda the ova are nurtured by the female within a pouch formed by a series of foliaceous plates, one of which is attached to each of the four anterior pairs of legs of the thorax. In the genus Podocerus the parent builds a nest in a very bird-like manner, amid the branches of the submarine zoophyte forests, and in one of these Mr Spence Bate met with two broods of different ages, clearly demonstrating that the maternal care for their young is continued long after birth. Similar ovigerous plates are developed in the fore-legs of females of the Isopoda. In all these sessileeyed forms the parent seems specially solicitous for the safety of its young. In Asellus, Talitrus, and Gammarus, they appear to quit the maternal pouch and return to it as to a place of safety. Caprella carries its young attached to its body; the female Arcturus supports them adhering to her large antennae.

In Daphnia, besides the several groups of ova which are successively hatched within the bivalved shell, and excluded during the spring and summer, giving rise to fertile females, there is formed each autumn an opaque layer within the incubatory cavity of the female, which hardens in two pieces like a small bivalved-shell, and is called the ephippium or saddle, and is placed on the dorsal surface of the Daphnia, but within the shell of the parent. Another structure, similar to the ephippiιvm, and called the “internal ephippium,” placed within it, is found to contain two bivalved capsules, in each of which a fertilised egg is lodged, which remains in a passive state through the winter, but hatches by the first warmth of spring, giving rise to females only (no males being hatched till autumn), these females in turn giving rise also to as many as six generations of fertile females. Their fecundity is so great as to bo almost beyond the power of figures to express. Development of the Crustacea. —In nearly all the Crustacea, the young undergo a series of metamorphoses after quitting the egg. This rule appears to apply more constantly among the truly marine forms. Among the stalk-eyed Crustacea some few species at least quit the egg in the form of their parents, with the full number of jointed appendages to the body. This is the case, according to Rathke, with Astacus fluviatilis, the common river crayfish (fig. 25), and according to Westwood in a West Indian land crab (Gecarcinufi). The present writer has also found an Australian Dromia protecting its brood on its false abdominal feet, the young differing from the parent only in point of size.

The young of the terrestrial Xso ∖ >ofa(Oniscus, Porcellio, and Armadillo) likewise nearly resemble their parents at birth.

In the king-crabs (Limιdus) the young undergo their principal moults before quitting the egg, when they differ but little in aspect from the adult. The metamorphosis undergone by the common lobster appears to be but slight. The young, according to Van Beneden, are distinguished from the adult by having their feet provided like those of Mysis with a swimming branch projecting freely outwards, whilst the abdominal and caudal appendages are undeveloped. In nearly all the marine Crustacea the young quit the egg in the condition of zoëa. We are acquainted with many examples in all three divisions of the Decapoda.

But we are indebted to Mr C. Spence Bate for the most complete series of observations on the development of one species, the common shore-crab, (Carcinus maenas) from the zoëa to the sexually mature animal.

He has shown that in this species the metamorphosis is a perfectly gradual one, and that, dissimilar as is the zoëa when it quits the egg from the adult animal, yet nevertheless the change at each moult is so small that it is only by a comparison between the earliest and the last stages that we perceive the amount of the change which has actually taken place.

“The most important peculiarities,” writes Fritz Müller, “which distinguish this zoëa-brood from the adult animal are as follows. The middle body (thorax), with its appendages, those five pairs of feet to which these animals owe their name of Decapoda, is either entirely wanting, or scarcely indicated; the abdomen and tail are destitute of appendages, and the latter consists of a single piece (fig. 26). The mandibles, as in the Insecta, have no palpi. The maxillipeds, of which the third pιir is often wanting, are not yet brought into the service of the mouth, but appear in the form of biramose natatory feet. Branchiae are wanting, or where their first rudiments may be detected as small verruciform prominences, these are dense cell-masses through which the blood does not yet flow, and which have therefore nothing to do with respiration. An interchange of the gases of the water and the blood may (and no doubt does) occur all over the thin-skinned surface of the body; but the lateral parts of the carapace may unhesitatingly be indicated as the chief seat of respiration.

“They consist, exactly as described by Leydig in the Daphniae, of an outer and inner lamina, the space between which is traversed by numerous transverse partitions dilated at their ends; the spaces between these partitions are penetrated by a more abundant flow of blood than occurs anywhere else in the body of the zoëa. A constant current of water passes beneath the carapace from behind forwards, maintained as in the adult animal by a foliaceous appendage from the second pair of maxillae.

“The zoëae of the crabs are usually distinguished by long, spini∙ form processes of the carapace (fig. 27). One of these projects up wards from the middle of the back, a second downwards from the forehead, and frequently there is a shorter one on each side near the posterior inferior angles of the carapace. But in the zoëa of Maia, Eurynome, and an allied genus to Achaeus, the spines are wanting in the first two genera, and but of inconsiderable size in the last-named genus.

“The following are the more important peculiarities in the zoëa of the crabs, although less striking than these processes of the carapace, which, in combination with the large eyes, often give them so singular an appearance. The anterior (inner) antennae are simple, not jointed, and bear two or three olfactory filaments at the extremity; the outer antennae frequently form a long spine-like process, and bear a minute squamiform process like the antennal scale in prawns. Of the natatory feet (afterwards maxillipeds) only two pairs are present, the third is entirely wanting, or present, like the five following pairs of feet, only as minute buds. The tail in zoëa is very variable in form, but nearly always bears three pairs of setae upon its hinder margin ’’

When the young zoëa first escapes from the egg, it is enveloped by a membrane veiling the spinous processes of the carapace, the setae of the feet, and the antennae; but this is cast off in a few hours. The zoëae of Porcellana (fig. 28) seem to differ widely from true crabs, but really approach them very closely. The dorsal spine is wanting, but the frontal and lateral spines are of extraordinary length, and directed straight forward and backward. The tail bears five pairs of setae.

The zoëa of Hippa eremita also resembles that of the crab in general appearance and in mode of locomotion (fig. 29). The carapace has only a short broad frontal process; and the caudal plate is edged with numerous short setae. The zoëae of hermit-crabs (fig. 30) have simple antennules like those in the Brachyuran zoëae. The antennae bear a scale-like appendage on the outside analogous to tlιat in the prawn. There are only two pairs of well-developed natatory feet (maxillipeds), but the third pair are present in the form of two-jointed rudimentary appendages destitute of setae. The hinder border of the tail bears five pairs of setae.

The zoëae of the shrimps and prawns agree closely with the Anomoura. They have a small median eye between the large compound ones. The third pair of maxillipeds are always present.

In investigating the development of the spiny lobster, Claus found embryos in the ova with completely segmented bodies, but wanting the abdominal and caudal appendages and the last two thoracic somites. They have a single median eye, the anterior antennae are simple, the posterior have a small secondary branch; the maxillipeds are divided into two branches.

The most singular example of lowly development recorded by Fritz Muller is that of a prawn of the genus Penaeus.

The young appear to quit the egg with an unsegmented ovate body, a median frontal eye, and three pairs of natatory feet, of which the anterior pair are simple and the others biramose, agreeing with the larval form common to the lower Crustacea, to which O. F. Müller has given the name of nauplius. In this stage there is no trace of a carapace, no trace of paired eyes, no trace of masticating organs, and the mouth itself is overarched by a helmet-like hood. In one of these species the intermediate forms which lead from the nauplius to the prawn have been discovered by Fritz Müller in a nearly continuous series (op. cit.

ρ, 57)

After successive moults the nauplius gives place to the zoëa period, during which the paired eyes, the segments of the thorax and abdomen, and the various appendages are produced in budlike succession. The zoëa next passes into the røysss-stage; the antennae cease to serve for locomotory organs, and their place is taken by the thoracic feet clothed with setae (fig. 34). The abdomen, furnished with powerful muscles, jerks the animal through the water in a series of lively jumps.

In the case of those Crustacea in which the young, as in Mysis, are retained within the incubatory pouch of the parent after quitting the egg, the larva emerges from the egg in a far more rudimentary and destitute condition than in those genera in which no such protective arrangement exists. Van Bernden, whose description of the development of Mtjsis is confirmed by Fritz Müller, mentions the very curious fact that the first segment that makes its appearance is the tail. In other stalk-eyed Crustacea the embryo has the ventral surface of the anterior and posterior halves of the body folded together, and the dorsal surface forms the external convexity of tl∙a young animal within the egg; but in Mysis the ventral surface is external and convex. The tail soon acquires the furcate form characteristic of the zoëa of the prawn; two thick ensiform appendages next make their appearance at the anterior end of the body, and behind these a pair of tubercles; these are the antennae and mandibles. At this immature stage of its development the egg-membrane bursts before any internal organ, or even any tissue except the cells of the cutaneous layer, is formed. The young animal may now be said to be in its nauplius-stage, but its nauplius-skin resembles more nearly a second egg-membrane within which its further development proceeds. The ten pairs of appendages of the cephalic and thoracic divisions of the body make their appearance simultaneously, and at a later period the five abdominal feet. Soon after the young Mysis has cast its nauplius envelope, it leaves the brood-pouch of its mother.

In Squilla mantis the eggs do not adhere to the abdominal feet of the parent (which in this genus are branchiferous), but are (says Fritz Müller) deposited in the form of thin, round, yellow plates within its submarine burrow. The spawn is consequently difficult to procure, and quickly dies when removed from its natural hatchingplace. In the embryo of Squilla the heart is short; the body is long and segmented, but without appendages; the tail is bilobate, and there are indications of the rudiments of six pairs of limbs. If it acquires more limbs before exclusion, the youngest larva must be on a par with the youngest of Euphausia observed by Claus. Only two larval forms of Squilla are mentioned by Fritz Müller; the elder of these zoëa (fig. 35) resembles the mature Squilla, particularly in the structure of the great raptorial thoracic feet and of the last cephalic pair; but the six pairs of feet which follow these are still wanting, although their somites are clearly seen. The abdomen shows rudiments of four pairs of branchial feet and one or two pairs of biramose natatory feet, but the tail has no appendages and still appears as a simple lamina.

The investigations of Goodsir, in 1843, made us acquainted with a most singular family of Crustacea, the Diastylidae, or Cumaceae, which have been placed in the Podophthalmia near to Mysis. In general aspect the adult animal presents the most larval and embryonic characters, and might with propriety have been treated as a larval form, had not Goodsir, and subsequently Kröyer, actually taken the young from the brood-pouch of the parent. The antennae are very small, the thoracic feet are, inmost, furnished with setae; in Cuma and Alauna the abdominal segments are moniliform and destitute of appendages. The caudal segment bears two long bifurcated styles. In Bodotria (fig. 36) five of the abdominal somites bear finlets. The young examined by Kröyer, taken from the brood-pouch of the female (which resembles that in Mysis), were already one-fourth the length of the parent, which they resembled in every respect. Whether or not there is a considerable development of the young of Cïimaceae within the brood-pouch of the parent is not certainly known. In the embryo the caudal portion is bent upwards as in the Isopoda, and the last pair of thoracic feet are wanting.

The development of the Edriophthalmia, or sessile-eyed Crustacea, is more simple than that of the stalk-eyed forms. In the “rock-slater,” Ligia (fig. 37), the embryo is bent upwards within the egg, as in Mysis, and has also, like Mysis, a larval membrane within which the young Ligia is developed. In Mysis this larval skin may be compared to a nauplius; in Ligia, however, it is destitute of appendages, and resembles a maggot with a long simple tail (fig. 37). The dorsal surface of the young Ligia is united to the larval skin a little behind the head. A foliaceous appendage is produced at this point, but exists only for a short time, and disappears before the young slater quits the brood-pouch of the mother. The young animal, when it commences to take care of itself, resembles the parent, save that it has only six, instead of seven pairs of ambulatory feet, and the last thoracic somite is but slightly developed and is destitute of appendages. The sexual peculiarities in this as in the young of other Crustacea are not developed at this early period; thus the males lack the hand-like enlargements of the anterior ambulatory feet, and the copula tory appendages are also absent.

The eggs and early stages of Asellus aquaticus have been investigated by numerous observers. De Geer, Rathke, Spence Bate, and Anton Dohrn have made careful observations on the embryology and development of this abundant freshwater Isopod. As in Ligia the embryo is bent upwards within the egg. It quits the egg in a most imperfect state, more so, says Rathke, than any other articulated or vertebrated animal. It is furnished in its earliest stages with two lateral external appendages, which probably are homologous with the foliaceous appendage observed by Fritz Müller at the back of the head in Ligia. These zoëal appendages are subsequently moulted. Moreover in the young Asellus there are only six leg-bearing segments, and six, instead of seven, pairs of legs as in the adult. The curvature of the embryo upwards, instead of downwards, seems to have been generally observed by Rathke, Dohrn, Fritz Müller, and others. The larval skin is in some genera so closely applied to the egg itself as possibly to be mistaken for an inner egg-membrane. The absence of the last pair of thoracic feet seems also a constant character; all the other limbs are usually well developed in the young of normal Isopods; but in the remarkable and aberrant genus Tanais (fig. 38) all the abdominal feet are wanting, but not the caudal appendages; they make their appearance, however, simultaneously with the last pair of the thoracic feet.

Among the many interesting facts relative to the development of the Crustacea not the least remarkable are the series of retrograde metamorphoses which certain Isopods undergo as a consequence of their assuming a parasitic mode of life when adult. Thus the Cymothoa∙, or “Fish-lice,” which in the adult state live parasitic on fishes, clinging firmly by means of their short recurved hook-like feet, arc lively free-swimming Crustacea in the larval state. Still greater is the metamorphosis which the adult female under goes in Bopyrus (fig. 39), Phryxus (fig. 40), Ione, Gyge, and several other allied genera, which are parasitic on crabs and lobsters, taking up their abode within the branchial cavity. The adult is usually quite destitute of eyes; the antennae are rudimentary; the broad and flat body is frequently unsymmetrically developed in consequence of the confined space in which it lives; its segments are more or less amalgamated together; the feet are stunted, and the abdominal appendages transformed into foliaceous or highly branched gills. The males are diminutive in size, but usually they have their eyes, antennae, and feet better preserved than the females; the abdomen is, however, rudimentary, and not unfrequently altogether destitute of appendages.

Among the Isopoda, in the remarkable genera Cryptothiria, Cryptoniscus, and Entoniscus, we meet with forms even still more debased in their adult parasitic condition than Bopyrns.

In the case of Cryptothiria Balani (fig. 41), first noticed as a male cirripede by Goodsir in 1843, but not rightly determined until 1851, by C. Spence Bate, the female is a large inert seven-lobed fleshy mass, destitute of exserted antennae, jaws, legs, or branchial appendages, lying within the shell, and attached to the base of the animal of Balanus balanoides. The male is free and resembles the male in the Bopyridae; its body is long and slender, and is furnished with seven pairs of legs; it has been met with by Spence Bate, Dana, and other observers within the body-cavity of Balani. Here then we have a Crustacean belonging to a higher order, viz., the Isopoda, living parasitic within the shell and deri ring its nourishment from one belonging to a lower order, viz., the Cirripedia.

The history of Cryptothiria pygmaea (Rathke, sp.) and Cryptoniscus planaridides (F. Müller) is perhaps still more remarkable. Professor Bell had long ago noticed the frequent presence of a singular parasite on the inner surface of the abdomen of Portunus and Carcinus on our coasts, having prima facie the aspect of a bag of immature eggs. This had keen described by Rathke in 1841 as an Entozoarian, but has since been proved by its transformations to be a Cirripede, and was named Peltogaster. In 1858 Lilljeborg found what he deemed to be a female Peltogaster with an egg-sac; but a careful dissection led to the discovery that another parasite of a higher order, namely a Cryptothiria, had become parasitic upon the parasite. The most curious part of this super-parasitic history is, that the roots of Sacculina and Peltogaster (two forms of rhizocephalous Cirripedia parasitic on crabs and hermit-crabs) seem constantly to be made use of by two parasitic Isopods, namely, a Bopyrus and the before-mentioned Cryptoniscus planaridides. These take up their abode beneath the Sacculina, and cause it to die away by intercepting the nourishment conveyed by the roots; the roots, however, continue to grow, even without the Sacculina, and frequently attain an extraordinary extension, especially when a Bopyrus obtains its nourishment from them (Fritz Müller, op. cit. p. 94). The free males and the young of Crvptothiria and Cryptoniscus are unlike young Cirripede⅛ but resemble the young of Bopyrus; they are, in fact, larval Isopods.

The female of Entoniscus (fig. 41) resides within the body of a species of Porcel- laιιa, lying in a thin-walled sac between the liver, intestine, and heart of its host, the head being destitute of eyes or antennae; the thorax has become an irregular inarticulate sac, beset with enormous brood laminae; the long vermiform and extremely mobile abdomen has sword-shaped legs; and swelling out above it in a globular form, as if in a hernial sac, the heart lies at the base of the first segment. The young of this singular parasite closely resembles that of Βopyvus and Cryptothiria.

The embyro of the Amphipoda can be distinguished from that of the Isopoda at a very early period; the former being bent downwards with the dorsal surface external, whilst the latter is bent backwards with the ventral surface external. The embryo in all the genera which have been examined is attached on the anterior part of the back to the inner egg membrane by a peculiar structure—reminding one of the union of the young Isopoda with the larval membrane, and of the unpaired “adherent organ” on the nape of the Cladocera, so remark ιbly developed in Eυadn,e, and persistent through its life in that genus; but though present in the young of Dj,p'ι>ιia, it disappears in the adult (Frits Müller).

The metamorphosis of the young Amphipod after it quits the egg seems greatly reduced and simplified, for before quitting the egg it acquires its full number of segments and limbs. In those instances in which certain of the segments are amalgamated together, or where one or more segments are deficient in the adult, we find the same fusion and the same deficiencies in the young animals taken from the brood-pouch of their m )ther. The development of the Ilyperiidae, an oceanic group of Amphipods found only in the gill-cavities of the Medusx, is very exceptional and remark ιble. Thus, in Hyperia the youngest larvae, taken by Fritz Muller from the brood-pouch of the mother, already possessed the whole of the thιracic feet; on the other hand, those of the abdomen were not as yet developed. All the feet are at first simple, but soon become converted into highly denticulated prehensile feet. In this state they remain for a very long time,—the abdominal appendages growing into powerful natatory organs, whilst the eyes, at first wanting or very minute, expand into large hemispheres occupying the entire lateral, and even encroaching upon the dorsal anil frontal walls of the head. The females (bi y per id) are distinguished by a very broad thorax, and the males (Leslrigonufi) by their long antenne. The youngest larvae c ιnnot swim, but are provided with chelate feet (as shown by Spence Bate) by which they cling firmly to the swimming-laminae of their host. The feet of the adults are simple, but they are then excellent swimmers, and are not unfrequently met with free in the open sea. The diversity in structure of the antennae in the adult male and female Hyρeriidae is so great as to have led naturalists to place them in separate genera or even families; but this difference is developed only when the animals are full-grown. Up to this period the young of both sexes resemble the females. In the male shore-hoppers (Orchestid) the second pair of the anterior feet is provided with a powerful hand (fig. 45), as in the majority of Amphipoda, but quite different from the fem ιle; the young nevertheless resemble the female. This is also the case in the adult male in Limulus, in which the second pair of appendages (antennae) are peculiarly modified (see 3ct in fig. 12 above); but in the young male they exactly resemble those of the adult female. According to Spence Bate and Fritz Müller, this second pair of antennae are absent in the females of Βrachyscelus, although the male possesses them, like other Amphipcds.

In the foregoing brief sketch of the evolution of the young in the Malaccstraca, it will be perceived that certain lines of development are followed, but these are subject to great diversity, and often vary greatly in the same order. Thus we have :—

I. The larval metamorphosis undergone within the egg. II. The larval metamorphosis undergone within the incubatory pouch of the mother.

III. I he young first appearing as free-swimming roi⅛ ’. IV. The young first appearing as nauplii.

These four stages in the larval development of the Crustaceae, it will be perceived, are not by any means strictly confined to particular orders of the Mclacostraca, nor do they hold good for all the members of the group in which they have been observed to occur. There is, in fact, no “hard and fast” rule in the class, but on the contrary, there would appear to be numerous exceptions and variations in every group.

In considering the larvi∙l development of the Entcmcs-traca, we shall find that their early history, when compared with that of the Malacostraca, is greatly simplified, and that the first er nauplius foim of 1he young, which Fritz Müller exceptionally met with in Fcuaus, has now become the rule almost without exception.

Embryology of Limulus. — Starting with that remarkable representative of a most ancient and now almost extinct order, the Merostomata, we find in Limulus a genus in which the young may be said to undergo all their earlier metamorphoses wûthintheegg, thusat once offering an exception to the general rule as regards the Ent¢mostraca. The embryology of Limulus has keen investigated by Dr Anton Dohm and Dr A. S. Packard. The natural history of the king-crab has been studied by the Rev. Samuel Lockwood, and its anatomy has quite recently formed the subject of two elaborate memoirs by Professor Owen, and by Dr Alphonse Milne-Edwards.

We can only very briefly notice these important contributions to our knowledge of the Xiphosura here.

The female Limulus of the north-east American coast spawns twice every year during the months of May, June, or July, at the great high tides. It comes up to near high-water mark, spawning under water; thus the eggs are daily exposed to the sun’s warmth for a short time at low water. Great numbers arrive in pairs, the male grasping the sides of the shield of the female with his strong and peculiarly modified chelate antennae. The eggs are deposited by the female in a hole in the sand, and are fecundated by the male after deposition, and are then left to hatch. Only one other similar case is on record, namely, that of the common freshwater cray-fish, in which, according to Μ. Chantran, the eggs are fecundated after expulsion from the oviducts. The eggs occupy from fifty to seventy days in hatching, according to the favourable or unfavourable conditions under which they are deposited; some which Dr Lock wood set aside in a jar of sea-water in a dark place hatched after 350 days!

The egg has two membranes, a dense inelastic chorion and an inner elastic protoderm. This chorion remains entire so long as development is arrested or is sluggish, but as soon as the embryo increases in size, the tough chorion splits asunder, and the inner elastic protoderm enlarges, becomes dense, and vicariously fulfils the duties of the former (fig. 46).

A similar splitting of the external egg-membrane has been noticed in A pus. Fritz Müller points out that in some Isopoda (as for instance PhHoscia) the larval skin is not only without any folds or sac-like diverticula, but is closely applied to the egg-membrane. This second eggmembrane in Limulus may perhaps therefore correspond to this first larval skin. Certainly, when the embryo first appears, its position is the same as in Asellus, Ligia, Philoscia, and other Isopcds, i.e., with its ventral sur face convex (figs. 47 and 48). In its first stage the larval Limulus has six bud-like indications on each side of the mesial line, where the paired cephalic appendages will be developed (fig. 47). In later stages (figs. 48-52) we have first two, then more, up to six pairs, of thoracic natatory feet (which in the adult become branchiferous), and traces of as many as nine post-cephalic somites, but the last three never attain appendages. As the young Limulus increases in size, the yolk gradually becomes absorbed, and the larva assumes the position of an Amphipod in the egg, having its dorsal surface convex, instead of its ventral, which is now concave (fig. 52). There appears to be no stage seen in larval Limulus which can be compared with the nauplius stage of Apus (as Dr Packard has supposed). The Trildbiten stadium of Dohrn resembles Prestwidιia rotundata from the Coal-measures far more than any known Trílobite. Packard has figured an earlier (?) stage of Limulus than Dohrn’s “Trilobiten-stadium,” which much more nearly resembles Trinucleus (compare figs. 51 and 55).

In its earlier stages the young Limulus can roll its posterior segments under its head-shield, and when it at last leaves the egg it can swim well, and has been captured by Alexander Agassiz, swimming freely on the surface of the ocean, three miles from Naushon Island, Buzzard’s Bay. At this period it has no caudal spine or telson; this is acquired only at a later moult, whilst a year or more elapses before the young males can be distinguished by their modified antennae from the females (see 3α in fig. 12).

In the shield-bearing, naked, and bivalved Phyllopoda, represented by Apus, Nebalιa, Branchipus, and Estheria, the embryological development is no doubt analogous, though less is known in regard to Estheria (fig. 56).

In Apus the male is not certainly known, all the progeny observed being fertile females. Probably, however (as is the case in Daphnia), males appear at a particular season of the year, and these suffice to render fertile several generations of females. The females let their eggs fall to the bottom of the water, where they remain until hatched by the sun’s warmth. The eggs of Apus occupy from two to there weeks in hatching; the chorion splits, as in Limulus, revealing a semi-transparent inner egg-membrane. This also soon after bursts, giving freedom to a simple nauplius like that exceptionally met with by Fritz Müller in a prawn allied to Penaeus. The large cephalic shield, so characteristic of the adult Apus, is not seen in these early stages, and when it first appears, it closely resembles that of Peltocaris, an extinct Silurian form (4 in fig. 57). The nauplius, being short-bodied, does not display those graceful undulatory motions in the water observed in the adult, but progresses rather by a series of jerks like the adult Cyclops. At the end of eight or ten days the young Phyllopod has acquired considerable size. The body-seg ments and feet, so numerous in Apvae, Artemia, and Branchipus (5α in fig. 57), are formed gradually (in repeated moults) from before backwards, without any sharply-defined regions of the body being discernible either by the time of their appearance or their form. All the feet are of the same pattern, and resemble the maxillae of the higher Crustacea.

The young animal exuviates about twenty times during the first two or three months; it is then full grown, and in every respect resembles the parent.

Nebalia (2 in fig. 57) presents a remarkable exception to the rest of its order, the young apparently (like Daphnia) undergoing no metamorphosis after they quit the egg. Metschnikoff, who has recently studied the development of Nebalia, states that he has observed that it passes through both a nauplius and a zoëa stage within the egg, and he therefore regards Nebalia as a Fhyllcpcdifoπn Decapcd. We hardly see sufficient grounds at present for assigning Nebalia to a higher order than that in which it is new placed.

The Cladocera do not afford any additional aid in embryological research. They appear to quit the egg enly smaller than the parent, but with their full number of limbs.

Of the developmental history of the Ostracoda but little is known. Zenker states that their anterior limbs are developed first, and the youngest stages, according to Claus, are shell-bearing nauplius-forms.

In the Copepoda (3-7 in fig. 58), the buckler doesnot cover more than the head and thorax, the abdeminal segments, which are nearly cylindrical, extending beyond it. They are met with both in fresh waters and in the sea all over the world, and are most numerously represented both in a free state and as parasites. The larvae of the non-parasitic forms (J>b-d in fig. 58), all possess at the earliest period the three anterior pairs of limbs, i.e. i the future antennae and mandibles,—the anterior pair with a single joint, the two following pairs being bifurcate. The eye is single, and the labrum and mouth already occupy their permanent positions. The hinder body is short, the abdominal segments not being yet developed. In subsequent moults these posterior segments appear, and new limbs sprout forth. In the second stage a fourth pair of extremities is added; these are the future maxillae; then follow three new pairs of limbs, the maxillae, and two anterior pairs of natatory feet. The three anterior pairs of appendages still represent rowing-feet. At the next moult the first cyclops-stage is arrived at, when there is a resemblance to the adult in the structure of the an t ennae and buccal organs, but the number of body-segments and appendages is still much less than in the parent. Only the rudiments of the third and fourth pairs of natatory feet are seen, and the body is made up of an oval cephalothorax, the second, third, and fourth thoracic segments, and an elongated terminal joint. In the Cgclo-pidae the posterior antennae have lost their secondary branch, and the mandibles have completely thrown off the previously existing character of natatory feet; whilst in other families these appendages are persistent, although more or less altered. Many of the parasitic Copepoda do not pass beyond this stage of free development. Such forms as Lernanthropus and Chondr acanthus never acquire the third and fourth pairs of limbs, nor does the fifth thoracic somite separate from the abdomen. Others, such as A htheres, even fall to a still lower grade, by the subsequent 1 J3s of the two pairs of natatory feet. But all free Copepoda and most of the parasitic Crustacea pass through a longer or shorter series of stages of development, in which the limbs acquire a higher degree of division into joints in continuous sequence, the posterior pairs of feet are developed, and the last thoracic segment and the different abdominal segments are successively separated from the common terminal portion (Claus). Some parasitic Copepods, such as Achtheres percarum, certainly quit the egg like the rest in the nauplius-stage; the oval astomatous body bears two pairs of simple rowing feet, and behind these are two inflations marking the third pair, each having a long seta. Beneath this nauplius-skin a very different larva lies concealed, which in a few hours bursts its clumsy envelope, and makes its appearance in a form which agrees both in the segmentation of its body and the development of its extremities with the first cyclops-stage. The entire series of nauplius-stages which are passed through by the free Copepods are in this case completely overleapt.

Although the parasitic species of Copepoda are all more or less permanently fixed when adult, they pass their youthful stages as freely locomotive larvae. To this rule there is a singular exception in the genus Caligus. The young animal (described by Burmeister as a peculiar genus, Chalimufi) lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When sexual maturity is attained the cable is cut, and the adult Caligus, which is an admirable swimmer, is not unfrequently captured swimming freely in the sea (Fritz Müller).

The animals belonging to the last division comprise two orders, the Rhizocephala and the Cirripedia. They have long been kept distinct from the Crustacea, and, together with various parasitic forms of Copepoda, whose developmental history w∙as not known, classed as Epizoa and Cirripedia.

By later zoologists the Rhizocephala have been placed with the Paecilopoda, but as this division includes many genera which prove to be merely parasitic forms of

Copepoda, it will be more convenient to separate them. All the animals of this last division, which for convenience wo would designate under the general name of Anchoracephala, are attached when adult;—in the Cirripedia by means of cement ducts which deposit calcareous matter, forming in the adult Balanid<x (figs. 59-60) a broad shelly base, an da simple attachment in the pedunculated Lepadùlae; in the Rhizocephala by ramifying nutritive roots, which sink deep into the interior of the body of the animal upon which they become parasitic (see figs. 82 and 83, p. 665). In all the members of this division the young appear as naupliiform larvae which speedily moult their first coat. The body is unsegmented and pyriform, having a median eye, a first pair of minute antennae, two anterior horns, which inclose the second pair of antennae, one pair of uniramous and two p1ir3 of biramous natatory legs, a forked terminal projection t > the b >dy, and a pj3terior point to the carapace (gee fig. 60 a, and fig.

61).

The nauplii of the Anchoracepha- 11 are distinguished from the Copepoda by possessing a d)rsal shield or ca-rapxc3, which sometimes, as in Sacculina purpurea, projects far beyond the body all round. They a’re also farther distinguished by possessing a pair of so-called “olfactory filaments,” which spring directly from the head. These filaments, or horns of the carapace (which are interpreted as the second pair of antennae by Darwin), are believed by Fritz Müller to be the homologue of the so-called “green gland,” which opens at the end of a conical process at the base of the inferior antennae in the Decapoda, and of the conical process, with an efferent duct traversing it, seen on the inferior antennae of the Amphipoda.

Tue abdomen of the young Cirripede is produced into a long tail-like furcate extremity, that of the young in Rhizocephala into a movable caudal fork.

The young Cirripedes have a mouth, stomach, <fcc., and their posterior pairs of limbs are fitted for organs of prehension and mtnducation. In the young of Rhizocephala all these organs are wanting. The young Cirripede, having to undergo several moults as a nauplius, is provided with organs to sustain its life. The young Rhizocephalon being astomatoιs, cannot sustain life in its nauplius state for long, and must therefore more rapidly pass through its transformitions. They both at length arrive at an equally ast imitons p;¢pa-stage. In this stage we see the young animιl with its carapace folded together like a bivalved shell; the foremost limbs become transformed into very peculiar adherent feet, and the two folle wing pairs, like the frontal horns, are cast off with the nauplius-skin. Behind these are six pairs of powerful biramose nιtatory feet, with long setae and two short setigerous caudal appendages. The young pupae of the Rhizocephala and Cirripedia agree in every particular, save that the latter possesses a pair of composite eyes; sometimes also traces of the frontal horns seem to persist.

When the proper time arrives the pupae of the Cirripedia attach themselves by means of their prehensile antennae to rocks, shells, turtles, cetacea, drift-wood, ships, &c.; the carapace becomes converted into the peculiar sessile-shell of the Balanus, or the pedunculated Lepus; the natatory feet grow into long cirri by which nourishment is whirled to the mouth, now open and furnished with mandibles and maxillae. The pupae of Rhizocephala in like manner attach themselves to the abdomen of crabs, Porcdlanae, and hermit-crabs; but they remain astomatous, lose all their limbs completely, and appear as sausage-like, sack-shaped, or discoidal excrescences upon their host, filled with ova (figs. 82 and 83); from the point of attachment closed tubes, ramifying like roots, sink deep into the interior of the host, twisting around the intestine, or are diffused among the sac-like tubes of the liver. The only manifesta tions of life which persist in this most retrogressively metamorphosed Crustacean are powerful contractions of the roots and alternate expansion and contraction of the body, causing water to flow into the brood-cavity, to be again expelled through a wide orifice (Fritz Müller).

Darwin has recorded various anomalous Cases of development in the Cirripedia; amongst others, that of Cryptophialus minutus (which forms a separate section Abdominalia, Darwin), parasitic in the shell of Concholepas peruviana. The egg, at first elliptical, becomes broader anteriorly, then acquires three club-shaped horns, one at each anterior angle and one behind. Subsequently the posterior horn disappears, and the adherent feet may be recognized within the anterior ones. From this “egg-like larva” the pupa is directly produced. Its carapace is but slightly compressed laterally, and is hairy as in Saccubna purpurea; the adherent feet are large, the natatory feet wanting, as are also the corresponding cirri in the adult animal. Mr Spence Bate mentions a similar case in a Rhizocephalon, in which the nauplius-stage is overleaped and the young quits the eggs as a pupa-form larva.

Exuviation in the Adult and Reparation of Injuries. —As we have already seen, the young Crustacean, on quitting the egg, usually undergoes a series of larval metamorphoses more or less numerous, and subject to considerable variation even among closely-allied forms. Eventually, whether by a direct or an indirect route, a form is attained in all, which, save in size, closely agrees with the adult.

Amongst the Insecta the larva usually undergoes repeated moults during its growth, from the time when it first quits the egg until it reaches the pupa-stage, a period of rest in most, but not in all insects, and an astomatous stage in some larval Crustacea. From the pupa springs the full-grown and perfect insect, when no further moult or change takes place—indeed, in some insects the parent only lives to deposit its eggs, and then dies. The immature Crustacean, in passing through its nauplial and zoëal stages, may moult its skin seven or eight times, or even more; nevertheless, when it reaches the imago stage, it has not nearly attained the size of the adult parent, but continues to grow and cast its calcareous envelope as often as its increased size necessitates its so doing. When adult, it still continues to moult probably through its entire lifetime, even to extreme old age.

In casting its-shell a crab not only parts with every joint and plate of its limbs and carapace, of its long and slender antennae, its external eyestalks, the plates of the tail, the appendages of the mouth, the lining of its gills, but even its stomach, with the gastric teeth and the slender apode ∙ mata, which give support to the muscles of the limbs and body—so that when the crab has escaped from its old suit, the cast-off shell seems nearly as perfect as the animal itself

When exuviating, the crab and lobster both escape from their old shells by a line of dehiscence which opens between the posterior border of the carapace and its union with the abdomen. Professor Bell also states that in the great crab (Cancer pagurtιs) and some other forms, the carapace divides at the junction of the epimera with the dorsal piece or tergum.

In Limulus the carapace splits all round the anterior border, at the union of the dorsal and ventral walls. Limulus sheds its shell five to six times during the fii¾t year, and probably once annually after that period.

Sir John Dalyell, Mr Couch, Mr Gosse, Mr Spence Bate, Mr Warrington, and others, have given excellent accounts of the process of moulting of various Crustacea. An accurate observer, Mr Harper, states that he confined six small specimens of the common shore-crab (Carcinus mamas) in separate glasses, and fed them daily, until one of them showed that something was amiss by refusing food. Soon after it cast its shell, an operation which only occupied five minutes. When very young this crab moults frequently. The same author registered the dates, and preserved the exuviae of one which moulted on April 11, 1858, andon May 22, July 3, August 30, and September 26 of the same year; the acceleration of the last moult is attributed to the creature having been fed daily, “like a prize beast,” on purpose to try the effect on its growth. Some of these little crabs had lost part of their limbs, but after a moult new limbs appeared of very diminutive size; after a second moult each new limb had increased to one-half as large as the rest, and in the third moult it had reached to its proper bulk and form. Hermit-crabs shed their hard shell before pulling off the exuviae of the tail; their increase at each moult is much less rapid than in the common crab. Prawns exuviate more frequently. Mr Warrington saw the change occur with much regularity every twelve days in the summer season. With the exception then of certain parasitic forms of Crustacea, which, like the Rhizocephala, have undergone such a complete retrograde metamorphism that no trace of articulations or appendages remain, all the Crustacea periodically exuviate their dermal covering, whether calcareous, chitinous, or membranaceous. In the Cirripedia it would not be possible to exuviate the adherent shell of the adult Balanus, or the peduncle of Lepas or Scalpellum, but, even in this aberrant division, the lining membrane of the shell and the many-jointed cirri are regularly moulted.

It has long been known that Crustacea possess the power of voluntarily casting their limbs, and of restoring such as have thus been lost by the animal’s will or by accident. If one or more distant phalanges of a limb be torn off, the animal has the power to throw off the remaining part of the limb also. This separation always occurs near the basal extremity of the first phalanx. When the limb is thrown off, the blood-vessels and nerve retract, thus leaving a small cavity; from this the germ of the future leg springs, and is at first seen as a nucleated cell. A cicatrix forms over the raw surface caused by the separation, which afterwards forms a sheath for the young leg. Principal Divisions of the Crustacea.

The subjoined table is intended to give only a general outline of the Crustacean class, with the sub-classes, legions, and orders.

Of the thirteen orders enumerated two only (printed in italics in the table) are extinct, namely the Trilobita and Euτypterida. These two lost orders disappeared in the Carboniferous epoch.

Table of Classification of the Crustacea, Class CRUSTACEA.

Sub-Class 1. THORACIPODÅ (or Malacostraca).

Legion I. Podophthalmia.

Order 1. Decapoda.

Sub-Order (a) Brachyura, Crabs. (b) Anomoura, Hermit-crabs.

(c) Macroura, Lobster, Prawn.

,, 2. Stomapoda, Squilla, Mysis, Diastylidae.

Legion II. Edriophthalmia.

,, 3. Isopoda, Oniscus, Idotea, Sphaeroma. ,, 4. Trilobitλ, Phacops, Asaphus, Calymene, de

,, 5. Amphipoda, Talitrus, Gammarus, d∙c.

Sub-Class 2. GN A TH0P0DA (or Entomostraoa).

Legion III. Merostomata.

,, 6, Xiphosl t ra, Limulus, Bellinurus, <L∙c.

,, 7. Evryγteridλ, Eurypterus, Pterygotus.

Legion IV. Branchiopoda

,, 8. Phyllopoda, Apus,Nebalia,Artemia.

,, 9. Cladocera, Daphnia, Lynceus, d~c.

Legion V. Lophyropoda.

,, 10. Ostracoda, Cypris, Candona, Cythere.

,, 11 Copepod a (a) Liberata: Cyclops, Cetochilus, Diaptσmus. (b) Parasita: Lernanthropus, Caligus, Nicothoe.

Legion VI. Anchoracephala.

,, 12. Rhizocepiiala, Sacculina, Pcltogaster.

,, 13. Cikripedia, (fl) Balanidce, Ac.

(ò) Lepadidae, &c.

If the old definition between the two great groups, tho Malacostraca and the Entomostraca, be maintained, namely, that the former shall consist only of forms having more than twenty-one segments, the introduction of the Trilobita among these may be looked upon as inappropriate. If, however, we admit that the Trilobita had (as there seem good grounds for allowing) true special locomotory appendages other than gill-feet or jaws, as in the Malacostraca generally, then we submit that they are appropriately classed. The main characteristic of the Malacostraca seems to be, not so much the possession of twenty-one segments, an inheritance really common to the whole class, but the presence of the seven anterior (cephalic) appendages especially set apart for the senses and nutrition, with separate post-cephalic organs of locomotion;—whereas the peculiarity of the Entomostraca seems to be that the seven anterior (cephalic) organs are not specially set apart as organs of sense and nutrition, but are employed in nearly all the class as the chief locomotory organs, the posterior feet being branchial or ovarian lamellae, or altogether want· ing. The writer ventures to propose therefore, instead of

Explanation.—The letters at the top of each column represent the several orders and sub-orders of Crustacea,—one family of the Arachnida, the Scorpionidae, being placed beside the Eurypterida to show its range in time and probable derivation from the Crustacea. The vertical lines show the actual range in time, the black dots denote the strata in which remains of each order have been met with. The curved lines, uniting order to order, are intended to show the probable evolution of the class fron a common great ancestor; but numerous as are the indications of affinities afforded by the Crustacea, the group is too ancient to be certainly traced back in time to a common parent; even the lowest Cambrian rocks have yielded evidence of two orders, namely, the Trilobita and Ostracoda, whilst the Middle Cambrian furnishes a Phyllopod Crustacean (Hymenocaris). As the branches of this genealogical tree of the Crustacea are not arranged in a circular manner, they cannot of course be made to show the affinities which each order presents to every other order or branch, any more than does the dried plant show the natural verticillate arrangement of its leaves and branches when pressed out flat upon the page of a Hortas Siccus.

L=the Lophyropoda. Of this legion, the order Ostιacoda is well represented throughout the entire series, from the Cambrian to the Tertiary and also in the seas and fresh-waters of to-day.

C=theCiRRtPEDiA.-Z=the Lepadidae, or pedunculated Cirripedes, appear first In the Wenlock Limestone (U. Silurian); represented by a single form (Turrilepas Wrightianus, H. Woodw.). Numerous forms occur in the Secondary and Tertiary strata, and they are abundantly distributed throughout the warmer seas of the world, δ=the Balanidae, or sessile Cirripedes, are represented by a single form, the Py∣∙goma cretacea, H. Woodw., in the Chalk, abundantly in the Tertiary rocks, and in recent deposits, and are distributed in the seas all over the world at the present day.

B=the Branchiopoda. —Of this division the order Phyllopoda appears in the Cambrian rocks, and is well represented in Silurian and Carboniferous strata; they are also met with in the Secondary and Tertiary formations, and living in fresh, brackish, and marine waters, widely distributed over the globe.

A=the Amphipoda. —This order has a single representative in the Upper Silurian, Necrogammarus Salweyi, H. Woodw.; it is represented by Gampsonyx? in the Coal, by Prosoponiscus in the Permian, and by several forms in the Secondary and Tertiary formations, and abundantly in recent freshwater and marine localities.

7=the Trìlobit^· This extinct order appears in the Cambrian, attains its maximum development in the Silurian, and terminates in the Carboniferous period.

I=the Isopod a.— This order is represented in.the Devonian by a single species, Praearcturus gigas, H.Woodw., and by remains in the Carboniferous strata. Many species occur in the Secondary and Tertiary strata. It is largely represented today by land, freshwater, and marine types.

S=Stomapoda.- Pygocephalus Huxleyi, H. Woodw., from the Coal-measures, probably belongs to this division, True Squillas and Jfysis-like Crustacea occur in the Jurassic rocks (Secondary). Forms of this order are abundant in our modern seas.

M=the Macroürak division of Decapoda. A single species, Anthrapal<emon Grossartii Salter, appears in the Coal-measures. This order is well represented from the Trias to the present day, and is now one of the prevalent types, occurring both in fresh and salt water.

Aa=the Anomoura, or irregular-tailed Decapoda. This sub-order embraces forms related both to the Crabs (e.g. Dromia, Porcellana, Dorippe) and to the Lobsters (e.g. Pagurus, Galathea, JUunida). Their earliest appearance is in the Cretaceous period; there are numerous living forms, both terrestrial and marine.

Ba=the Brachtüba. —The oldest known crab is the Palaeinachus longipes, H. VVoodw., from the Gt. Oolite. Crabs are well-represented from the Upper Secondary to the present day, when they attain their maximum within the warmer latitudes, being represented by land, freshwater, and marine forms.

X=the Xiphosura, or king-crabs. These are remarkable for their longevity; they appear first in the Upper Silurian (Neolimulus falcatus, H. VVoodw.); again in the Coal-measures, next in the Oolite and Tertiaries, and living to-day in the Old and New worlds.

E=EL nvpτEnrt)A.-This extinct order contains some of the largest known members of the Crustacean class (e.g, Pterygotus anglicus, Devonian). It ranges from the Upper Silurian to the Coal-measures. On morphological grounds there is good reason to conclude that the Eurypterida are the ancestors of the Scorpionidae, to which they present the strongest affinity.

A=ARACHNIDA. S= Scorpionidae.— The scorpions range from the Coal-measures, apparently unaltered, to the present day.

the terms Malacostraca and Entomostraca, which convey no idea of any structure or function common to either division to which they are applied, the adoption of the terras Thoracipoda and Gnathopoda, which embody the salient character in each sub-class.

The Trilobita are probably represented to-day by the Isopoda, to which doubtless they are closely related. There is reason to believe the members of the other extinct order, the Euryρteιfida, to have been the aquatic branchiferous ancestors of the terrestrial tracheated air-breathing Scorpionidae; nevertheless they need not on that account be removed from their present position in the Crustacean class; and the Scorpionidae should of course still form a part of the class Arachnida, which may, however, be conveniently placed beside the Crustacea, as in the annexed diagram (fig. 64), in which their probable morphological and ancestral relationship is indicated.

Types of Existing Crustacea.

Sub-class 1. Thoracipoda (or Malaeostraca).

I. Podophthalmia : (1.) Decapoda —(a) Brachyura. —Crabs are certainly the highest representatives of the Crustacean class, and in this ten-footed order are included some of the most active and intelligent members of the community,—the “land-crabs,” and “shore-crabs,” and also the largest living representative of the class, the Inachus Kempf eri from Japan.

Crabs furnish the best illustration among the Crustacea of that concentration of organs around a single nerve-centre, which Professor Dana aptly terms cephalization.

Instead of a long vermiform body composed of a large number of annuli, each having its own nerve-ganglion, vve have in the crab one large cephalo-thoracic ganglion representing nearly the entire nerve force of the body, the supraaesophageal ganglion only giving rise to the nerves of sense and volition. (See fig. 9, nerves of Maia. )

This highest cephalized type is exemplified by Maia, but as a matter of fact the triangular crabs, of which Maia squinado and Inachzt,s Kempferi are examples, do not embrace, by any means, the liveliest and most intelligent of the order; on the contrary, we should decidedly award the highest place for intelligence to the quadrangular land and shore-crabs; indeed, it is amongst such genera as Grapsus, Gelasimus, Ocypoda, Gecarcinus, &c., that we find the most rapidly moving terrestrial forms of Crustacea. Most of the land-crabs retreat to burrows in the ground during the heat of the day, and issue forth at dusk to feed on the growing crops of sugar-cane, rice, or maize. The Gecarcinus ruricola (see ante, fig. 21) is peculiarly destructive to the young sugar-canes in the West Indies. In the highlands of the Deccan land-crabs are most abundant. Gecarcini are found at Mahableshwar at an elevation of 4500 feet above the sea. These land-crabs probably do not visit the sea at all, as do the Jamaica land-crabs, but deposit their eggs, when near the time of hatching, in the freshwater streams, the banks of which they are known to frequent. Many of the land-crabs have the chelate limbs largely developed, usually more strongly so in the males, e.g., the male of Macrophthalmus Latreillii; in others one claw only is very disproportionately enlarged, as in the males of the “calling-crab” (Gelasimus), which are said in running to carry this claw elevated as if beckoning with it. Fritz Müller says, however, that the species common in Brazil (a small Gelasiτnus with one claw very large) always holds it closely pressed against, its body. Vast numbers of landcrabs are met with on the sea-shore and among rocks along the coast, especially in the warmer temperate and sub-tropi cal regions of the earth. Of the genus Thelphusa one freshwater species ( 7 τ . fluviatilis) is a native of the rivers of southern Europe. It is eaten by Catholics during Lent, and hence called “Lenten crab.” This crab is also common to the rivers of India.

Although some land-crabs are certainly vegetarians in diet, the class, as a whole, are carrion feeders, greedily devouring animal matter even in a putrescent state. The Portunidae and Carcinidae perform the duties of sanitary police around our coasts between tide-marks, being assisted by swarms of “sand-hoppers” (Talitrus locusta) ; whilst below low-water mark the prawns, Maias, great crabs, and lobsters share the task. Many sea-side resorts would be extremely unwholesome were it not for the labours of these useful but unpaid scavengers.

The swimming-crabs are mostly predaceous; forms like Portunus pelagicus and Polybius IIenslowii (ng. 66) have exceedingly thin shells, and all the feet, save the great chelate claws, are modified into oars. They are thus enabled to live and hunt at their ease, often hundreds of miles from land. The writer has seen Henslow’s swimming-crab in the middle of the Bay of Biscay far out of sight of land. Crouch, the Cornish naturalist, states that they fasten upon pilchards and mackerel with their knife-like claws, and never relax their hold until the terrified victim floats exhausted on the surface.

Two genera of Tertiary Land-crabs have been described from English localities (Goniocypoda and Litoricola) f. Macrophthalmus occurs fossil in China, where it is prized as a valuable materies medicaβ Panina,“ the frog-crab ’ of the Indian Ocean and Japan, occurs in Tertiary rocks β in Bunde and Ebenda, Germany, in San Stefano, Italy, and in Malta. Rumphius says, “It loves to climb upon the roofs of houses, n

More than fifty genera of fossil Crustacea, referable to the Decapoda-Brachyura, have been described by Milne-Edwards, Bell, Reuss, M i Coy, H. Woodward, and others The oldest known crab is the Palaeinachus longipes, H. Woodw., from the forest marble, Wilts (Quart. Journ. Geol. Soc., 1866, vol. xxii. Pl. xxiv. ρ. 493).

I. Podophthalmia: (1.) Decapoda —(δ.) Anomoura.— The irregular-tailed or Anomourous Crustaceans, of which the hermit-crab is a type (figs. 20 and 67), are excellent examples both of arrested development and retrograde metamorphosis in the adult, resulting from disuse and consequent atrophy of particular parts or organs.

We have seen among the Isopoda the females of the Bopyridae, which live parasitic within the branchial chamber of other Crustaceans, or under their abdomen, or within the parietes of a Balanus, or actually within the body of a Porcellana. In the Rhizocephala we have seen the free.- swimming pupa become attached to the soft body of the Pagurus, cast off its shell, lose all its limbs, and appear as a sausage-like, sack-shaped, or discoidal excrescence, without even a mouth, its body, filled with ova, attached by its antennae, which are modified into roots, that anchor it and at the same time bring it ready prepared nourishment from the juices of its hosts (see figs. 82 and 83). After such extreme retrogression, the depauperization of certain parts and organs observable in the Anomoura is easily to be understood and admitted.

If we bring together for study a series of Anomourous Crustacea, we shall be at once able superficially to divide them into two sections, the Macrourous and the Brachyurous Anomoura. [table]

We have also burrowing forms which obviously are near to these, although not actually classed with them, viz., [table]

In both these sections of the Anomoura we find the same peculiarity, namely, that the fifth pair of (thoracic) legs, and sometimes indeed the fourth and fifth hinder pairs, are not formed for walking, but are minute and rudimentary, and are placed above the level of the other legs.

In Porcellana and Lithodes, in Galathea and Munida, the posterior legs are simply rudimentary. In Dorippe and Dromia, in Pagurus and Birgus, though still disproportionately small, they are modified into organs for holding on with. This rudimentary condition of the posterior thoracic feet in the Anomoura at once recalls the last larval stages of nearly all the Malacostraca, in which the hinder thoracic somites are not yet developed, or if so, are either destitute of appendages, or have only rudimentary ones.

The chelate form of the two posterior pairs of feet in Dromia and Dorippe, in Homola and Birgus, being a variable characteristic, and not present in all Anomoura, is doubtless developed as an individual specific modification, like the chelate penultimate feet in Brachyscelus and the ante-penultimate pair in Phronima s.

Some Anomoura have chelate terminations to a pair of their rudimentary feet, others have both pairs simple. This is at once elucidated when we inquire into their economy. Dromia does not carry about a turbinated shell like Pagurus, but clothes itself with the skin of its victim, a “sea-lemon” (Doris) for example, or encourages a parasitic sponge of showy colour to grow upon its back, holding it in its place with its two hind pairs of rudimentary feet, just as the other true hermits hold their shells on over their soft-skinned bodies (Gosse).

Professor Verrill has described a Dorippe (D. facchino), which always carries an Actinia (the Cancrisocia expansa, St.) upon its back. Like most other cases of commensalism this friendly association of the crab and sea-anemone w as begun long ago and has been regularly adhered to. When young the Dorippe carries a small shell (1 the half of a bivalve), which it holds in position by means of its two hind pairs of legs. The Actinia fixes itself when young to the shell, and afterwards by its growth completely conceals the carapace of the crab, replacing the shell which is, no doubt, after a time either disintegrated or abandoned when the Dorippe moults. A Sagartia, associated with a hermit-crab on our own coasts, is said entirely to dissolve away the Buccinum in which the Pagurus is lodged, and to supply its place, as in the case of Dorippe, with its expanded foot.

Several specimens of a small species of Pagurus, common on the French coast, have been brought to the writer, each crab tenanted in what appeared, to be the shell of a small Buccinum or Nassa ; but the whole was so completely encrusted by a sponge as to leave no part visible externally. On cutting one open vertically the spiral form of the interior cavity of the shell was very distinctly seen, but the shell itself had been entirely dissolved away by the sponge.

A Zoanthus has been described by Duben and Koren under the name of Mammalifera incrustata, which is commonly found parasitic on shells that are tenanted by a species of Pagurus. In all cases the shell is destroyed after a while, not by the hermit-crab, but by some process of disintegration or absorption, the diffused basal crust of the Zoophyte forming a perfect cast of it, and affording shelter to the crab. This form occurs in Shetland and in the North of England, as well as in Norway, and is regarded by Mr Hincks as distinct from Zoanthus Couchii, Gosse (Rev. T ¾ Hincks, Ann. and Mag. Nat. Hist. 1862, p. 304).

The Adamsia palliata always selects shells tenanted by Pagurus Prideauxii, but instead of adhering to the spire the “cloak-anemone” fixes itself to the smooth inner lip of the shell, so that when the hermit is feeding, the mouth of the anemone is just below that of the crab, and ready to receive any fragment he may let fall. When the Adamsia is very young, less than half-an-inch in diameter, its outline is circular; but as it grows older it expands laterally, forming two lobes, which creep along the mouth of the shell, until they meet and coalesce on its outer lip. The base of the cloak-anemone is then perforated, and through this opening the hermit puts out or retracts his head and legs.

In all the Anomoura the abdomen is more or less modified; for instance, in the Brachyurous type, it is not closely bent under the body, as in ordinary crabs, some Porcellanae (fig. 68) carrying it extended straight out; whilst in the long-tailed forms, like Galathea, the epimera are shorter, and the segments are less arched than in the lobster. The caudal plates are also more rudimentary. In Pagurus (fig. 67), the abdomen is naked, only a mere trace of the shelly plates remaining. In Birgus latro the sternal portion only of the abdominal somites remains.

If we turn for an instant to the Thalassinidae, a family of burrowing Macroura, we find the hard and shelly epimeral pieces of the body-segments are not properly developed, and the lobes of the tail are in like manner rudimentary; the integument of the body is extremely thin and soft (approaching that of the hermit-crabs, which like themselves live concealed in various foreign substances living and dead, e.g., shells, sponges, Actiniae, <fcc.).

There can be no doubt that both in the case of the true hermit-crabs, and in that of the burrowing Crustacea, the non-development of the hard calcareous covering to the abdomen (in Callianassa extending to the whole body except the legs and chelae) is due to the same cause, viz., disuse of the abdomen and caudal fin for natation, and to their constant habit of living in concealment.

In the case of Birgus latro, when quite young it pro· bably conceals itself in some shell, but as it grows to such a large size and becomes so enormously fat, from feeding upon the cocoa-nut, it must abandon its early disguise and conceal itself within a burrow instead. The writer has received a small hermit-crab very like Birgus, the abdomen of which was concealed in a sponge.

If then the Anomoura be the descendants of certain Crustacea in which an arrested stage of development in the young has become a persistent character in the adult, or in which organs atrophied by disuse have at last come to bo suppressed or greatly modified, the conclusion seems obvious that we are dealing not with a distinct sub-order equivalent to the Brachyura or the Macrura, but with a group composed of various irregular forms at present placed intermediate to, but originally belonging to both these divisions.

Forms allied to Homola and Dromia—Homolopsis and Dromilites— occur fossil, the former in the Gault and the latter in the London Clay (in fig. 70); no other Anomourous forms have been met with, save some legs and chelae in the Chalk, which have been attributed to a Pagurus.

Remains of Callianassa are very abundant in the Cretaceous and Tertiary rocks of Europe, and have been found fossil even as far off as Japan. It is always the hands which are preserved, the body being usually too delicate for fossilization.

I. Podophthalmia : (1.) Decapoda —(c.) Macroura. —The common lobster and prawn are excellent examples of the Macroura. In this truly aquatic type the abdomen is no longer rudimentary, as in the crabs, but is developed into a powerful organ for leaping and swimming, r lhe body-segments are of nearly equal growth, and being compressed at the sides, or cylindrical in form, they present a well-marked contrast to the crabs, or Brachyura, in which the segments are expanded laterally. The abdomen is terminated by a broad swimming tail. The Macroura are numerically very abundant in both marine and fresh water.

The lobster prefers a rocky coast, and being somewhat of a gourmand in his tastes is tempted by the fisherman c n our shores to such good purpose, that as many as 25,000 live lobsters are often delivered at Billingsgate in a day. If only as many are eaten in the whole of England as in London, this would be at the rate of 50,000 per day, or 18,250,000 annually. March to August is the period of greatest catch.

The river cray-fish, Astacus fluviatilis (fig. 25), also common to the rivers of Europe, is largely caught, and when fresh boiled is not to be despised. It is largely imported into London, and is used by the chefs at the West End to garnish dishes. The writer with a friend caught as many as 900 cray-fish in a single evening from 8 till 12 (with a series of simple scale-like nets baited with liver), along the bank of the Thames and Severn Canal, Gloucestershire. The Murray river cray-fish from Australia (Potamobius serratus) is as large as a fine sea-lobster, and has its segments ornamented with spines, reminding one of the epiny lobster (Enoploclytia sussexienaeis) from the chalk of Sussex and Kent.

More than fifty genera of fossil Macroura have been met with and described; the earliest known is the Anthra∙ Isilaemon GrQssartii from the Lower Carboniferous series near Glasgow. Similar forms have been obtained from the Coal-measures in England; from Illinois, U.S.; from B shemia, &c.

I. Podophthalmia : (2.) Stomapoda. —All the members embraced within the three divisions of the preceding order (Decapoda) were cryptobranchiate, in this order they are nudibranchiate, iae., the gills are composed of plates or simple filaments attached to the feet, whilst the carapace, so largely developed in the order Decapoda. is here both shorter and narrower, and the body less complet. Taking Squilla (fig. 71) as an example, the segments are much less coalesced than in the lobster. Those bearing the eyes and the antennules are readily separable from the front of the head, and are not covered by the carapace, which only conceals eight segments, whereas in the lobster it covers fourteen, and in the crab twenty-one. The gills are borne by the abdominal swimming feet, free and uncovered. The first pair of thoracic limbs are developed into a pair of large and formidable claws, the terminal joint of which bears a row of long, sharp, and recurved teeth; these double back upon the edge of the penultimate joint which has a groove to receive them, like a pocket-comb.

In Mysis, “the opossum shrimp,” another member of this order, the two posterior pairs of feet only are branchiferous; all the feet are biramous and flagellate; in the female the hinder feet are modified into broad plates which, uniting beneath the body, form a pouch or marsupium in which the eggs are protected and the young pass through their infancy.

These opossum-shrimps, which are pelagic in their habits, are frequently met with in countless myriads towards the surface of the Greenland Sea, and, though small, they form the chief part of the food of the common whale (Balaena mystiaeetuae)β

Some forms of Erichthys are included in this division; these, like Mysis, are also pelagic, and occur abundantly on the surface of the Indian and Atlantic Oceans, where, together with the larvae of Cirripedes and many other oceanic cosmopolites, they may be taken with the towing net in abundance.

Numerous specimens of true Squilla (Sculda pennata, Münst) and of a ½/ys√s-like Crustacean have been found fossil in the Solenhofen limestone, of Oolitic age, in Bavaria. With the Stomapoda are also placed a group of very anomalous and larval-looking Crustacea (the Diastylidae) originally noticed in 1843 by Mr Harry Goodsir, who obtained them from the Firth of Forth. They closely resemble Copepoda in aspect, and might readily be confounded with the larval stages of some Decapod. They have, however, been found with their eggs borne by the female in an incubatory pouch beneath the thorax, as in Mysis.

The branchiae are situated on each side of the thorax immediately above the insertion of the legs, and approach in their comb-like appearance to those of the higher Crustacea.

Three genera have been established for these singular forms, namely, Guma, Alauna, Bodotria (see fig. 36).

II. Edriophthalmia:(3.)Isopoda. —From the stalk-eyed Podophthalmia we pass now to the sessile-eyed Edriophthalmia, in which the eyes with one exception are fixed immovably on the surface of the head. As in the higher forms, the eyes are compound, consisting in the young of some ten or twelve lenses only, but in the adult of as many as sixty to eighty. In nearly all, the body is distinctly divisible into three parts—the head usually very small, the seven thoracic segments well and evenly developed, the abdominal somites more or less coalesced. The general conformity in size and function of the thoracic somites and their seven pairs of legs characterizes the majority of the Isopoda. These legs are nearly uniform, and are fitted either for walking or for swimming, or as powerful hooklike organs to enable them to adhere to the fishes on which they are parasitic. The branchiae in this order are transferred from the thoracic legs to the abdominal appendages, which are converted into special organs of respiration.

One group of Isopods, the Oniscidae (forming Spence Bate’s and Westwood’s family Aerospirantia), familiar in our gardens under the names of “woodlouse,” “sow-bug,” and “armadillo” (fig. 22), are all air-breathers, incapable of existing in water, but breathing air which, however, it is necessary must be saturated with moisture. Several of the species which inhabit caves are destitute of eyes (e.g., Titanethes albus, Schrodte). The “great sea-slater” (Ligia oceanica) is common on all our coasts, running with agility and feigning death when attacked. The genus Armadillo, found commonly in our gardens and woods, and so called from the perfect way in which the segments roll together, forcibly reminds one of “the great Barr Trilobite” (Illaenus Barriensis), from the Silurian of Staffordshire (7 in fig. 73).

A very interesting little Isopod (presented to the British Museum by Dr Milligan of Tasmania), from Flinders Island, Bass’s Straits, and named in MS. by Mr Adam White Ceratocephalus Grayanus, offers many points of analogy with the extinct Trilobites. The fragile mode of articulation of its walking legs, and their entire concealment beneath the body-segments, are very suggestive. This Isopod is near to the Sphaeromidae, but will form a distinct family, as the antennae are inserted beneath and within the margin of the head-shield; apparently it does not roll itself into a ball.

The Sphaeromidae are very littoral in their habits; they range from the equatorial latitudes to the colder temperate zones, but are not found in Polar regions. They are vegetable feeders, and some (like Limnoria) are guilty of destroying timber. When molested or alarmed they roll themselves up into a ball. The Sphaeromidae present many points of analogy, if not of affinity, with the extinct Trilobites.

In this order we find the Limnoria terebrans (or the “gribble,” as it is called by the fishermen). It is one of the most destructive creatures, attacking all woodwork below tidemarks; the only wood which it cannot destroy is teak. Although its ravages had gone on for ages, it was only made known to the scientific world and described by Dr Leach in 1811.

In the aberrant genus Tanais (fig. 38) the first pair of thoracic legs are converted into chelae, and the head-shield is covered by a carapace, abundantly traversed by currents of blood, beneath which a stream of water passes, maintained as in the zoëae and adult Decapoda by a flabelliform appendage of the second pair of maxillae, which is wanting in all other Edriophthalmia. The abdominal feet, which in other Isopoda act as respiratory organs, are simple natatory feet in Tanais. These characters, together with the pedunculated eyes and the great chelate hands, give to Tanais a very decapod-like aspect (see fig. 38).

The Idoteidae contain representatives of some of the largest known Isopoda, some of which are above 4 inches in length.

The Aegidae x and Cgmothoidae have all the feet furnished with a robust finger, sharp at the tip, for seizing and holding on to fishes upon which they are parasitic. Another family, the Bopyridae (fig. 39) are parasitic chiefly on members of their own class, frequently occupying the branchial chamber of the common prawn, and distorting the carapace on one side. A fossil Bopyrus is observed lying in the branchial cavity of a crab (Palaeocorystes) from the Gault and Greensand. We have already referred to these and other parasitic forms in the earlier part of this article.

Isopoda are met with as far back as the Old Red Sandstone, where remains of a gigantic species allied to Arcturus have been discovered; others occur in the Carboniferous and Oolitic periods. The Purbeck beds of Swanage, Dorset, also yield abundance of a freshwater form, the Archaeoniscus Brodiei ; species of Oniscus and Sphaeroma are found fossil in the Cretaceous, the Eocene, and the Miocene of Europe.

II. Edriophthalmia : (4.) Trilobita. —So long ago as 1821 Audouin placed the Trilobites with the Isopoda, whilst Macleay assigned them a distinct order between the Isopoda and the Phyllopoda. Later researches by Milne-Edwards and others have caused the Trilobita to be referred to the Entomostraca, on account of the very variable number of body-rings observed in the several genera (from six to twenty-six) evidencing a much lower type of structure than the Isopoda, in which the thorax is composed of seven free and movable segments with a head-shield and anchylosed caudal somites. Moreover, until the discovery of presumed ambulatory appendages in an Asaphus from the Trenton Limestone in 1870, the only appendage previously observed was the hypostome or lip-plate. There seems, however, no good reason to urge against the conclusion that the Trilobita were an earlier and more generalized type of Crustacea, from which the later and more specialized Isopoda have arisen,—a view which the writer is glad to say he shares with the distinguished carcinologists, Professor Dana in America, and Mr C. Spence Bate in England, although at present more evidence is needed as to the nature of the locomotory appendages in this extinct group. If we range the characters of Trilobita and Isopoda side by side, we shall find there are sufficiently good grounds for placing them in the Edriophthalnüa together, [table]

Perhaps no investigator of fossil forms has devoted so much careful research to any group as Μ. Barrande has expended upon the extinct Trilobita. Writing recently upon the divisions of their body he arranges them in four groups, according to the number of their free movable thoracic segments. [table]

We thus perceive that the number of those forms of Trilobites which have a great excess of free segments is not large when we consider the group as a whole.

In the higher and more specialized forms of Isopoda of the present day, we do not find the number of segments absolutely adhered to without any variations; on the contrary, we constantly meet with individuals in which more or fewer segments are welded together, so as to conceal the normal number of seven thoracic somites between the head and the abdomen. Such being the case, we cannot be surprised to find considerable variation in a group like the Trilobita, which, if they really are the remote ancestors of the recent Isopoda, must, according to the views suggested above, be the prototypes of the larvae rather than of the adult stage of the living Isopoda.

In his researches among the Trilobites of Bohemia Μ. Barrande has discovered forms which, there is every reason to believe, exhibit (as he has so admirably shown in his great work) the gradual development of the Trilobite from the earliest form on quitting the egg to the adult. We give seven of the earliest stages of Trinucleus ornatus and seven of Sao hirsuta, copied from Μ. Barrande’s monograph.

One most striking feature in the Trilobita is the remarkable development of their compound eyes (fig. 11), a subject ably discussed and illustrated long ago by Dr Buckland in his Bridgewater Treatise (1836).

Perhaps the eye of the Trilobite may be best compared with that of Limulus, but there are forms like Aeglina in which the eyes are enormously developed, occupying nearly the entire head-shield with their facetted surfaces.

We have an analogous development of the organs of vision amongst some of the pelagic Amphipoda, the Hyperiidae, and in a very singular form brought home by the “Challenger,” the Thaumops pellucida (Phil. Trans. 1873). The “facial suture” in the head-shield of the Trilobita, which separates the lateral genal portion from the glabella, was for a long time considered as peculiar to Trilobites and Limuli, but C. Spence Bate has ably shown that it homologizes with the suture which traverses the inferior surface in the carapace of the Brachyurous Decapod and the cervical suture in the Macrouran type (Reports Brit. Assoc., Bristol, 1875, p. 46.)

The Trilobita are the chief representatives of the Crustacean class in Cambrian times. More than 500 species have been described; out of these 350, representing 42 genera, have been recorded from the Lower Palaeozoic rocks of Bohemia alone by Barrande.

About 51 genera and 304 species are British in Cambrian and Silurian rocks; ten are Devonian, and four Carboniferous. A gigantic Paradoxides, nearly two feet in length, occurs in the Middle Cambrian, and large forms of Asaphus, Homalonotus, Lichas, <fcc., are met in the Bala group. Phacops, Sphaerexochus, Encrinurus, Calymene, Illaenus, and Acidaspis are among the Upper Silurian forms,—some, like Acidaspis, being extravagantly ornamented with spines and tubercles. The Devonian has fewer and less varied forms of Trilobites. Those in the Carboniferous belong nearly all to two genera (Phillipsia and Grissithides), both small, neat, and simple forms. None are met with in rocks of later date.

II. Edriophthalmia : (5.) Amphipoda.— This order, as Spence Bate has well observed, constitutes a group among the Edriophthalmia, parallel with the Macreura among the Podophthalmia, whilst the Isopoda may represent the broad and flattened Brachyura, the Caprellae offering a kind of parallelism with Squilla and its allies. As in the Isopoda, the head is small and carries only the organs of sense and nutrition; the sessile eyes are generally small, yet in a few instances they are extremely large (e.g., Lestrigonus and Thaumops), covering the entire sides of the head.

The seven thoracic segments, constituting the middle-body, are well developed and nearly equal in size; all the segments are compressed laterally as in the Palaemonidae.

The two anterior pairs of the seven thoracic legs (see Th. 8, 9 in fig. 1), which are jaw-feet in the Podophthalmia, are here developed into arm-like legs, having an enlarged penultimate joint or hand, against which the seventh and terminal joint doubles back, like a finger against the palm, and so forms a prehensile organ similar in form to the claws in the Crangonidae. The best-formed claws are seen in Orchestia Darwinii (fig. 45), and in Melita exilii (fig. 76).

The ova are nourished within a pouch formed by a series of foliaceous plates attached to the four anterior pairs of legs; except in the Hyperiidae, which are parasitic on Medusae, as already mentioned. The males in the Amphipoda closely resemble the females (save in those forms in which the hands are enlarged in the male), but contrary to the general rule the females are much smaller than the males.

This division, like the preceding one, has its terrestrial representatives, Talitrus and Orchestia, the “sandhoppers,” living out of the sea, but choosing moist places. Orchestia with us loves to live within reach of the sea spray, but some species in the southern hemisphere (0. tahitiensis, telluris, and sylvicolast live many miles inland, some under plants at an elevation of more than a thousand feet above the sea. But by far the largest section are natatorial in their habits, being most active and untiring swimmers. One form, Gammarus pulex (fig. 77), is most common in our freshwaters, two other genera, both blind, Niphargus, with three species, and Crangonyx, with one species, are found in wells in England, and from their structure there is every reason to conclude they are as truly indigenous to these underground water-courses in the Chalk, Oolite, or Carboniferous Limestone, as are the numerous species of blind Crustacea met with in the waters flowing through the Mammoth Cave, Kentucky.

A curious subdivision of Amphipods is formed by the Podocerides (Amphithoë and Podocerus), all the species of which invariably construct nests in which they take shelter and nourish their young. These abodes are built of wood or stones, mud, clay, <fcc., united together by a cement excreted by the animals themselves. Some closely resemble miniature birds’-nests, others are in the form of tubes.

This division includes another most destructive woodborer, the Chelura terebrans, so devastating to piles and submarine timber all round the shores of Europe, but not recorded from other lands.

Finally, we come to the minute aberrant forms of Dulichiidae and Caprellidae (fig. 78), in which the body is reduced to a slender elongated cylindrical form, the· thorax having only about six somites (one being absent and two soldered together), and the abdomen being quite rudimentary. They have long antennae and feet, all fitted for climbing and holding on by. Their singular appearance has caused them to be called “spectre shrimps,”

With these aberrant forms are associated the Cyamidae (fig. 79), a family which affix themselves by their strong recurved legs to the rough portions of the Cetacea upon which they feed. The feet are all prehensile; the third and fourth somites bear the branched or simple branchiae. The abdomen, as in the Caprellidae, is rudimentary; the eggs and young are sheltered by four broad lamellar plates, developed from the appendages on the under side of the body of the female. Spence Bate and Westwood have figured five species. They approach in many respects to the Pycnogonidae, which also live parasitic on Cetacea (see Arachnida, vol. ii. p. 276-77), but we must not attempt to discuss their affinities here. A fragment of a presuined Amphipodous Crustacean has been described by the writer from the Upper Silurian (the Necrcgammarus Salweyi) another, the Gampsonyx fimbriatus, occurs in the Coal-measures of Germany, Bohemia, and America. Mr Spence Bate has described one from the Permian of Durham, the Prosoponiscus problematicus, its modern living representative, the Sulcator, making peculiar tracks upon our shores today like those met with upon the surfaces of slabs of Palaeozoíc rocks. Several other Amphipod-like forms occur in the lithographic stone of Bavaria. The world-wide distribution of the Amphipoda accords well with their range in time, which was as great or even greater than the Isopoda.

Sub-class 2. Gnathopoda (or Entomostraca).

III. Merostomata : (6.) Xiphosura. —The king-crab (Limulufi) is a remarkable type of crustacean closely related to the extinct Eurypterida. Found living in the seas of China and Japan and on the north-east coast of North America, it exemplifies a peculiar and most ancient order, the affinities of which are not at first readily recognized because its nearest allies have passed away. The head-shield is enormously expanded so as to shelter all the anterior appendages beneath it; and the succeeding segments are so soldered together as to appear like one piece, although all the hind-segments are free and movable in the larva. The eyes are fixed on the head-shield; the antennules are chelate, and placed in front of the mouth. The antennae, mandibles, maxillae, maxillipeds, are all converted into walking legs, forming also chelate appendages and, at their bases, jaws; thus serving admirably to illustrate the most prominent characteristic of the sub-class Gnathopoda, “mouth-footed.”

The thoracic feet are flattened out into broad bilobed plates which cover the branchiae and the egg-pouches. The abdomen is rudimentary, being partly represented by the posterior portion of the hinder shield, and partly by the long ensiform tail-spine (?) (See Owen’s Memoir, Trans. Linn. Soc., vol. xxxviii., 1873.) But in the larvae, as has been already shown, these post-cephalic somites are free and unanchylosed, and the tail-spine is undeveloped, thus connecting the modern king-crab with its far-off ancestors in the Coal and Silurian periods. The oldest species known is the Neolimulus falcatus, H. Woodw. (õ in fig. 80), from the Upper Silurian of Lanarkshire, in which the segments are apparently all free and unanchylosed

In the Coal-measures no fewer than three genera and eight species of small Limuloid Crustaceans have been met with, viz. Bellinurus (four species), Prestwichia. (three species), and Euproops (one species),—the last named an American form. Many of these closely resemble young larval Limuli. The Oolitic Limuli found in the lithographic stone of Solenhofen agree closely with existing species, one form even equalling in size the living Limulus polyphemj'S from the American coast (1 in fig. 80; see also fig. 12).

III. Merostomata : (7.) Eurypterida. —In this order we become acquainted with the second extinct type of the Crustacean class, and by far the most interesting, because all the appendages as well as the bodyrings have been preserved to us, whereas in the Trilobita the former are remarkable by their almost entire absence.

Unlike Limulus, in which all the segments in the adult are soldered together into a fore and hind body and telson, in the Eurypterida the body is long and well adapted for swimming, the segments being quite distinct and well developed; the feet are also fitted for natation (see 5 in fig. 73, and fig· 81).

We again observe the reiteration of the same well-marked characteristics in the legion Merostomata—already noticed in the Edriophthalmia and Podophthalmia; namely, the division into Brachyuran and Macrouran forms which exemplify the crawling and swimming types, by the soldering together of the body-segments in the one and the retention of free movement in the somites in the other. The characters of these two orders of the Merostomata are summarized in the subjoined table. [table]

Numerous species of these ancient extinct long-bodied Merostomata have been met with and described by Hall in America, by Fischer in Russia and Sweden, and by Huxley, Salter, and Woodward in Britain (see H. Woodward’s monograph in Pal. Soc., 4 parts, 1865-1872).

The most perfect specimens of the genera Slimonia, Pterygotus, and Eurypterus have been obtained by Dr J. Sli-ιnon of Lesmahagow, Lanark. The largest known remains, representing specimens from 5 to 6 feet in length, are from the Devonian of Forfarshire, belonging to the great Pterygotus anglicus and to Stylonurus scoticus obtained by Mr James Powrie, F.G.S., Reswallie, Forfar.

In the Upper Silurian we have one English genus, Hemiaspis (6 in fig. 80), and three Russian forms, Exapinu-rus, Pseudoniscus (7 in fig. 80), and Bunodes, which, like the Anomoura, serve to bridge over the interval between the Limuli and Pterygoti, the hind-body being partially developed. The best illustration of the Eurypterida is to be found in the zoëa of the common shore-crab, Carcinus maenas (figs. 26 and 27), in which the principal locomotory organs are the maxillipeds, and the abdominal somites are destitute of all appendages. The latest representative of this extinct order has been found in the Lower Carboniferous series of West Lothian, the Eurypterus Scouleri, which however differs greatly from all the other forms.

IV. Branchiopoda : (8.) Phyllopoda. —This order includes not only the bivalved Estheria and Nebalia, and the shield-bearing Apus, but two forms of naked gill-footed Crustacea, Branchipus and Artemia (5 in fig. 57). Of the shieldbearing forms the fresh-water Apus may serve as a good example. The eyes are placed in front on the dorsal surface of the carapace and are nearly confluent. The first pair of feet (maxillipeds) are long and branching; to these succeed about sixty pairs of branchial feet. The thoracic and abdominal somites are nearly cylindrical and are composed of about thirty articulations, terminated by two long, many jointed tail-spines.

Probably Apus has more articulations to its appendages and body than any other Crustacean. Schaffer tabulated them, and found they numbered 1,802,604; Latreille puts them down at not less than 2,000,000. Apus affords an excellent illustration of a form in wlxich the mere vegetative repetition of parts is carried to an extreme distance beyond the normal number of body-rings so characteristic of the class.

In Nebalia, the marine type (1 in fig. 57), the carapace or head-shield has a well-marked rostrum, and is more compressed laterally than in Apus, covering the head and thorax and even a part of the abdomen. The eyes in Nebalia are placed on peduncles beneath the carapace; the number of segments is not excessive as in Apus.

In Estheria (fig. 56), the carapace is composed of two valves, subovate in outline, like a bivalve molluscan shell, which it also resembles in being united by the umbones of each valve on the anterior dorsal border, and in each valve being marked by regular concentric lines of growth.

Branchipus (5 in fig. 57) and Artemia are destitute of any carapace, so that the elegant wave-like motion of their many-jointed transparent bodies and branchial feet can be freely observed. The former inhabits our fresh waters, the latter is marine, being peculiarly prolific in the brine-pans at Lymington, where the workmen firmly believe that the “brine-shrimp” aids in some way the rapid deposit of salt, through the constant agitation caused by such myriads of these minute and restless Entomostraca in the water.

Chirocephalus or Branchipus is believed to be only a variety of Artemia, resulting from change of conditions between a fresh and an extremely saline fluid medium.

In Professor T. Rupert Jones’s monograph on fossil Estheriae, out of thirty-eight localities recorded for that genus living, three only were met with in brackish water.

Mr John Arthur Phillips, F.G.S., states that the Artemia fertili» is exceedingly abundant in the highly-saline waters of Mono and Owen’s Lakes, California. These little Crustacea congregate in such dense serpentine or annular masses in the water that a breeze sufficient to ruffle the surface of the lake scarcely affects the water filled by the Artemiae, which remains perfectly smooth. The only other inhabitant of these salt-lakes is the larva of a dipterous insect, the Eρhydra californica, Torrey, which is collected by the Indians, and, when dried in the sun, forms an important article of food. Myriads of gulls and other aquatic birds visit these lakes in summer to feed upon the Artemiae and larval Ephydrae. Nebalia, at the present day, seems but the puny and degenerate representative of the once giant pod-shrimps of Silurian times, the caudal somites of one of which measured 8 inches and the tail spines 6 to 7 inches in length, the carapace not being preserved. The ancestors of Nebalia date back to the Menevian group, Hymenocaris major being the earliest known. Ceratiocaris papilio is so abundant in the Upper Silurian of Lesmahagow, Lanark, as to cover entire beds with its remains,

Many fossil forms, as Dithyrocaris, Discinocaris, Apty∙ chopsis, and Peltocaris, carried their head-shield flat and expanded like Apus at the present day; one of these, a Discinocaris from Moffat, Dumfriesshire, had a carapace 7 inches in diameter. These forms occur in all the Palaeozoic rocks. Esthaeriae are found in the various strata from the Carboniferous Limestone to the Tertiary. Although Branchipus or Chirocephalus is destitute of any head-shield, its fossil remains have just been transmitted to the writer together with numerous Dipterous and Coleopterous insects from a freshwater deposit, associated with plan* remains, at Gurnet Bay, in the Isle of Wight,

IV. Branchiopoda : (9.) Cladocera. —In the Cladoccra are placed a number of minute animals furnished with branching natatory antennae and five or six pairs of short f<_ li-aceous feet; the body (except the head which is distinct and projecting) is entirely inclosed within a carapace, formed of two valves joined together on the back; the eye is single and very large. The Cladocera are chiefly freshwater, and are distributed over the w hole world. Of this order the Daphnia pulex (1 in fig. 58), so abundant in our fresh waters, is a good example. So numerous are they in our ponds in summer as frequently to impart a blood-red hue to the water for many yards in extent. In order to realize the wonderful fecundity of this and allied genera, it is necessary to realize that when a Daphnia is only ten days old, eggs commence to be formed within the carapace, and under favourable conditions of light and temperature, it may have three broods a month or even a greater number,—the larger species having as many as forty or fifty eggs at once. No males appear until the autumn, so that all the earlier broeds are derived from females, wdιose parent was fertilized, and died after depositing its eggs, the year before, and these continue to reproduce fertile female offspring throughout the summer.

F. Goldenberg has lately described a fossil form belonging to this division (Lyncites ornatus) from the Coal-measures of Saarbruck.

V. Lophyropoda: (10.) Ostracoda. —In the Ostracoda, of which Cypris, Candona (2 in fig. 58), and Cythere are examples, the body is entirely enclosed in a carapace composed of two nearly equal parts like a bivalve shell. Two pairs of antennae, one pair long, with hairy filaments, one pair short, stout, and recurved (like feet), and three pairs of short feet, may be seen protruded from the carapace, which is compact and brittle, yet is capable of protecting the living animal during long periods of drought, buried in the dried-up mud of pools. Cÿpris frequents stagnant water, living on dead animal matter. Cythere is found in pools along the sea-shore.

Like the Phyllopoda, the Ostracoda are of immense geological antiquity, Primitia prima occurring in the Lower Cambrian of St David’s. There is abundant evidence, in almost every stratum, of the former existence of these little bivalved Crustacea, often in vast numbers; their size in early times was much larger, Leperditiae as big as large horse beans being found in Silurian strata, but the living forms are all microscopic. Μ. Ch. Brongniart has just described a fossil Ostracod, Palaeocypris Edwardsi, inclosed in silex from the coal of St Etienne, in which all the organs are most perfectly preserved. It closely resembles the modern form. The Ostracoda help with other microscopic organisms to build up the Chalk. They make up the great mass of the Cypris shales of the Wealden, Isle of Wight, and many Tertiary beds are largely composed of their remains.

V. Lophyropoda : (11.) Copepoda —(α.) Liberata. — The free Copepods, of which Cyclops, Canthocamptus, and Cetochilus may servo as examples, have the head and thorax closely enveloped in a carapace with which the front rings of the thorax are confluent. The abdominal somites are much diminished in sizo and cylindrical. The single or paired external ovisacs are attached to two of the posterior somites, which are usually welded together. The single sessile median eye is situated near the front of the head; in the males of Diaptomus and Anomalocera the eye is pedunculated. The antennae are very long and powerful natatory organs (in Cetochilus, 4 in fig. 58, and Diaptomus equalling the entire length of the animal’s body); in the males one or both of the antennae have a swelling near the centre or towards the extremity, followed by a movable joint which acts like a hinge and serves as a clasper to detain the female. There are five pairs of rowing feet, one pair of which are usually rudimentary.

The species belonging to this family are to be found in both fresh water and the sea. In the muddiest and most stagnant pools and in the clearest springs Cyclops abound (3α in fig. 58). The marine species frequent the pools along shore and the open ocean in equal abundance. They assist in producing that luminous appearance in the sea called “phosphorescence,” for want of a better name (5 in fig. 58).

The. fecundity of this order is truly surprising. Cyclops quadricornis is often found with thirty or forty eggs on each side; and though those species which have but a single ovisac do not carry so many, their number is still very considerable. Jurine isolated female specimens of Cyclops, and found them to lay eight to ten times within three months,—each time about forty eggs. At the end of a year one female would have produced 4,442,189,120 young! Cetochilus (4 in fig. 58) is so abundant, both in the Northern Seas andin the South Atlantic, as to serve for food to such an immense animal as the whale. They colour the sea for many miles in extent, and when the experienced whaler sees this ruddy hue upon the ocean he knows he has arrived at the “pasture of the whales.” They are to be seen in vast quantities off the Isle of May in the Firth of Forth during the summer months; many Cetacea are attracted thither, and vast shoals of fish also come to feed upon them. One anomalous type of free Copepods is the Notodelphys acidicola, described by Allman, which is found swimming freely in the branchial sac of the Ascidia communis. In this species the female has the fourth anterior segment of the body peculiarly modified, so as to form on its dorsal surface a marsupium for receiving and retaining the ova until they are hatched, when they escape by an opening between the sac and the upper surface of the body-ring.

We have no positive records of Copepoda occurring in a fossil state.

V. Lophyropoda: (11.) Copepoda —(δ.) Parasita.— The parasitic Copepods are divisible into two groups, the fir.>t comprising the peripatetic genera, in which the male and female both retain their organs of locomotion in the adult state, and can change their habitat whenever needful; this division would include the fresh-water Argulidae and the marine Caligidae. The second division embraces the fixed parasites, in which the females when adult lose their locomotory appendages and become fixed, deriving their nourishment by a true suctorial mouth, armed with styliform mandibles, from the fishes and other animals upon which they are parasitic. The larvae when they emerge from the egg are nauplii, like those of other Copepods. The males and females are then alike; after attachment the female often attains a large size, and is soon litt 1 e more than a maggot-like body, with immense paired ovaries attached to her abdomen. The male is very small and resembles a fat Acarus; he is usually parasitic on the female, adhering to the vulva.

Almost every fish has some form of these Copepod parasites, either on its skin, it~ eyes, or its gills.

Argulus foliaceus is of a rounded oval shape, the carapace inclosing the thoracic somites in a deep notch behind, and the body terminating jvith a bilobed telson. The antennae are formed into recurved hooks for holding on by, when the animal shifts its position. The second pair of footjaws are converted into powerful suckers by which it attaches itself to its host. The mouth is tubular, and has a sharp styliform organ within it, affording good evidence of its suctorial habits. There are four pairs of biramose natatory feet. The animal, when detached, swims with extreme rapidity and elegance, and no fish, however rapid, can escape from its adherence.

The female is much larger than the male. She leaves the fish on which she is parasitic when desirous to deposit her eggs, which she fixes to a stone or other inorganic body at the bottom of the water. As many as 400 are deposited at one time by a single female. Argulus catostomi is said to lay 1500 eggs at once. The young are hatched in thirty-five days, and after about three moults as free Copepod larvae they put on the adult form.

It would be impossible to give a detailed account of all the varied forms of Copepoda in an article like the present; we therefore must refer the reader to the works of Baird, Claus, and others for fuller information.

VI. Anchoracephala : (12.) Rhizocephala. —These have been referred to under Metamorphosis, so that we need not allude to them further here, save only to give illustrations of two genera, Peltogaster (fig. 82) and Sacculina (fig. 83).

VI. Anchoracephala: (13.) Cirripedia. —Forty years ago the Cirripedia were still arranged as Mollusca in many public museums; nor is this surprising, considering the fixed condition of their shells and the degree of external resemblance between Lepas and Teredo on the one hand, and Balanus and a compound of a Patella and a Chiton shell on the other. Strauss in 1819 first affirmed that Cirripedes were Crustacea. But this view was disregarded until J. Vaughan Thompson’s capital discovery in 1830 ρf their metamorphosis, since which time they have been almost universally placed with the Crustacea.

The Cirripedia are classed by Darwin in three great divisions :—(1) Thoracica (limbs thoracic); (2) Abdominalia (limbs abdominal); (3) Apoda (appendages wanting). In the first division are embraced the Balanidae, the Verrucidae, and the Lepadidae; in the second a single genus, Cryptophialus minutus; the third is also represented by one form, the Proteolepas bivincta.

(1) Thoracica. —Cirripedes ordinarily are bisexual, in which respect they differ from all other Crustacea,—the male (where it exists distinct) being minute and rudimentary in structure and permanently epizoic on the female. In these latter facts we find an analogy in the Copepoda Parasita just noticed. The male has the excretory organ single, median, and probosciform, and placed at the extremity of the abdomen,—in all these respects differing from other Crustacea in which the male organ is laterally double. In the female organs the ovarian tubes and caeca inosculate together; there are no oviducts, the ova connected together by membrane, and so forming the “ovigerous lamellae,” become exposed by the exuviation of the lining tunic of the carapace or sack, and by the formation of a new tunic on the under side of the lamellae, a process unknown in any other Crustacean. The Thoracica are mainly divided into Balanidae and Lepadidae; in the former, the animal when adult is inclosed in the parietes of its shell, and fixed to some living or dead object by a broad shelly basis, the aperture being protected by the opercular valves. In the Lepadidae the animal is attached by the extremity of a more or less long muscular peduncle, and its body is lodged within the shelly valves of the capitulum. In some species, as in Pollicipes and Scalpellum, the peduncle is covered with more or less numerous rows of scales or squamae. This peduncle in Pollicipes and Scalpellum corresponds with the basis in Balanus, as may clearly be seen if a Pollicipes with a short peduncle and a Balanus with a deep cup-formed or cylindrical basis be compared, the animal being in part lodged in both, as in Ibla and Lithotrya. The scales which surround the base of the valves in Pollicipes correspond with the parietes of the walls of Balanus, the valves of the capitulum of the former being homologous with the opercular valves in the latter (fig. 84). The body consists of six, perhaps of seven, posterior thoracic somites. In the division Thoracica the abdomen is undeveloped. The thoracic segments support six pairs of cirri. Each cirrus consists of a two-jointed pedicel, carrying two multi-articulated rami. The mouth has a labrum, palpi, mandibles, and two pairs of maxillae; within the sack a folded membrane forms the branchiae. Darwin concludes that in the Cirripedia the body may be said to be composed, at most, of bat seventeen segments.

In order to indicate the homologies which still exist between the parts of an adult Cirripede and an ordinary free

Crustacean, we give the accompanying illustrations from Darwin (fig. 85). The upper figure is a Stomapod Crustacean (Leudfef) , , the abdomen, being rudimentary in the adult Cirripedia, is only shown in faint outlines. The lower figure is of a mature Lepas with the antennae and eyes, which are actually present in the larva, retained for the sake of completing the comparison. “All that we externally see of a Cirripede, w hether pedunculated or sessile, is the three anterior segments of the head of a Crustacean, with its anterior end permanently cemented to a surface of attachment, and with its posterior end projecting vertically from it” (Darwin).

The thoracic appendages of the Cirripedia present us with yet another wonderful modification of the Crustacean type; these biramous multi-articulated cirri are neither natatory, nor ambulatory, nor branchial, but “captorial,” or fitted for sweeping the water, and thus catching prey,— their alternate extensile and retractile wave-like movements bringing all floating particles and minute organisms within reach of the inclosed mouth.

(2) Abdominalia. —The single Cirripede, the Cryptophialus minutus, forming this section is the smallest known, being less than τ 1 δ th of an inch in length. It is met with imbedded in vast numbers in the living shell of Concholepas Peruviana, the crypts almost touching each other. The three abdominal somites bear three pairs of cirri, the thoracic somites being apodal. The sexes are distinct, the minute almost globular male being lodged within the crypt occupied by the female. Darwin found from one or two up to seven males attached to the same female.

(3) Apod a. —This section, like the last, contains only a single form, the Proteolepas bivincta, resembling the larva or maggot of a fly attached by two threads; the mouth is suctorial; it has no limbs; its body is shell-less, and it lies parasitic within the sack of Alepas cornuta.

Cirripedia occur attached to the most varied objects, living and dead, throughout the seas of the globe. Sessile forms like Tubidnella, Coronula, Plaiylepas, Chelonobia, <fcc., are found attached to, and imbedded in, the epidermis of the whale, and on the shell of the turtle, &c. Pedunculated forms are similarly widely distributed. Fritz Müller calls attention to one anomalous form described by Darwin, the Anelasma squalicola, parasitic upon sharks in the North Sea, which seems to offer a remarkable analogy to the Rhizocephala. This Lepadide, he says, seems in a fair way to lose its cirri and buccal organs altogether. “The widely-cleft shell-less test is supported upon a thick peduncle, which is imbedded in the skin of the shark The surface of the peduncle is beset with much-ramified hollow filaments, which penetrate the shark’s flesh like roots.” Cement glands and cement were not visible. “It seems to me,” says Fritz Müller, “hardly doubtful that the ramified hollow filaments are themselves nothing but the cement ducts converted into nutritive roots, and that it is in consequence of the development of this new source of nourishment that the cirri and buccal organs are in the highest degree aborted.” All the mouth organs are minute and rudimentary; the cirri thick, inarticulate, and destitute of bristles; the muscular tissue without transverse striation, and the stomach perfectly empty.

“The Lepadidaef writes Darwin, “include a much greater range of forms than the Balanidae, and this is what might have been expected, for it is the most ancient family, and extinction has done its work in separating genera which, according to analogy, were once more nearly connected by intermediate forms.”

The most ancient sessile Cirripede found fossil is the Pyrgoma cretacea, H. Woodw., from the Chalk. Previous to 1865 the oldest-known pedunculated Cirripede was the Pollicipes rhaeticus, Moore. In that year the writer described a curious and most anomalous form of Cirripede, from the Upper Silurian of Dudley, with imbricated calcareous plates (fig. 86), under the name of Turrilepas Wrightii, previously described as a Chiton. The fossil form with which it has been compared is more perfect and equally bizarre, viz., the Loricula pulchella (see 6 in fig. 70), originally discovered in the Chalk of Rochester and since in that near Norwich. It affords evidence of a most aberrant form of Lepadide, in which the capitulum is very small; the body of the animal was lodged in a broadly-expanded peduncle, clothed in five rows of smooth loricate! calcareous scales, which, if both sides were perfect, would have possessed ten rows and the plates would have exceeded 200 in number. In Bate and Westwood’s Sessileeyed Crustacea (vol. ii. p. 268) is figured a larval form of Cryptothiria Balani (reproduced in fig. 41 above), which seems to afford evidence of a similar arrangement of plates. Possibly Loricula was parasitic like Bopyrus.

In conclusion, sufficient evidence has been adduced to show that the normal development of the Crustacean class has been one of progressive advancement, the forms of to-day, when viewed as a whole, being more highly developed and more differentiated than those which the geological record has preserved to us. But in any large community or class it is only the few that outstrip the many in the struggle for existence. Thus the Podophthalmia and Edriophthalmia present numerous examples of high advancement, both in intelligence and in attaining to a terrestrial life, especially in the Dec.ιpoda-Brachyura. The Ostracoda, Phyllopoda, and Xiphosura are good instances of merely persiàtent forms. They are orders the members of which have branched out long since into byways of their own, where, being checked from further progress, they have, by their great tenacity of life and large powers of reproduction, held their ground through the long lapse of ages from Silurian times to the present day, whilst higher orders have been modified or swept away.

But the history of some Crustacea has been retrogressive, probably in a few instances from arrested larval development, as, for example, in the case of the imperfectly-developed fifth pair of legs in Porcellana, Galathea, and Munida; in most instances, however, retrogression seems clearly traceable to the parasitic or sedentary mode of life which the members have adopted. We have examples of this in the loss of eyesight in Crustacea passing their lives in subterranean caverns, wells, and streams; the loss and atrophy of a part of the defensive armature of the body in the burrowing Thalassinidae; the complete loss of the abdominal calcareous covering in Pagurus; the general atrophy of limbs and loss of symmetry of the body in the Bopyridae through residence within the branchial chambers of other Crustacea; and the complete or partial loss of all or nearly all recognizable Crustacean characters in the adult female, or in both sexes, by parasitism on Crustacea, fishes, <fcc., in certain of the Bopyridae, in the Copepoda, in the Rhizocephala, and in the Cirripedia.

Viewed as a whole, the Crustacea probably offer the best illustration of a class constructed on a common type, retaining its general characteristics, but capable of endless modifications of its parts, so as to suit the extreme requirements of every separate species.

The outline of this great class here attempted necessarily lacks many important details; these must be filled in by the reader from the various works referred to throughout the article. (h. W.)

68), which lives concealed, holding tightly to the under side of flat stones at low water, does exactly the same thing with its maxillipeds as the b .rnacles do with their cirri; it keeps up a semicircular sweeping movement, so that a constant current conveys all the small living and dead objects within reach of its mouth.