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...