Sponges

SPONGES. The great advance which has been made during the past fifteen years in our knowledge of the sponges is due partly to the vivifying influence of the evolutional hypothesis, but still more to the opportunities afforded by novel methods of technique. To the strength and weakness of the deductive method Haeckel’s work on the Kalkschwämme (6) is a standing testimony, while the slow but sure progress which accompanies the scientific method is equally illustrated by the works of Schulze (fio), who by a, masterly application of the new processes has more than any one else reconstructed on a sure basis the general morphology of the sponges. In the general progress the fossil sponges have been involved, and the application of Nicol’s method of studying fossil organisms in thin slices has led, in the hands of Zittel and others (24, 35 ), to a complete overthrow of those older classifications which relegated every obscure petrifaction to the fossil sponges and consigned them all to orders no longer existing. But, whilst many problems have been solved, still more have been suggested. An almost endless diversity in details differentiates the sponges into a vast number of specific forms; the exclusive possession in common of a few simple characters closely unites them into a compact group, sharply marked off from the rest of the animal kingdom. of the Ascetta, as of all sponges, is most obviously manifested, as Grant (5 ) first observed, by a rapid outflow of water from the oscule and a gentle instreaming through the pores,—a movement brought about by the energetic action of the flagella of the endodermic cells. The in streaming currents bear with them into the cavity of the sac (paragastric cavity) both protoplasmic particles (such as Infusoria, diatoms, and other small organisms) and dissolved oxygen, which are ingested by the flagellated cells of the en doderm. The presence of one or more contractile vacuoles in these cells suggests that they extricate water, urea, and car bonic acid. The insoluble re sidue of the introduced food, together with the fluid excreta, is carried out through the os cule by the excurrent water.

New individuals are produced from the union of ova and spermatozoa, which develop from wandering amoeboid cells in the mesoderm. The walls of Ascetta are strengthened by calcareous scleres, more especi ally designated as spicules, which have the form of tri-

radiate needles. If we make abstraction of these we obtain an ideal sponge, which Haeckel has called Olynthus (6), and which may be regarded as the ancestral form from which all other sponges have been derived. To give greater exactness to our abstraction we should perhaps stipulate for the Olynthus a somewhat thicker mesoderm and more spherical form than a decalcified Ascon presents.

Canal System.— We shall now trace the several modifications which the Olynthus has undergone as expressed in the different types of canal system.

The simple paragaster of Ascetta may become complicated in a variety of ways, such as by the budding off from a parent form of stolon-like extensions, which then give rise to fresh individuals, or by the branching of the Ascon sac and the subsequent anastomosis of the branches; but in no case, so long as the sponge remains within the Ascon type, does the endoderm become differentiated into different histological elements. The most interesting modification of the Ascon form occurs in Homoderma sycandra (12 ), in which from the walls of a simple Ascon caecal processes grow out radiately in close regular whorls, each process reproducing the structure of the parent sponge (figs. 2, 3). From this it is but a short step to the important departure which gives rise to the Sycons.

In the simplest examples of this type the characters of Homoderma sycandra are reproduced, with the important exception that the endoderm lining the paragastric cavity of the original Ascon form loses its primitive character, and from a layer of flagellated cells becomes converted into a pavement epithelium, not in any distinguishable feature different from that of the ectoderm. The flagellated cells are thus restricted to the caecal outgrowths or radial tubes. Concurrently with this differentiation of the endoderm a more abundant development of mesoderm occurs. In some Sycons (Sycaltis, Hk.) the radial tubes remain separate and free; in others they lie close together and are united by trabeculae, or by a trabecular network, consisting of mesodermic strands surrounded by ectoderm (fig. 4). The spaces between the contiguous radial tubes thus become converted into narrow canals, through which water passes from the exterior to enter the pores in the walls of the radial tubes. These canals are the “intercanals” of Haeckel, now generally known by their older name of incurrent canals. The openings of the incurrent canals to the exterior are called pores, a term which we have also applied to the openings which lead directly into the radial tubes or paragastric cavity; to avoid ambiguity we shall for the future distinguish the latter kind of opening as a prosopyle. The term “pore” will then be restricted to the sense in which it was originally used by

Grant. The mouth by which a radial tube opens into the paragaster is known as a gastric ostium. In the higher forms of Sycons the radial tubes no longer arise as simple outgrowths of the whole sponge-wall, but rather as outgrowths of the endoderm into the mesoderm, which, together with the ectoderm, exhibits an independent growth of its own; and this results in the formation of a thick investment, known as the cortex (fig. 5), to the whole exterior of the sponge. The radial tubes may branch, Heteropegma (fig. 4). If the branches are given off regularly, as the radial tubes were in the first plan, and if at the same time the original radial tube exchanges its flagellated for a pavement epithelium, a structure as shown in fig. 6 (Polejna connexiva, Pol.) will result. This form might also be brought about by unequal growth of the gastral endoderm leading to a folding of the inner part of the sponge-wall. Very little direct evidence exists as to which of these two plans has actually been followed. Phylogenetically the transition from a simple Ascon to the most complicated Sycon can be traced step by step; and ontogeny shows that such a Sycon form as Grantia raphanus passes through an Ascon phase in the course of its larval development.

Returning to the ancestral form of sponge, Olynthus, let us conceive the endoderm growing out into a number of approximately spherical chambers, each of which communicates with the exterior by a prosopyle and with the paragastric cavity by a comparatively large aperture, which we may term for distinction an apopyle ; at the same time let the endoderm lose its flagellated character and become converted into a pavement epithelium, except in the spherical chambers. Such a form, called by Haeckel “dyssycus,” may be more briefly named a Rhagon from the grape-like form of its flagellated chambers, which differ from those of a Sycon both by their form and their smaller dimensions. The Rhagon occurs as a stage in the early development of Plakina monolopha (Schulze) and Reniera fertilis (9 ) (fig. 7); a calcareous sponge which appears to approach it somewhat is Leucopsis pedunculata, Lfd. By the folding of the wall of a Rhagon, or by its outgrowth into lobes, a complicated structure such as that of Plakina monolopha (20) (see fig. 26 f ) results. This is characterized by the chambers retaining their immediate communication with the incurrent and excurrent canals, opening into the latter by the widely open apopyle and receiving the former by one or several prosopyles. This may be termed the eury- pylous type of Rhagon canal system. The fold ing of the sponge-wall may be simple, as in the example given, or too complex to unravel. In higher forms of sponges (Geodinidae, Stellettidge) the chambers cease to open abruptly into the excurrent canals: each is prolonged into a narrow canal, aphodus, or abitus, which usually directly, sometimes after uniting with one or more of its fellows, opens into an excurrent canal. The prosopyles, now restrict ed to one for each chamber, may remain unchanged in character, or at the most be prolonged into very short tubes, each a prosodus or aditus (fig. 8). This may be termed the aphodal or racemose type of Rhagon system, since the chambers at the ends of the aphodi radiating from the excurrent canal look like grapes on a bunch. As Haeckel, however, has used “racemose” in a different sense, we shall adopt here the alternative term. By the extension of the prosodal or adital canals into long tubes a still higher differentiation is reached (fig. 9). This, which from the marked presence of both prosodal and aphodal canals may be termed the diplodal type of the Rhagon canal system, occurs but rarely. Chondrosia is an example.

The following scheme will render clear the foregoing distinctions :—

1. Ascon type: simple, ex. Ascetta, Hk.; strobiloid, ex. Homo derma, Lfd.

2. Sycon type: simple radial tubes, ex. Sycetta, Hk.; branched radial tubes (cylindrical chambers), ex. Heteropegma, Fl.; chamber-layer folded, ex. Polejna, Pol.

3. Rhagon type: eurypylous, with several prosopyles to each chamber, ex. Spοngelia; with a single prosopyle to each chamber, ex. Oscarella, Thenea ; aphodal, aphodal canals well developed, ex. Geodia, Lmk.; diplodal, with both aphodal and prosodal canals well developed, ex. Chondrosia, O.S.

In the case of the calcareous sponges Polejaeff has argued forcibly that the eurypylous type arises directly from the Sycon and not from the Rhagon. It is therefore doubtful how far the Rhagon in other sponges is a primitive form derived directly from an Olynthus, or whether it may not be a secondary larval state resulting from the abbreviated development of a former Sycon predecessor. Whatever may have been its past history, the Rhagon serves now at all events as a starting-point for the development of the higher forms of canal system.

In the higher Rhagons, as in the Sycons, further complications ensue, owing to an independent growth of the external ectoderm and the adjacent mesoderm. While the endoderm, with its associated mesoderm, is growing out or folding to form the excurrent canal system, the superficial mesoderm increases in thickness, and the ectoderm, extending laterally from the sides of the incurrent sinuses, burrows into it, parallel to the surface of the sponge. Thus it forms beneath the skin (i.e ., the layer of superficial mesoderm and investing ectoderm) cavities which may be either simple and spacious or be broken up into a number of labyrinthine passages by a network of mesoblastic strands (invested with ectoderm) which extend irregularly from roof to floor of the chamber. These cavities are known as subdermal chambers.

With the appearance of subdermal chambers the sponge becomes differentiated into two almost independent regions, an outer or ectosome and an inner or choanosome, which is characterized by the presence of flagellated chambers. The ectosome forms the roof and walls of the subdermal chambers, and is in its simplest form merely an investing skin; but in a large number of sponges it acquires considerable thickness and a very complicated histological structure. It is then known as a cortex. The thickening which gives rise to a cortex takes place chiefly beneath those parts of the skin which are not furnished with pores. Beneath the pores—in this case collected into sieve-like areas—dome-like cavities are left in the cortex; they open freely into the subdermal cavities below and their roof is formed by the cribriform pore membrane above. In many sponges (Geodia, Stelletta) the cortical domes are constricted near their communication with the subdermal cavity (subcortical crypt) by a transverse muscular sphincter, which defines an outer division or ectochone from an inner or endochone (fig. 10), the whole structure being a chone. The endochone is frequently absent (fig. 10). The early development of the cortex has scarcely yet been studied. In Stelletta phrissens (Soll.), one of the “Challenger” Stel lettidae, an early form of the sponge (fig. ll), shows the choanosome already characteristically folded within the cortex, which forms a com plete not-folded envelope around it. The roots of the incurrent sinuses form widely open spaces imme diately beneath the cortex and are the rudiments of subcortical crypts. Again, in some sponges a part of the endoderm and asso ciated mesoderm may like wise develop independ ently of the rest of the sponge, as in the Hexac tinellida, where the choa nosome forms a middle layer between a reticulation of ectosome on the one side and of endoderm and mesoderm, i.e ., endo some, on the other. Fin ally, the attached or lower half of a Rhagon may de velop in an altogether dif ferent manner from the other or upper half, the endoderm not producing any flagellated chambers. In this case the upper portion alone is characterized by the flagellated chambers, which are the distinctive mark of a sponge, and hence may be called the spongomere ; the lower half, which consists of all three fundamental layers, may be called the hypomere.

The form and general composition of sponges are exceedingly various and often difficult to analyse, presenting, along with some important differences, a remarkable general resemblance to the Coelentera in these respects. Like them, some sponges are simple, and others, through asexual multiplication, compound. The only criterion by which the individual sponge can be recognized is the osculum; and, as it is frequently difficult, and in many cases impossible, to distinguish this from the gastric opening of a large excurrent canal, there are many cases in which the simple or compound nature of the sponge must remain open to doubt. The oscule may also fail (lipostomosis), and so may the paragastric cavity (fipogastrosis); the problem then becomes insoluble. The loss of the oscule may in some cases be due to the continued growth of several endodermal folds towards the exterior, with a corresponding absorption of the mesoderm and ectoderm which lie in the way, till the folds penetrate to the ectoderm and open at the exterior, thus giving rise to excurrent openings, which are not readily distinguishable from pores. At the same time the original osculum closes up and entirely disappears. Lipogastrosis, on the other hand, may be produced by the growing together of the roots of the choanosomal folds, thus reducing the paragastric cavity to a labyrinth of canals, which may easily be confounded with the usual form of excurrent canals. While in some sponges the original oscule is lost, in others secondary independent openings, deceptively like oscules, are added. This pseudostomosis is due to a folding of the entire sponge, so as to produce secondary canals or cavities, which may be incurrent (vestibular) or excurrent {cloacal), the opening of the latter to the exterior being termed a false oscule or pseudostome. The faulty use of the term oscule for what is neither functionally nor morphologically a mouth is here obvious, for in one sense the oscule is always a pseudostome; it would be better if the term pseudoproct could be substituted.

Skeleton.— All sponges, except three or four genera belonging to the Myxospongiae, possess some kind of skeletal structures. They may be either calcareous or silicious or horny scleres, the latter usually having the form of fibres, which sometimes enclose silicious needles (spicules) or foreign bodies introduced from without. Foreign bodies also contribute to the formation of the skeleton of some silicious sponges, and occasionally form the entire skeleton, no other hard parts being present.

Mineral scleres usually occur in the form of spicules. The spicules of calcareous sponges consist of carbonate of lime, having the crystalline structure and other properties of calcite (29 ). Each spicule, so far as its mineral component is concerned, is a single crystal, all the molecules of calcite of which it is built up being similarly oriented. On the other hand, its form and general structure are purely organic. Its surfaces are always curved, and usually it has the form of a cone or combination of cones, each of which consists of concentric layers of calcite surrounding an axial fibre of organic matter,—probably of the same nature as spongiolin or spongin, the chief constituent of the fibres of horny sponges. A thin layer of organic matter, known as the spicule sheath, forms an outer investment to the spicule and is best rendered visible as a residue by removing the calcite with weak acid. Silicious spicules consist of colloid silica or opal, and hence can be distinguished from calcareous by having no influence upon polarized light. Structurally the two kinds of spicules present no important difference. The spicules of different sponges differ greatly both in form and in size. They may be conveniently divided into two groups,—minute or flesh spicules, which usually serve as the support of a single cell only {microscleres), and larger or skeletal spicules, which usually contribute to the formation of a more or less consistent skeleton {megascleres). The distinction is not one that can be exactly defined, and must so far be regarded as of a provisional nature. There is usually but little difficulty in applying it in practice, except in some doubtful cases where large spicules do not form a continuous skeleton, or in others where flesh spicules appear to be passing into those of larger size. It is indeed highly probable that all large spicules have originated from flesh spicules (12 ).

(1) Monaxon Biradiate Type {rhabdus).— By far the commonest form is the oxea, a needle-shaped form pointed at both ends and produced by growth from a centre at the same rate in opposite directions along the same axis. It is therefore uniaxial and equibiradiate (fig. 12 a ). (2) Mon axon Uniradiate Type (stylus).— By the suppression of one of the rays of an oxea, an acuate spicule or stylus results (fig. 12 b). (3) Triaxon Tmradiate Type.— Linear growth from a centre in three directions inclined at an angle of 120° to each other gives rise to the primitive form of triradiate spicule so eminently characteristic of the calcareous sponges, but by no means confined to them (fig. 12 c). (4)

Tetraxon Quadriradiate Type *Calthrops).— Growth from a centre in four directions inclined at about 110° to each other produces the primitive quadriradiate form of the Tetractinellida and of some calcareous sponges (fig. 12 d). (5) Sexradiate Type.— Growth in six directions along three rectangular axes produces the primitive sexradiate spicule of the Hexactinellida sponges (fig. 12 e). (6) Multiradiate Type.— Extensions radiating in many directions from a centre produce a stellate form (fig. 12 f). (7) Spherical Scleres.— Concentric growth of silica about an organic particle produces the sphere, which occurs as a reduction of the rhabdus in some species of Poecillastra, or as an overgrown globule (flesh spicule) in Caminus.

Usually conical, the spicular rays often become cylindrical; usually pointed (oxeαte) at the ends, they are also frequently rounded off (strongylate), or thickened into knobs (t ylotate), or branched (cladose). Their growth is not always rigorously confined to a straight line: frequently they are curved or even undulating. They are also liable to become spined, either by mere superficial thickening or by a definite outgrowth involving the axial fibre (fig. 13 g, h).

The rhabdus if pointed at both ends is known as an oxea (fig. 13 c ); if rounded at both ends as a strongyle (fig. 13 a ); if knobbed at both ends as a tylote (fig. 13 b ); the tylote if pointed at one end is a iylotoxea (fig. 13 d ); the strongyle similarly becomes a slrongyloxea. These last two forms are with difficulty distinguished from the stylus, which is usually pointed at the end, and strongylate (fig. 13 f ) or tylotate (fig. 13 e) about the origin. A particular case of the cladose rhabdus, but one of the most frequent occurrence, is the triaene ; in this form one ray of a rhabdus ends in three branches, which diverge at equal angles from each other. The rhabdus then becomes known as the shaft or rhabdome, and the secondary rays are the arms or cladi, collectively the head or cladome of the spicule. The arms make different angles with the shaft: when recurved a grapnel or anatmaene is produced (fig. 13 k), when projecting forwards a protriaene (fig. 13 l ), and when extended at right angles an ortho-triaene (fig. 13 m ). The arms of a triaene may bifurcate (dichotriaene) once (fig. 13 n ), twice, or offener, or they may trifurcate. Again, they may extend laterally into undulating lamellae, or unite to form a disk, the triaene character of which is indicated by the included axial fibre. The shaft may also become trifid at both ends, arnphitriaene (fig. 13 p), and the resulting rays all bifurcate, or the cladome may arise from the centre of the rhabdome, eentrotriaene (fig. 13 o). Amongst one group of Lithistid sponges {Rhabdocrepida) the normal growth of a strongyle is arrested at an early stage; it then serves as a nucleus upon which further silica is deposited, and in such a manner as to produce a very irregularly branching sclere or desma (fig. 13 s ), within which the fundamental strongyle can be seen enclosed. In such a desma no axial fibre besides that of the enclosed strongyle is formed.

The chief modification of the triradiate spicule is due to an elongation of one ray, distinguished as apical, the shorter paired rays being termed basal, and the whole spicule a sagittal triradiate. The angle included by the basal rays is usually over 120° (fig. 14 a).

Some or all of the rays of the primitive calthrops (fig. 14 b ) may subdivide into a number of terminal spines candelabra (fig. 14 c ); or some or all of them may bifurcate once or twice and finally terminate by subdividing into numerous variously shaped processes; such a tctracladine desma (fig. 14 e) characterizes one division of the Lithistid sponges.

By the excess or defect of one or more rays a series of forms such as are represented in fig. 15 arise. In the oxea, which results from the suppression of all rays but two, the Sexradiate character is sometimes preserved by the axial fibre, which gives off two or four processes in the middle of the spicule where the defective arms would arise. Let fig. 12 e represent a regular Sexradiate spicule with its four horizontal arms extended beneath the dermis of its sponge; the over-development of the proximal ray and a reduction of the distal ray produce a form known as the dagger (fig. 15 a ); the suppression of the proximal ray and the development of spines projecting forwards on the distal ray produce the pinnulus (fig. 15 b, c ); the suppression of both proximal and distal rays gives the staurus (fig. 15 f ), and the suppression of two of the remaining horizontal rays a dermal rhabdus (fig. 15 g). The suppression of a distal ray, excessive development of a proximal ray, and recurved growth of the remaining rays produce an anchor. In Hyalonema (glass rope sponge) anchors over a foot long occur, but their arms or teeth are not restricted to four, and the axial fibre gives off its processes before reaching the head of the spicule. Such a grapnel helps to support the sponge in the ooze of the sea-bed. Other characteristic spicules belonging to sponges distinguished by Sexradiate spicules are the following :—the uncinaria (fig. 16 a ), a spinose oxea with the spines all pointing one way; the clavula, a tylotate form with a toothed margin to the head (fig. 16 b ); the scapularia (fig. 16 c ), a besom-shaped spicule with tylotate rays, which vary in number from two to eight; the amρhidisk (fig. 15 d ), a shaft terminating at each end in a number of recurved rays. When the Sexradiate spicules of the Hexactinellida unite together in a manner to be described later, the rays may be bent in a variety of ways out of the triaxial type, so that the Sexradiate character alone remains.

Multiradiate Type.— The rays of an aster as of other spicules may be spined or tylotate. In one remarkable form known as a sterraster (fig. 12 g, h), and characteristic of the family Geodinidae, the rays are almost infinite in number, and coalesced for the greater part of their length; the distal ends, however, remain separate, and, becoming slightly tylotate, are produced into four or five recurved spines, which give attachment to connective tissue fibres by which adjacent sterrasters are united together.

In one aberrant group of Lithistid sponges (Anomocladina) the skeleton is formed of desmas, which are multiradiate, each presenting a massive centrum (with an included cavity) produced into a variable number (4 to 8) of rays, which rays terminate in expanded ends (fig. 12 f ).

It is doubtful whether a distinction between megascleres and microscleres can be maintained in the calcareous sponges, unless the minute oxeas which occur in Eilhardia sehulzei, Pol. (16 ), are to be referred to this group. They are widely distributed throughout the silicious sponges, and by their different forms afford characters of the highest importance in classification.

One of the simplest forms is the sigmaspire (fig. 17 a , b ); it looks like the letter C or S, according to the direction in which it is viewed, its actual form being that of a single turn of a cylindrical spiral. A turn and a part of a turn of a spiral of somewhat higher pitch than that of a sigmaspire gives the toxaspire (fig. 17 c ); a continued spiral growth through several revolutions gives the pοlyspire. The sigmaspire becoming spined produces the spiraster or spinispirula (fig. 17 d) ; this, by losing its curvature, becomes the sanidaster (fig. 17 e), and by simultaneous concentration of its spines into a whorl at each end, the amphiaster (fig. 17 f ). By reduction of the spire the spiraster passes into the stellate or aster (fig. 17 n ). A thickening about the centre of the aster produces the sρheraster (fig. 17 m), allied to which is the sterraster. By a reduction in the number of its rays the aster becomes a minute calthrops, from which, by increased growth, the skeletal calthrops may very well be derived; by further reduction to two rays a little rhabdus or microrabd results, and of this numerous varieties exist, of which the oxeate microrabd is the most interesting, since it only differs in size from the commonest of all skeletal spicules, the oxeate or acerate rhabdus. The sigmaspire is formed as a superficial spiral thickening in the wall of a spicule cell or scleroblast; as superficial deposits also the next group of spicules, the so-called anchorates, arise. Take a hen’s egg as the model of a scleroblast, draw round it a broad meridional band, interrupted only on one side, for 30° above and below the equator; this will represent a truly C-shaped spicule, which differs from a sigmaspire by the absence of spiral, twist. It may be termed a cymba (fig. 17 g). The back of the “C” is the keel or tropis ; the points are the prows or prorae. Now broaden out the prora on the eggshell into oval lobes (p roral pteres) ; and from each pole draw a lobe midway between the prora and the tropis (pleural pteres), and a common form of anchorate, the pterocymba results (fig. 17 h ). The pterocymba is subject to considerable modifications: the prows may be similar (homoproral) or dissimilar (heteroproral) ; the pteres may be lamellar or ungual; additional lamellae tropidial pteres) may be produced by a lateral outgrowth of the keel (fig. 17 k) ; and by growing towards the equator the opposed proral and pleural pteres may conjoin, producing a spicule of two meridional bands (oocymba ; fig. 17 l). A curious group of flesh spicules are the trichites. In this group silica, instead of being deposited in concentric coatings around an axial fibre, forms within the scleroblast a sheaf of immeasurably fine fibrillae or trichites, which may be straight (fig. 17 m) or twisted. The trichite sheaf may be regarded as a fibrillated spicule. Trichite sheaves form in some sponges, as Dragmastra (25 ), a dense accumulation within the cortex. In Hexactinellid sponges the rays of the aster are limited to six, arranged as in a primitive Sexradiate spicule, but divided at the ends into an indefinite number of slender filaments, which may or may not be tylotate, rosettes (fig. 17 t).

Spongin is a horny substance, most similar to silk in chemical composition, from which it differs in being insoluble in an ammoniacal solution of copper sulphate (cuproso-ammonium sulphate). In Darwinella aurea, F. Müller, it occurs in forms somewhat resembling tri-, quadri-, and sex-radiate spicules. But usually the spongin skeleton takes the form of fibres, consisting of a central core of soft granular substance around which the spongin is disposed in concentric layers, forming a hollow cylinder (fig. 23 b ). The relative diameters of the soft core and of the spongin cylinder differ greatly in different sponges. The fibres branch so as to form antler-like twigs or bushy tree-like growths, or anastomose to form a continuous network, as in the bath sponge (Euspongia officinalis). The detailed characters of the network differ with the species, and are useful in classification. In Ianthella certain cells (sponginblasts) become included between the successive layers of the spongin cylinder, and their deep violet colour, contrasting with the amber tint of the spongin, renders them very conspicuous.

In some sponges the scleres are simply scattered through the mesoderm and do not give rise to a continuous skeleton,— Corticium, Chondrillα, Thrombus. In the Cαlcαreα and many silicious sponges they are dispersed through the mesoderm, but so numerously that by the overlapping of their rays a loosely felted skeleton is produced. In the calcareous sponges the spicules are frequently regularly disposed; and in the Sycons in particular a definite arrangement, on two plans, the articulate and inarticulate, can be traced in the skeleton of the radial tubes. On the latter plan the triradiate or quadriradiate spicules, the apical rays of which are of considerable length, are arranged in two sets, one having the basal rays lying in the mesoderm of the paragastral wall and the other with the corresponding rays in the dermal mesoderm. The apical rays of each set lie in the mesoderm of the radial tubes parallel to their length, but pointing in opposite directions (fig. 18 b). In the articulate division numerous spicules, small in comparison with the size of the radial tubes, form a series of rows round the tubes, their basal rays lying parallel to the paragastric surface and the apical pointing towards the ends of the radial tubes (fig. 18 a).

In the Silicispongia sheaves of long oxeate spicules radiate from the base of the sponge if of a plate-like form, or from the centre if globular, and extend to the surface. If triaenes are present their arms usually extend within the mesoderm immediately below the dermal surface (fig. 19). Single spicules reach from centre to surface only in small sponges. As the sponge increases in size the spicules must either correspondingly lengthen, or fresh spicules must be added, if a continuous skeleton is to be formed. The latter is the plan fol lowed in fact: the ad ditional spicules over lap the ends of those first formed like the fusiform cells in a woody fibre. With the formation of a fibre, often strengthened by spongin or bound to gether with connective tissue, there appears to be a tendency for the constituent spicules to diminish in size, and the length of each in the most markedly fibrous sponges is insignificant when compared with the length of the fibre. The spicular fibre thus formed may be simple or echinated by spicules either similar to those which form its mass or different. More usually they are different, and generally styles, often spinose about their origin. The spongin which sometimes cements together the spicules of a fibre may progressively increase in quantity and the spicules diminish in number, till a horny fibre containing one or more rows of small oxeas results. In an echinated fibre the axial spicules may disappear and the echinating spicules persist. Finally all spicules may be suppressed and the horny fibre of the Ceratose sponges results. The horny fibres may next acquire the habit of embedding foreign bodies in their substance, though foreign enclosures are not confined to the Ceratosa but occur in some Silici-spongiae as well. The included foreign bodies may increase in quantity out of all proportion to the horny fibres; and finally the skeleton may consist of them alone, all spongin matter having disappeared.

In the Lithistid sponges a skeleton is produced by the articulation of desmas into a network. The rays of the desmas (figs. 12 f , 13 s, 14 e) terminate in apophyses, which apply themselves to some part of adjacent desmas, either to the centrum, shaft, arms, or similar apophyses, and then, growing round them like a saddle on a horse’s back, clasp them firmly without anchylosis. Thus they give rise to a rigid network, in conjunction with which fibres composed of rhabdus spicules may exist. In the Hexactinellida both spicular felts and fibres occur, and in one division (Dictyonina) a rigid network is produced, not, however, by a mere clasping of apophyses, but by a true fusion. The rays of adjacent spicules overlap and a common investment of silica grows over them.

Histology.

The ectoderm usually consists of simple pavement epithelial cells (pinnacocytes), the margins of which can be readily rendered visible by treatment with silver nitrate, best by Harmer’s method. The nucleus and nucleolus are usually visible in preparations made from spirit specimens, the nucleus being often readily recognizable by its characteristic bulging beyond the general surface. In some sponges (Thecaphora) the epithelium may be replaced locally by columnar epithelium, and the cells of both pavement and columnar epithelium may bear flagella (Aplysilla viοlaceα, Oscarella lobularis). The endoderm presents the same characters as the ectoderm, except in the Ascons and the flagellated chambers of all other sponges, where it is formed of collared flagellated cells or choanocytes,— cells with a nearly spherical body in which a nucleus and nucleolus can be distinguished and one or more contractile vacuoles. The endoderm extends distally in a cylindrical neck or collum, which terminates in a long flagellum surrounded by a delicate protoplasmic frill or collar (fig. 21 g). In Tetractinellida, and probably in many other sponges—certainly in some—the collars of contiguous choanocytes coalesce at their margins so as to produce a fenestrated membrane, which forms a second inner lining to the flagel lated chamber (fig. 20, ii.). The presence of this membrane enables us readily to distinguish the excurrent from the incurrent face of the chamber, since its convex surface is always turned towards the prosopyle. In sponges with an aphodal canal system the flagellated chambers usually pass gradually into the aphodal canal, but the incurrent canal enters abruptly. This abrupt termination of the incurrent canal appears to mark the termination of the ectoderm and the commencement of the endoderm. The flagellated chambers differ greatly in size in different sponges, and evidently manifest a tendency to become smaller as the canal system increases in complexity; thus Sycon are always larger than Rhagon chambers, and eurypylous than aphodal Rhagon chambers. In most sponges except the Ascons the mesoderm is largely developed, and in many it undergoes a highly complex histological differentiation. In its commonest and simplest form it consists of a clear, colourless, gelatinous matrix in which irregularly branching stellate cells or connective tissue corpuscles are embedded; these may be termed collencytes (fig. 21 a) and the tissue collen-chyme. In the higher sponges (Geodia, Stelletta) it consists of small polygonal granular cells either closely contiguous or separated by a very small quantity of structureless jelly, and in this form may be termed sarcenchyme (fig. 20). Collenchyme does not originate through the transformation of sarcenchyme, as one might expect, for it precedes the latter in development. Schulze (20 ) , who has compared collenchyme to the gelatinous tissue which forms the chief part of the umbrella of “jelly-fish,” describes it as becoming granular immediately in the neighbourhood of the flagellated chambers in the bath sponge, the granules becoming more numerous in sponges in which the canal system acquires a higher differentiation, till at length the collencytes are concealed by them. According to this view, sarcenchyme would appear to originate from a densely granular collenchyme. Amoeboid wandering cells or archae-ocytes (fig. 22) are scattered through the matrix of the collenchyme. They evidently serve very different purposes: some appear to act as carriers of nourishment or as scavengers of useless or irritant foreign matter; others may possibly contribute to the formation of higher tissues, some certainly becoming converted into sexual products. Their parentage and early history are unknown.

A tissue (c ystenchyme) which in some respects resembles certain forms of vegetable parenchyme occurs in some sponges, particularly Geodinidae and other Tetractinellida. It consists of closely adjacent large oval cells, with thin well-defined walls and fluid contents. Somewhere about the middle of the cell is the nucleus with its nucleolus, supported by protoplasm, which extends from it in fine threads to the inner side of the wall, where it spreads out in a thin investing film (fig. 21 c ). Cystenchyme very commonly forms a layer just below the skin of some Geodinidae, particularly of Pachymatisma, and, as on teasing the cortex of this sponge a large number of refringent fluid globules immiscible with water are set free, it is just possible that it is sometimes a fatty tissue, and if so the contained oil must be soluble in alcohol, for alcoholic preparations show no trace of it. A tissue resembling cartilage, chondrenchyme, occurs in Corticidae (fig. 21 b ).

Connective-tissue cells or desmacytes are present in most sponges; they are usually long fusiform bodies, consisting of a clear, colourless, often minutely fibrillated sheath, surrounding a highly refringent axial fibre, which stains deeply with reagents (fig. 21 d). In other cases the des-macyte is simply a fusiform granular cell, with a nucleus in the interior and a fibrillated appearance towards the ends. The desmacytes are gathered together, their ends overlapping, into fibrous strands or felted sheets, which in the ectosome of some sponges may acquire a considerable thickness, often constituting the greater part of the cortex. The spicules of the sponge often furnish them with a surface of attachment, especially in the Geodinidae, where each sterraster of the cortex is united to its neighbours by desmacytes, in the manner shown in fig. 10.

Contractile fibre cells or myocytes occur in all the higher sponges. They appear to be of more than one kind. Most usually they are fine granular fusiform cells with long filiform terminations, and with an enclosed nucleus and nucleolus (fig. 21 e). In the majority of sponges both excurrent and incurrent canals are constricted at intervals by transverse diaphragms or vela, which contain myocytes concentrically and sometimes radiately arranged. The excessive development of myocytes in such a velum gives rise to muscular sphincters such as those which close the chones of many corticate sponges, such as Pachymatisma. In this sponge, which occurs on the British shores, the function of the oscular sphincters can be readily demonstrated, since irritation of the margin of the oscule is invariably followed after a short interval by a slow closure of the sphincter.

Supposed sense-cells or aesthacytes (fig. 22) were first observed by Stewart and have since been described by Von Lendenfeld (12 ). According to the latter, they are spindle-shaped cells, 0·01 mm. long by 0·002 thick; the distal end projects beyond the ectodermal epithelium in a fine hair or palpocil; the body is granular and contains a large oval nucleus; and the inner end is produced into fine threads, which extend into the collenchyme and are supposed—though this is not proved—to become continuous with large multiradiate collencytes, which Von Lendenfeld regards as multipolar ganglion cells (fig. 22).

More recently he has described an arrangement of these cells curiously suggestive of a sense-organ. Numerous aesthacytes are collected over a small area, and at their inner ends pass into a granular mass of cells with well-marked nuclei, but with boundaries not so evident; these he regards as ganglion cells. From the sides of the ganglion other slender fusiform cells, which Von Lendenfeld regards as nerves, pass into the mesoderm, running tangentially beneath the skin. The inner end of the ganglion is in communication with a membrane formed of fusiform cells which Von Lendenfeld regards as muscular. If his observations and inferences are confirmed, it is obvious that we have here a complete apparatus for the conversion of external impressions into muscular movements.

In most sponges a direct connexion can be traced by means of their branching processes between the collencytes of the mesoderm and the cells of the ectodermal and endodermal epithelium and the choanocytes of the flagellated chambers. As the collencytes are also united amongst themselves, they place the various histological constituents of the sponge in true protoplasmic continuity. Hence we may with considerable probability regard the collencytes as furnishing a means for the transmission of impulses: in other words, we may attribute to them a rudimentary nervous function. In this case the modification of some of the collencytes in communication with the ectoderm might readily follow and special aesthacytes arise. Fusiform collencytes perpendicular to the ectoderm, and with one end touching it, are common in a variety of sponges; but it is difficult to trace the inner end into connexion with the stellate collencytes, so that precisely in those cases in which it would be most interesting to find such a connexion absolute proof of it is wanting.

The colour of sponges usually depends on the presence of cells containing granules of pigment; though dispersed generally through the mesoderm, these cells are most richly developed in the ectosome. Pigment granules also occur in the choanocytes of some sponges,— Oscarella lobularis and Aplysina aerophoba, for instance. In the latter the pigment undergoes a remarkable change of colour when the sponge is exposed to the air, and finally fades away. In many cases sponges borrow their colours from parasitic algae (O scillatoria and Hostoc) with which they are infested. The colours of sponge-pigments are very various. They have been examined by Krukenberg and Merejknovsky. Zoonerythin, a red pigment of the lipochrome series, is one of the most widely diffused; it is regarded as having a respiratory function. Reserve cells or thesocytes (fig. 21 f ) have been described in several sponges as well as amylin and oil-bearing cells.

Each spicule of a sponge originates in a single cell (fig. 21 h-n), within which it probably remains enclosed until it has completed its full growth; the cell then probably atrophies. During its growth the spicule slowly passes from the interior to the exterior of the sponge, and is finally (in at least some sponges, Geodia, Stelletta) cast out as an effete product. The sponge is thus constantly producing and disengaging spicules; and in this way we may account for the extraordinary profusion of these structures in some modern marine deposits and in the ancient stratified rocks. Within the latter these deciduous spicules have furnished silica for the formation of flints, which have been produced by a silicious replacement of carbonate of lime (26 ) .

The horny fibres of the Ceratosa are produced as a secretion of cells known as sponginblasts, which surround as a continuous mantle the sides of each growing fibre, and cover in a thick cap each growing point (fig. 23). The lateral sponginblasts are elongated radially to the fibre; the terminal cells are polygonal and depressed. The latter give rise to the soft granular core and the former to the spongin-walls of the fibre. Cells similar to the lateral sponginblasts, and regarded as homologous with them, occur in a single layer just below the outer epithelium of some horny sponges (Aplysilla and Dendrilla ) and under certain circumstances secrete a large quantity of slimy mucus (11 ). Classification.

The phylum Parazoa or Spongiae consists of two main branches, as follows :—

Branch A. — MEGAMASTIC- Branch B. — MICROMASTICTORA. TORA.

Class Calcarea, Grant. Class I. — Myxospongiae,

Order 1.— Homocoela, Pol. Haeckel.

Order 2.— Heterocoela, Pol. Order 1.— Halisarcina.

Order 2.— Chondrosina.

Class II.— Silicispongiae. Sub-class i.— Hexactinellida,

O. Schmidt.

Order 1.— Lyssacina, Zittel. Order 2.— Dictyonina, Zittel.

Sub - class ii. — Demospongiae, Sollas.

Tribe a.— Monaxonida.

Order 1.— Monaxona.

Order 2.— Ceratosa, Grant.

Tribe b. — Tetractinellida, Marshall.

Order 1.— Choristida, Sollas. Order 2.— Lithislida, O.S.

By the possession of both sexual elements and a complex histological structure, and in the character of their embryological development, the sponges are clearly separated from the Protozoa ; on the other hand, the choanoflagellate character of the endoderm, which it retains in the flagellated chambers throughout the group without a single exception, as clearly marks them off from the Metazoa. They may therefore be regarded as a separate phylum derived from the choanoflagellate Infusoria, but pursuing for a certain distance a course of development parallel with that of the Metazoa.

Different views have been propounded by other authors. Savile Kent regards the sponges as Protozoa (10 ); Balfour suggested that they branched off from the Metazoan phylum at a point below the Coelentera, and considered them as intermediate between Protozoa and Metazoa; Schulze regards them as derived from a simple ancestral form of Coelentera (23 ); Marshall advocates the view that they are degraded forms derived from Coelenterates which were already in possession of tentacles and mesenteric pouches (14 ).

As a phylum the Spongiae are certainly divisible into two branches, one including the Calcarea and the other the remaining sponges, which Vosmaer has termed Non-Calcarea, and others Plethospongiae. Since, however, the choanocytes of the Calcarea are usually, if not universally, larger than those of other sponges, we may make use of this difference in our nomenclature, and distinguish one branch as the Megamastictora (μaστίκτωp, “scourger”) and the other as the Micromastictora.

Branch A. —MEGAMASTICTORA.

Sponges in which the choanocytes are of comparatively large size, 0·005 to 0·009 mm. in diameter (Haeckel, ό).

Class Calcarea. Megamastictora in which the skeleton is composed of calcareous spicules.

Order 1. Homocoela.— Calcarea in which the endoderm consists wholly of choanocytes. Examples: Leucosolenia, Bwk.; Homoderma, Lfd.

Order 2. Heterocoela.— Calcarea in which the endoderm is differentiated into pinnacocytes, which line the paragastric cavity and excurrent canals, and choanocytes, which are restricted to special recesses (radial tubes or flagellated chambers). Examples: Sycon, O.S.; Grantia, Fl.; Lcuconia, Bwk.

Branch B. —MICRΟMASTICTORA. (Non-Calcarea, Vosmaer; Plethospongiae, Sollas.) Sponges in which the choanocytes are comparatively small, 0·003 mm. in diameter.

Class I. MYXOSPONGIAe. Micromastictora in which a skeleton or scleres are absent.

Order 1. Halisarcina.— Myxospongiae in which the canal system is simple, with simple or branched Sycon or eurypylous Rhagon chambers. An ectosome sometimes and a cortex always absent. Examples: Halisarca, Duj.; Oscarella, Vosm.; Bajalus, Lfd.

Order 2. Chondrosina.— Myxospongiae in which the canal system is complicated, with diplodal Rhagon chambers and a Well-developed cortex. Example: Chondrosia, O.S.

The Halisarcina are evidently survivals from an ancient and primitive type. The simplicity of the canal system is opposed to the view that they are degraded forms; we may therefore regard the absence of scleres as a persistent primary and not a secondary acquired character. They are as interesting, therefore, from one point of view (absence of scleres) as the Ascons are from another (undifferentiated endoderm). With the Chondrosina the case is different; they differ only from Chondrilla and its allies by the absence of asters; these differ only from the Tethyidae by the absence of strongyloxeas; and we may very reasonably assume that in these three groups we have a series due to loss of characters, the Chondrillae being reduced Tethyidae and the Chondrosina reduced Chondrillae. Still, as Huxley has well remarked, “classification should express not assumptions but facts”; and therefore till we are in possession of more direct evidence it will be well to exclude the Chondrosina from the Silicispongiae.

Class IL Silicispongiae. Micromastictora possessing a skeleton or scleres which are not calcareous.

Sub-class i. HEXACTINELLIDA. Silicispongiae characterized by Sexradiate silicious spicules. Canal system usually simple, with Sycon chambers. Sponge differentiated into ecto-, choano-, and endo-some.

Order 1. Lyssacina.— Hexactinellida in which the skeleton is formed of separate spicules, or, if united, then by a subsequent not a contemporaneous deposit of silica. Examples: Euplectella, Owen; Asconema, S. Kent; Hyalonema, Gray; Rossella, Crtr.

Order 2. Dictyonina. —Hexactinellida in which Sexradiate spicules are cemented together by a silicious deposit into a continuous network pari passu with their formation. Examples: Farrea, Bwk.; Eurete, Marshall; Aphrocallistcs, Gray; Myliusia, Gray; Dactylοcalyx, Stutchbury.

The Hexactinellida are a very sharply defined group, impressed with marked archaic features. No other Silicispongiae possess, so far as is known, so simple a syconate canal system. The oldest known fossil sponge is a member of the Lyssacina (7 and 24), viz., Protospongia, Salter, from the Menevian beds, Lower Cambrian, St David’s Head, Wales. The group is almost world-wide in distribution, chiefly affecting deep water, from 100 to 300 fathoms, but often extending into abyssal depths; occasionally, however, though rarely, it frequents shallow water (Cystispongia superstes dredged off Yucatan in 18 fathoms).

Sub-class ii. DEMOSPONGIAe.

Silicispongiae in which Sexradiate spicules are absent.

Tribe a . MONAXONIDA. Demospongiae in which the skeleton consists either of silicious spicules which are not quadriradiate, or of horny scleres or included foreign bodies, or of one or more of these constituents in conjunction.

Order 1. Monaxona. —The skeleton is characterized by either uniaxial or polyaxial spicules. Examples: Amorphina, O. S. (“crumb of bread” sponge); Spοngilla, Lmk. (“freshwater” sponge); Chalina, Bwk.; Tethya, Link.

Order 2. Ceratosa. —The skeleton consists of horny scleres which never include “proper” spicules, or of introduced foreign bodies, or of both these in conjunction. Examples: Darwinella, F. Miiller; Euspongia, Bronn (the “bath” sponge).

Tribe b. TETRACTINELLIDA. Demospongiae possessing quadriradiate or triaene spicules or Lithistid scleres (desmas).

Order 1. Choristida. — Tetractinellida with quadriradiate or triaene spicules, which are never articulated together into a rigid network. Examples: Tetilla, O.S.; Thenea, Gray; Geodia, Lmk.; Dercitus, Gray.

Order 2. Lithistida.— Tetractinellida with branching scleres (desmas), which may or may not be modified tetrad spicules, articulated together to form a rigid skeleton. Triaene spicules may or may not be present in addition. Examples: Theonella, Gray; Coral-listes, O.S.; Azorica, Crtr.; Velulvna, O.S.

This large sub-class embraces the great majority of existing sponges. Its external boundaries are fairly well defined, its internal divisions much less so, as its various orders and families pass into each other at many points of contact. Although there does not appear to be much resemblance between a Lithistid sponge, such as Theonella, a Monaxonid such as Amorphina, and an ordinary "bath” sponge (Euspongia), yet between these extremes a long series of intermediate forms exists, so nicely graduated as to render their disruption into groups by no means an easy task. If the delimitation of orders is difficult, that of genera is often impossible, so that they are reduced to assemblages depending on the tact or taste of the author. Thus Polejaeff states that with a single exception “none of the genera of Ceratosa are separable by absolute characters.” The chief spicules of Monaxona are uniaxial, often accompanied by characteristic microscleres. Although distinguished as a group by the absence of quadriradiate or triaene spicules, two exceptions are known in which these occur (Tricentrion, Ehlers, and Acarnus, Gray); these, however, present unusual characters which suggest an independent origin. The canal system of Monaxona has not yet been fully investigated; it appears usually to follow the eurypylous Rhagon type, but the aphodal is not unknown. The Ceratosa contain all sponges with a horny skeleton, except those in which the homy fibres are cored or spined with silicious spicules secreted by the sponge (“proper” spicules); these are arbitrarily assigned to the Monaxona. There is convenience in this proceeding, for horny matter is widely disseminated throughout the Demospongiae, occurring even in the Lithistida, and it frequently serves to cement the oxeate spicules of the Monaxona into a fibre, without at the same time forming a preponderant part of the skeleton. It would be wellnigh impossible to say where the line should be drawn between a fibre composed of spicules cemented by spongin and one consisting of spongin with embedded spicules, while there is comparatively no difficulty in distinguishing between fibres containing spicules and fibres devoid of them. That the distinction, however, is entirely artificial is shown by the fact that, after spicules have disappeared from the horny fibre, they may still persist in the mesoderm; thus Von Lendenfeld announces the discovery of microscleres (cymba) in an Aplysillid sponge and of strongyles in a Cacospongia, both horny sponges. (A form intermediate between this Aplysillid and the Desmacidonidae would appear to be Toxo-chalina, Ridley.) The Ceratosa frequently enclose sand, Foraminifera, deciduous spicules of other sponges and of compound Ascidians, and other foreign bodies within the horny fibres of their skeleton; they also sometimes attach this material, probably by a secretion of spongin, to their outer surface, and thus invest themselves in a thick protective crust. In some Ceratosa no other skeleton than that provided by foreign enclosures is present. The canal system is syconate or eurypylous in the simpler forms and diplodal in the higher. The Monaxonida make their earliest appearance in the Silurian rocks (Climacospongia, Hinde), and are now found in all seas at all depths. The only sponges inhabiting fresh water belong to this group. The Tetractinellida adhere to the Monaxonida at more than one point, and one of these groups has probably been a fruitful parent to the other, but which is offspring and which parent is still a subject for discussion. The Choristida in its simplest forms presents a eurypylous Rhagon system, in the higher an aphodal system. It is in this group that the most highly complex cortex is met with; in the Geodinidae, for instance, it consists usually of at least five distinct layers. Thus, proceeding outwards, next to the choanosome is a layer of thickly felted desmachyme, passing into Collenchyme on its inner face; then follows a thick stratum of sterrasters united together by desmacytes; this is succeeded by a layer of cystenchyme or other tissue of variable thickness; external to this is a single layer of small granular cells and associated dermal asters; and finally, the surface is invested by a layer of pavement epithelium. The Lithistida, like the Ceratosa, are possibly of polyphylitic origin; in one group (Tctracladina) the articulated scleres are evidently modified calthrops spicules (see fig. 14 e), and associated with them are free triaenes, which support the dermis and resemble precisely the triaenes of the Choristida. In another group (Rhabdocrepida) the scleres are moulded on a Monaxonid base (see fig. 13 q-s); but, associated with them, triaenes sometimes occur similar to those of the Tctracladina. Both these groups are in all probability derived from the Choristida, and a distinct passage can be traced from the Tetracladose to the Rhabdocrepid group. In the Rhabdocrepida we find forms without triaenes; these may possibly be degenerate forms. The third group of Lithistids is derived from the Rhabdocrepida, the Anomocladine desma being derivable from the Rhabdocrepid by a shortening of the main axis into a centrum. The thick centrum, from which the arms, variable in number, originate, is hollowed out by a cavity, which appears during life to have been occupied by a large nucleus, like that of a scleroblast, and it is quite conceivable that the scleroblast, which in the Tetracladine Lithistids lies in an angle between the arms, may have become enclosed in an overgrowth of silica, from which additional arms were produced. The constancy with which spicules in other sponges maintain their independence is very striking. When once a persistent character like this is disturbed, excessive variability may be predicted, as in the Anomocladine scleres.

The classification of the sponges into families is shown in the following scheme.

Class CALCAREA.

Order 1. Homocoela , Pol.

Family 1. Asconidae, Hk.— Homocoela which are simple or composite, but never develop radial tubes. Examples: Ascetta, Hk. (fig. 1); Lcucosolenia, Bwk.

Family 2. Homodermidae, Lfd.— Homocoela with radial tubes. Example: Homoderma, Lfd. (figs. 3, 4).

Order 2. Heterocoela , Γol.

Tribe a. †Syconaria. An * indicates that the group is only known in the fossil state, a † that it is both recent and fossil. 1

The flagellated chambers are either radial tubes or cylindrical sacs.

Family 1. Sycoxidae. —The radial tubes open directly into the paragastric cavity.

Sub-family a. Syconina.—The radial tubes are free for their whole length, or at least distally. Examples: Sycetta, Hk.; Sycon, O.S.

Sub-family b. Uteina, Lfd.—The radial tubes are simple and entirely united. The ectosome is differentiated from the choanosome and sometimes develops into a cortex. Examples: Grantissa, Lfd.; Ute, O.S. (fig. 5); Sycortusa, Hk.; Amphoriscus, Pol.

Sub-family c . Grantina, Lfd.—The radial tubes are branched. The incurrent canal system is consequently complicated. An ectosome is present. Examples: Grantia, Fl.; Heteropegma, Pol. (fig. 4); Anamaxilla, Pol.

Family 2. Sylleibidae, Lfd.—The choanosome is folded. The flagellated chambers (which are partly rhagose in Vosmaeria) communicate with the paragastric cavity by excurrent canals. Examples: Polejna, Lfd. (fig. 6); Vosmaeria, Lfd.

Family 3. Teichonellidae, Carter.—Composite Sylleibidae with the oscules and pores occurring on different parts of the surface. Example: Teichonella, Crtr.

Tribe b. † Leucoxaria,

The canal system belongs to the eurypylous Rhagon type.

Family 1. Leucoxidae, Hk.—The outer surface is not differentiated into osculiferous and poriferous areas. Examples: Leucetta, Hk.; Leucaltis, Hk.; Leucortis, Hk.

Family 2. Eilhardidae, Pol.—Composite Leuconaria, with the outer surface differentiated into special osculiferous and poriferous areas. Example: Eilhardia, Pol.

The arrangement adopted above is founded on Von Lendenfeld’s revision (11 ) of the classification propounded by Polejaeff (16 ), who in a masterly survey has thrown an unexpected light on the structure and inter-relationships of a group which Haeckel has rendered famous. It should not be overlooked that Vosmaer (31 ) had previously explained the structure of the Leucones. However erroneous in detail, Haeckel’s views are confirmed in their broad outlines, and it was with true insight that he pronounced the Calcarea to offer one of the most luminous expositions of the evolutional theory. In this single group the development in general of the canal system of the sponges is revealed from its starting-point in the simple Ascon to its almost completed stage in the Leucon, with a completeness that leaves little further to be hoped for, unless it be the requisite physiological explanation.

Class MYXOSPONGLAe. Order 1. Halisarcina .

Family 1. Halisarcidae, Lfd.—The flagellated chambers are syconate. Examples: Halisarca, Duj. (with branched chambers); Bajal·us, Lfd. (with simple chambers).

Family 2. Oscarellidae, Lfd.—The flagellated chambers are eurypylous and rhagose. Example: Oscarella, Vosm.

Order 2. Chondrosina.

Family 1. Choxdrosiidae.— With the characters of the order. Example: Chondrosia, O.S.

Class SILICISPONGIAe.

Sub-class I. HEXACTINELLIDA.

Order 1. † Lyssacina .

Family 1. Euplectellidae. —The spicules of the dermal membrane are “daggers” (fig. l5 a). Examples: Euplectella, Owen; Holascus, E. Sch.; Habrodiclyum, W. T.

Family 2. Ascoxematidae. —The dermal spicules are “pinnuli” (fig. 15 b, c). Examples: Asconema, S. Kent; Sympagella, O.S.; Caulophaeus, Schulze.

Family 3. Hyalonematidae. —The dermal spicules are pinnuli and amphidisks (fig. l5 d). Example: Hyalonema, Gray.

Family 4. † Rosselidae. —The dermal spicules are gomphi, stauri (fig. 15 f ), and oxeas. Examples: Rossella, Crtr.; Crateromorpha, Gray; Aulochoma, E. Sch.

Family 5. *Receptaculidae, Hinde.—The distal ray of the dermal spicules is expanded horizontally into a polygonal plate. Example: *Receptaculites, Defr.

Order 2. † D ICTYONINA.

Sub-order 1. UNCIN1TARIA.

Uncinate spicules are present.

Tribe a. Clavularia.

Clavulae (fig. 16 c) are present.

Family 1. Farreidae. —Characters those of the tribe. Example: Farrea, Bwk.

Tribe b. Scopularia.

The dermal spicules are scopulariae (fig. 16 b).

Family 1. † Euretidae. —Branched anastomosing tubes, or goblet-shaped, with lateral outlets. Examples: Eurete, Marshall; Peri-phragella, Marshall; Lefroyella, Schulze.

Family 2. † Mellittonidae. —Tubular or goblet-shaped, with honeycomb-like walls. Example: Aphrocallistcs, Gray. Family 3. † Chonelasmatidae. —Flat or beaker-shaped; straight funnel-shaped canals perforating the wall perpendicularly and opening laterally on each side. Example: Chonclasma, Schulze.

Family 4. † Volvulindaae .—Tubular, goblet-shaped, or massive; crooked canals more or less irregular in their course. Examples: Volvulina, Schulze; Fieldingia, S. Kent.

Family 5. Sclerothamxidae. —Arborescent body; perforated at the ends and sides by round narrow radiating canals. Example: Sclerolhamnus, Marshall.

Sub-order 2. INERMIA. Dictyonina without uncinati, clavulae, or scopulariae.

Family 1. † Myliusidae. —Depressed cup-shaped; a complex folding of the wall produces lateral excurrent tubes. Example: Myliusia, Gray.

Family 2. † Dactylocalycidae. —Goblet-shaped or pateriform, with a thick wall consisting of numerous parallel anastomosing tubes, of uniform breadth, which terminate at the same level within and without. Examples: Daclylocalyx, Gray; Sclcroplegma, O.S.; Margaritella, O.S.

Family 3. † Euryplegmatidae. —Goblet-shaped or resembling ear-shaped saucers; the wall deeply folded longitudinally so as to produce a number of dichotomously branched canals or covered-in grooves. Example: Euryplegma, Schulze.

Family 4. † Aulocystidae. —Of massive rounded form, with an axial cavity; wall consisting of a system of obscurely radiating anastomosing tubes and intervening inter-canals; both inter-canals and the external terminations of the tubes are covered by a thin membrane, which is perforated by slit-like openings over the lumina of the tubes, and thus assumes a sieve-like character. Examples: Aulocyslis, Schulze; C ystispongia, Roemer.

This arrangement of the Hexactinellida is taken from the latest work on the subject, Schulze’s Preliminary Report on the "Challenger" Hexactinellida. The reference of fossil forms to the families here instituted is rendered difficult by the disappearance of the requisite “guiding” spicules in the process of mineralization. A revision of the fossil families to bring them into harmony with the recent has certainly been rendered necessary, but this is too large a task to undertake in this place.

Sub-class II. DEMOSPONGIAe.

Tribe a. Monaxonida.

Order 1. ΜΟΝΑΧΟΝΑ.

Family 1. Tethyidae. —Skeleton consisting of radiately arranged strongyloxeas (except in the genus Chondrilla, which is without megascleres) and large spherasters. The ectosome is a thick fibrous cortex. Example: Tethya, Link.; Chondrilla, O. S.

Family 2. Polymastidae. —Skeleton consisting of styles radiately arranged and cortical tylostyles. The oscules in many cases open at the ends of long papillae. Examples: Polymastia, Bwk.; Thcca-phora, O.S.; Trichostemma, Sars.

Family 3. Suberitidae. —Skeleton consisting of strongylate or tylotate styles, arranged to form a felt. The flesh spicules when present are usually microrabds or spirasters. Examples: Suberites, Nardo; Cliona, Grant; Poterion, Schlegel.

Family 4. Desmacidonidae. —The flesh spicules are cymbas. Examples: Esperella, Vosm.; Dcsmacidon, Bwk.; Cladorhiza, Sars.

Family 5. † Halichondridae. —The flesh spicules when present are never cymbas. Examples: Halichondria, Fl.; Reniera, O.S.; Chalina, Bwk.; *Pharetrospοngia, Soil.

Family 6. Ectyonidae.— The skeleton consists of fibres echinated by projecting spicules. Examples: Plocamia, O.S.; Eetyon, Gray; Clathria, O.S.

Family 7. † Spongillidae.— Halichondridae which are reproduced both sexually and by statoblasts. Habitat freshwater. Examples: Spongilla, Lmk.; Eρhydatia, Link.; Parmula, Crtr.; Potamolepis, Marshall.

The foregoing classification is purely provisional; the group requires a complete revision.

Order 2. C ERATOSA.

Family 1. Darwinellidae. —Canal system of the eurypylous Rhagon type. Flagellated chambers, pouch-shaped, large; the surrounding Collenchyme not granular. Horny fibres with a thick core. Examples: Darwinella, Fritz Muller; Aplysilla, F.E.S.; Ianthella, Gray.

Family 2. Spoxgelidae. —Canal system as in the Darwinellidae, but the flagellated chambers more or less spherical. Horny fibres with a thin core, and usually containing foreign enclosures. Examples: Velinea, Vosm.; Spongelia, Nardo; Psammoclema, Marshall; Psammopemma, Marshall.

Family 3. Spoxgidae. —Canal system aphodal. Chambers small and spherical; surrounding Collenchyme granular. Fibres with a thin core. Examples: Euspongia, Bronn; Coscinoderma, Crtr.; Phyllospongia, Ehlers.

Family 4. Aplysixidae.— Canal system diplodal; Collenchyme surrounding the flagellated chambers densely granular. . Fibres with a thick core. Examples: Luffaria, Duch, and Mich.; Vcron-gia, Bwk.; Aplysina, Nardo.

The species of sponge in common use are three,— Euspongia officinalis (Linn.), the fine Turkey or Levant sponge; E. zimocca (O.S. ), the hard Zimocca sponge; and Hippospongia equina (O.S.), the horse sponge or common bath sponge. The genus Euspongia is distinguished by the regular development of the skeletal network throughout the body, its narrow meshes, scarcely or not at all visible to the naked eye, and the regular radiate arrangement of its chief fibres. Hippospongia is distinguished by the thinness of its fibres and the labyrinthic character of the choanosome beneath the skin As a consequence its chief fibres have no regular radiate arrangement. The species of Euspongia are distinguished as follows. In E. officinalis the chief fibres are of different thicknesses, irregularly swollen at intervals, without exception cored by sand grains; in E. zimocca they are thinner, more regular, and almost free from sand. In E. officinalis, again, the uniting fibres are soft, thin, and elastic; whilst in E. zimocca they are denser and thicker, to which difference the latter sponge owes its characteristic hardness. Finally, the skeleton of E. officinalis is of a lighter colour than that of E. zimocca. The common bath sponge (Hippospongia equina) has almost always a thick cake-like form; but its specific characters are not yet further defined.

Tribe b. Tetractinellida. Order 1. C HORISTIDA.

Sub-order 1. SIGMATOPHORA.

The microsclere is a sigmaspire.

Family 1. Tetillidae. —The characteristic megasclere is a protriaene. Canal system in the lower forms eurypylous, in the higher aphodal. The ectosome in the simpler forms is a dermal membrane, in the higher a highly differentiated cortex. Examples: Tetilla, O.S.; Craniella, O.S. (fig. 21 h, l).

Family 2. Samidae. —The characteristic megasclere is an ampħitriaene. Example: Samus, Gray.

Sub-order 2. ASTEROPHORA.

The microsclere is an aster.

Group 1. Spirastrosa. —A spiraster is usually present.

Family 1. Theneidae, Carter.—The flesh spicule is a spiraster. Canal system eurypylous. Ectosome not differentiated to form a cortex. Examples: Thenea, Gray (fig. 21 a, f); Poecillastra (Normania), Bwk.

Family 2. † Pachastrellidae. —Canal system eurypylous in the lower, aphodal in the higher forms. Examples: Plakortis, F.E.S.; Dercitus, Gray.

Group 2. Euastrosa. —Spirasters are absent.

Family 1. † Stellettidae. —Canal system aphodal, but approaching the eurypylous in the lower forms. The cortex chiefly consists of Collenchyme in the lower forms; in the higher it is highly differentiated. Example: Stelletta, O.S. (fig. 11); Ancorina, O.S.; Myriastra, Soll.

Family 2. Tethyidae. —Although this family has been placed in the Monaxonida, this seems to be its more natural position.

Group 3. Sterrastrosa. —A sterraster is present, usually in addition to a simple aster.

Family 1. † Geodinidae. —The megascleres are partly triaenes. Canal system always aphodal. Cortex highly differentiated. Examples: Geodia, Lmk. (fig. 21 n); Pachymatisma, Bwk. (fig. 21 c); Cydonium, Miiller (fig. 10); Erylus, Gray.

Family 2. Placospongidae. —The megasclere is a tylostyle. Triaenes are absent. Example: Placospongia, Gray.

Sub-order 3. MICROSCLEROPHΟRA.

Microscleres only are present.

Family 1. Plakindae, Schulze.—Canal system very simple, belonging to eurypylous Rhagon type. Characteristic spicules candelabra. Examples: Plakina, F.E.S. (fig. 26).

Family 2. Corticidae. —Canal system aphodal or diplodal. Mesoderm a Collenchyme crowded with oval granular cells; the spicules either candelabra, amphitriaenes, or triaenes irregularly dispersed in it. Example: Corticium, O.S. (figs. 9, 21 b).

Family 3. Thrombidae. —Canal system diplodal. Spicules tricho-triaenes. Example: Thrombus, Soil.

The Pachastrellidae or the Corticidae are probably the families from which the Tetracladine Lithistids have been derived. In the Tetillidae the characteristic microsclere may occasionally fail, but there is never any difficulty in identifying the sponge in this case, as the triaenes are of a very characteristic form: the aims of the protriaenes are slender, simple, and directed very much forwards, making a very large angle with the shaft. Microscleres, Having the form of little globules, are sometimes present with the sigmaspires.

Order 2. L ITHISTIDA, O.S.

Sub-order 1. TETRACLADINA, Zittel.

The desmas are modified calthrops spicules. Family 1. Tetracladidae. —With the characters of the suborder. Examples: Theonella, Gray (fig. 21 k) ; Discodermia, Bocage; *Siphonia, Parkinson.

Sub-order 2. RHABDOCREPIDA.

The desmas are of various forms, produced by the growth of silica over a uniaxial spicule.

Family 1. Megamorinidae. —The desmas are comparatively large. Triaenes, usually dichotriaenes, help to support the ectosome. Microscleres usually spirasters. Examples: Corallistes, O. S.; * Hyaloimgos, Zittel; Lyidium, O. S.; * Dorydermia, Zittel.

Family 2. Micromorinidae. —The desmas are comparatively small. Triaenes and microscleres are both absent. Examples: Azorica, Crtr.; * Verruclina, Zittel.

Sub-order 3. ANOMOCLADINA.

Desmas with a massive nucleated centrum, from which a variable number of arms (28) extend radiately (see fig. 12 f ). Examples: Velulina, O. S.; *Asiylospongia, Roemer.

Reproduction and Embryology.

Fresh individuals arise by asexual gemmation, both external and internal, by fission, and by true sexual reproduction.

Fission is probably one of the processes by which compound sponges are produced from simple individuals. Artificial fission has been practised with success in the cultivation of commercial sponges for the market. External gemmation has been observed in Thenea, Tethya, Polymastia, and Oscarella. A mass of indifferent sponge-cells accumulates at some point beneath the skin, bulges out, drops off, and gives rise to a new individual. Internal gemmation, which results in the formation of a statoblast, is only known to occur in the freshwater Spongillidae. The statoblasts consist of a mass of yolk-bearing mesoderm cells, invested by a capsule, which in Ephydatia fluviatilis is composed of an inner cuticle of spongin separated from a similar outer layer by an intermediate zone of amphidisks and interspersed protoplasmic cells. On one side of the capsule is a hilum which leads into the interior.

Their development has recently been studied by Götte, with results that confirm the conclusions of Carter (3 ) and Lieberkühn (13 ). The process commences with an accumulation of amoeboid cells within the mesoderm to form a globular cluster; yolk granules develop within them, especially in those that lie nearer the centre. The external cells give rise to the investing capsule; some resemble sponginblasts and secrete the inner and outer horny cuticle; others give rise to the amphidisks and interspersed cells of the middle layer. Under favourable conditions the interior cells creep out through the pore of the capsule, and form a spreading heap, which by subsequent differentiation gives rise to a young Spongilla.

Since the freshwater sponges can only be regarded as modified descendants of ancient marine species (probably of the family Halichondridae), we may consider the internal gemmules, like the similar statoblasts of the freshwater Polyzoa, as special adaptations to a changed mode of life. They appear primarily to serve a protective purpose, ensuring the persistence of the race, since they only appear in extreme climates on the approach of drought, and in cold ones on the approach of winter.

As a secondary function they serve for the dispersal of the species; some are light enough to float down a stream, but not too far, so that there is no danger of their being carried to sea; others, which are characterized by large air-chambers, are possibly distributed by the wind.

Both sexual elements may be formed in the same individual, e.g., Oscarella lobularis, Grantia raphanus, and many others; but even in hermaphrodites one or other element usually occurs to excess in different individuals, so that some are predominantly male and others predominantly female. Polejaeff found only one such male form to 100 female forms in Grantia raphanus. In other sponges— Reniera fertilis, Euspongia officinalis— the sexes are distinct. The ova develop from archaeocytes or wandering amoeboid cells, which increase in size and acquire a store of reserve nourishment in the form of yolk granules; at first they exhibit lively amoeboid movements, but later pass into a resting stage. The cavity of the mesoderm within which they are situated becomes lined by a layer of epithelium, which may not appear, however, till a late stage of segmentation. In Euspongia officinalis the ova occur congregated in groups within the mesoderm, thus presenting an early form of ovary. The spermatozoa, which also develop from wandering amoeboid cells, are minute bodies with an oval or pear-shaped head and a long vibratile tail (fig. 24 k ). Each amoeboid cell produces a large number of spermatozoa, which occur in spherical clusters or sperm-balls. The heads of the spermatozoa, as in the Metazoa, are produced from the nucleus of the mother-cell, the tails from the surrounding protoplasm.

The development in detail is upon two plans. In Grantia raphanus (15 ) the nucleus of the mother-cell divides into two (fig. 24 b); one of the resulting daughter nuclei undergoes no further change, but with a small quantity of peripheral protoplasm forms a “cover-cell” to the other or primitive sperm nucleus and its associated protoplasm. The sperm nucleus repeatedly divides, with out involving the surrounding protoplasm (fig. 24 c - f ). The resulting nuclei at length cease to exhibit a nucleolus, and become directly transformed into the heads of spermatozoa; the tails are appropriated by each head from the common protoplasmic residue. The mothercell in this case undergoes no increase in volume as development proceeds, and it is not enclosed within an “endothelial” layer. In the second and apparently more usual case (20 ) no "cover-cell” is formed, but the mother-cell divides and subdivides, protoplasm as well as nuclei, till a vast number of minute cells results; the nucleus of each becomes the head of a spermatozoon and the protoplasm its tail. In this case the sperm-ball does increase in bulk: it grows as it develops, and the cavity containing it becomes lined by epithelium, or so-called “endothelium” (fig. 24 j ). No doubt (15 ) the development of the epithelium stands in direct physiological connexion with the growth of the sperm-ball.

Obscure as are the details of this subject, sufficient is known to enable us to make out two chief types of development. One, common amongst the calcareous sponges, and possibly occurring in a single genus (Gummina) of the Micromastictora , is characterized by what is known as the “amphiblastula” stage; the other, widely spread amongst the Micromastictora (Reniera, Dcsmacidon, Euspongia, Spongelia, Aplysilla, Oscarella), is characterized by a “planula” stage.

The first has been most thoroughly investigated in Grantia raphanus by Schulze (20 ). The ovum by repeated segmentation gives rise to a hollow vesicle, the wall of which is formed by a single layer of cells— blastosphere (fig. 25 d ). Eight cells at one pole of the blastosphere now become differentiated from the rest; they remain rounded in form, comparatively large, and become filled with granules (stored nutriment), while the others, rapidly multiplying by division, become small, clear, columnar, and flagellated. By further change the embryo becomes egg-shaped; the granular cells, now increased in number to thirty-two, form the broader end, and the numerous small flagellated cells the smaller end. Of the granular cells sixteen are arranged in an equatorial girdle adjoining the flagellate cells. A blastosphere thus differentiated into two halves composed of different cells is known as an amphiblastula. The amphiblastula (fig. 25 e) now perforates the maternal tissue, and is borne along an excurrent canal to the oscule, where it is discharged to the exterior and swims about in a whirling lively dance.

It then assumes a more spherical form, a change premonitory of the next most remarkable phase of its career. In this the flagellated layer becomes flattened, depressed, and finally invaginated within the hemisphere of granular cells, to the inner face of which it applies itself, thus entirely obliterating the cleavage cavity, but by the same process originating another (the invagination cavity) at its expense (fig. 25 f ). The two-layered sac thus produced is a paragastrula ; its outer layer, known as the epiblast, gives rise to the ectoderm, the inner layer or hypoblast to the endoderm. The paragastrula next becomes somewhat beehiveshaped, and the mouth of the paragastric cavity is diminished in size by an ingrowth of the granular cells around its margin. The larva now settles mouth downwards on some fixed object, and exchanges a free for a fixed and stationary existence (fig. 25 y ). The granular cells completely obliterate the original mouth, and grow along their outer edge over the surface of attachment in irregular pseudopodial processes, which secure the young sponge firmly to its seat (fig. 25 h ). The granular cells now become almost transparent, owing to the exhaustion of the yolk granules, and allow the hypoblast within to be readily seen; a layer of jelly-like material, the rudimentary mesoderm, is also to be discerned between the two layers. The spicules then become visible; slender oxeas appear first, and afterwards tri-and quadri-radiate spicules. The larva now elongates into a somewhat cylindrical form; the distal end flattens; and an oscule opens in its midst. Pores open in the walls; the endodermal cells, which had temporarily lost their flagella, reacquire them, at the same time extending the characteristic collar. In this stage (fig. 25 h, j) the young sponge corresponds to a true Ascon, no trace of radial tubes being visible; but as they characterize the parent sponge they must arise later, and thus we have clear evidence through ontogeny of the development of a Sycon sponge from an Ascon.

The three most striking features in the history of this larva are, first, the amphiblastula stage; next the invagination of the flagellate cells within the granular, instead of invagination in the reverse order; and third the attachment of the larva by the oral instead of the aboral surface. Should Schulze be correct in deriving the sponges from the Coelentera, it is probable that the reversal of the

Coelenterate history as exemplified in the last two events will furnish an explanation of the remarkable divergencies which distinguish the two phyla. The history of the second or planula type has been thoroughly worked out by Schulze (20 ) in a little incrusting Tetrac-tinellid sponge (Plakina monolopha, Schulze). The ovum by regular segmentation produces a blastosphere, the blastomeres of winch increase in number by further subdivision till they become converted into hyaline cylindrical flagellated cells (fig. 26 f ). Thus a blastosphere is produced consisting wholly of similar flagellated cells. It becomes egg-shaped, and, hitherto colourless, assumes a rose-red tint, which is deepest over the smaller end. The larva (now a planula, fig. 26 a, by the filling in of the central cavity) escapes from the parent and swims about broad end foremost. In this stage thin sections show that the cleavage cavity is obliterated, its place being occupied by a mass of granular gelatinous material containing nuclei (fig. 26 b ). In from one to three days after hatching the larva becomes attached. It then spreads out into a convex mass, and a cavity is produced within it by the splitting of the central jelly (fig. 26 c , d ; compare Eucope and others amongst the Coelenterates). This cavity becomes lined by short cylindrical cells (endoderm), while the flagellated cells of the exterior lose their flagella and become converted into pinnacocytes (ectoderm). The gelatinous material left between the two layers now formed acquires the characters of true Collenchyme and thus becomes the mesoderm. The endoderm then sends off into the mesoderm, as buds, rounded chambers, which communicate with the paragastric cavity by a wide mouth and with the exterior by small pores (fig. 26 e ). An oscule is formed later, and the sponge enters upon the Rhagon phase. Subsequent foldings of the sponge-wall give rise to a very simple canal system (fig. 26 f ). In addition to these two well-ascertained modes of development others have been described which at present appear aberrant. In Oscarella lobularis, O.S. (27), a curious series of early developmental changes results in the formation of an irregular paragastrula, the walls of which become folded (while still within the parent sponge) in a complex fashion, so as to produce a form in which the incurrent and excurrent canals appear to be already sketched out before the flagellated chambers are differentiated off. In Spongilla Götte describes the ectoderm as becoming entirely lost on the attachment of the larva, so that the future sponge proceeds from the endoderm alone. As Spongilla, however, is a freshwater form, anomalies in its development (which remind us of those in the development of the freshwater Hydra) might almost be expected.

Probably in no other single group is the doctrine of homoplasy enunciated by Lankester more tellingly illustrated than in the sponges. The independent development of similar types of canal system in different groups, sometimes within the limits of a single family, is a remarkable fact. In the following table the sign × shows independent evolution of similar types of canal system in different groups :— [table]

In the gross anatomy of the canal system similar homoplasy obtains; thus, to cite one case amongst many, a peculiar type of canal system characteristic of Siphonia (Lithistid) occurs also in Emploca (Hexactinellid), Schmidtia (Monaxonid), and other apparently unrelated genera. The development of a cortex has likewise taken place independently, but on parallel lines, in the Syconidae, Leu-conidae, Monaxona, Tetillidae, and Stellettidae. Calcareous and silicious spicules have evidently an independent history, and yet all the chief forms of the former are repeated in the latter. Quite as remarkable is the similarity of the independently evolved horny spicules of Darwinella aurea to the quadri-and sex-radiate silicious spicules. We have now sufficient knowledge of the morphology and evolution of the sponge to furnish the physicist with data for an explanation of the skeleton, at least in its main outlines. The obvious conclusion from this is that variation does not depend upon accident, but on the operation of physical laws as mechanical in their action here as in the mineral world. Another important consequence follows: if homoplasy— i.e., the independent evolution of similar structures —is of such certain and quite common occurrence in the case of the sponges, it is also to be looked for in other groups, and polyphylitic origin, so far from being improbable, is as likely an occurrence as monophylitic origin.

Physiology and Aetiology.

Under the head of “physiology” we have almost a blank. At present we do not even know what cells of the sponge are primarily concerned in the ingestion of food. If a living sponge, such as Spongilla, be fed with carmine for a few minutes, then immersed in dilute osmic acid, and examined in thin sections, its flagellated chambers are found to be all marked out as red circular patches, and a closer investigation shows that the choanocytes, and they alone, have ingested the carmine. In this way we confirm the earlier observations of Carter made by teasing carmine-fed sponges. This might be thought to decide the question; but, though it effectually disposes of Polejaeff's argument that the choanocytes do not ingest nutriment because mechanical disadvantages (conceived a priori) make it impossible, it has not proved a final solution. Von Lendenfeld, by feeding sponges such as Aplysilla with carmine for a longer interval—a quarter of an hour—finds that amoeboid cells crowd about the sides and particularly the floor of the subdermal cavities, and are soon loaded with carmine granules; after a time they wander away to the flagellated chambers and there cast out into the excurrent canals the carmine they have absorbed, apparently in an altered state. On the other hand, the choanocytes, though they at first absorb the carmine, soon thrust it out, apparently in an unaltered state. Hence Von Lendenfeld concludes that it is the epithelium of the subdermal cavities which is charged with the function of ingestion, and that the amoeboid cells subsequently digest and distribute it, and finally cast out the worthless residues. There may be much truth in this view, but it requires to be supported by further evidence. (1) Sufficient proof is not adduced to show that the carmine granules expelled from the amoeboid cells are really more decomposed than those rejected by the choanocytes. (2) There is at present no proof that carmine is a food, or that if it is sponges will readily feed upon it. In either case one would expect the amoeboid cells to play the part which they perform in other organisms and to remove as soon as possible useless or irritant matter from the surface which it encumbers; at the same time the choanocytes, not having found the food to their liking, would naturally eject it. (3) If the choanocytes do not ingest food, how does the Ascon feed, since in this sponge all the pinnacocytes are external? It is, however, a very noticeable fact that, as the organization of a sponge increases in complexity, the choanocytal layers become reduced in volume relative to the whole bulk of the individual; and it is quite possible that as histological differentiation proceeds it may be accompanied by physiological differentiation which relieves the choanocytes to some extent of the ingestive part of their labours.

The origin of the sponges is to be sought for among the choanoflagellate Infusoria ; and Savile Kent has described a colonial form of this group which is suggestively similar to a sponge. Its differences, however, are as marked as its resemblances, and have been sufficiently pointed out by Schulze (23 ). Kent has called this form Protospongia, a name already made use of, and fortunately, as the organism is not in any sense a true sponge; the present writer proposes, therefore, to call it Savillia, in honour of its discoverer. It consists of choanoflagellate Infusoria (see Protozoa, vol. xix. p. 858, fig. XXI., 15), half projecting from and half embedded in a structureless jelly or blastema, within which other cells of an amoeboid character and reproductive function are immersed. Professor Haddon arrives at the generalization that conjugation amongst the Protozoa always takes place between individuals of the same order: flagellate cells conjugate with flagellate, amoeboid with amoeboid, but never with flagellate; while in true sexual reproduction the conjugation occurs between two individual cells in different stages of their life cycle: a flagellate cell conjugates with a resting amoeboid cell. Now Savillia would appear to be extremely near such a true sexual process, since the simultaneous coexistence of cells in two different stages of life and within easy reach of each other—a necessary preliminary, one would think, to the union—has already been brought about. That coalescence between two different histological elements should result in products similarly histologically differentiated (compare amphiblastula stage of Calcarea) has in it a certain fitness, which, however, has still to be explained. The mode by which an organism like Savillia might become transformed into an Ascon cannot be suggestively outlined with any satisfactory results till our knowledge of the embryology of sponges is more advanced. The minute characters of the flagellate cells of the amphiblastula and other sponge larvae are still a subject for research. They often possess a neck or collum; but the existence of a frill or collar is disputed. Kent asserts that it is present in several embryos which he figures; and Barrois makes the same assertion in respect to the larva of Oscarella, and illustrates his description with a figure. On the other hand, Schulze and Marshall both deny its existence, and the former attributes Kent’s observations to error. One constant character they do possess: they are provided with flagella at some stage of their existence, but never with cilia. Ciliated cells, indeed, are unknown amongst the sponges, and, when pinnacocytes exceptionally acquire vibratile filaments, as in Oscarella and other sponges, these are invariably flagella, never cilia. An Ascon stage having been reached at some point in the history of the sponges, the Sycon tubes and Rhagon chambers would arise from it by the active proliferation of choanocytes about regularly distributed centres, possibly as a result of generous feeding. Vosmaer recognized as the physiological cause of Sycon an extension of the choanocytal layer. Polejaeff, relying on Von Lendenfeld’s experiments, which seem to prove that it is the pinnacocytes and not the choanocytes which are concerned in the ingestion of nutriment, argues that, as in Sycon the pinnacocytal layer is increased relatively to the choanocytal, we have in this a true explanation of the transition. The existence of Homoderma, Lfd., however, shows that in the first stage there was not a replacement of choanocytes by pinnacocytes, but that this was a secondary change, following the development of radial tubes, and therefore cannot be relied upon to explain them. The radial tubes having been formed by a proliferation of choanocytal cells, the reduction of those lining the paragastric cavity to pinnacocytes would follow in consequence of the poisonous character of the water delivered from the radial tubes to the central cavity, since this water not only parts with its dissolved oxygen to the choanocytes it first encounters, but receives from them in exchange urea, carbonic acid, and faecal residues. The development of subdermal cavities is explicable on Von Lendenfeld’s hypothesis.

Distribution.

Our knowledge of this subject is at present but fragmentary; we await fuller information in the remaining reports on the sponges obtained by the “Challenger.” The sponges are widely distributed through existing seas, and freshwater forms are found in the rivers and lakes of all continents except Australia, and in numerous islands, including New Zealand. Many genera and several species are cosmopolitan, and so are most orders.

As instances of the same species occurring in widely remote localities we take the following from Polejaeff :— Sycon arcticum is found at the Bermudas and in the Philippine Islands, as also are Lcuconia multiformis and Lcucilla, uter; Sycon raphanus occurs at Tristan da Cunha aud the Philippines; Heteropegma nodus-gordii and Lcuconia dura at the Bermudas and Torres Straits. We do not know, however, whether these species are isolated in their distribution or connected by intermediate localities. Of the Calcarea about eighty-one species have been obtained from the Atlantic, twenty-two from the Pacific, and twenty-two from the Indian Ocean; but these numbers no doubt depend largely on the extent to which the several oceans have been investigated, for the largest number of species has been found in the ocean nearest home. Schulze states that the Hexactinellida brought home by the “Challenger” were obtained at seventeen Atlantic stations, twenty-seven Pacific, and nineteen in the South Seas. In the last the number of species was greatest, in the Atlantic least. They flourish best on a bottom of diatomaceous mud. The Calcarea and Ceratosa are most abundant in shallow water and down to 40 fathoms, but they descend to from 400 to 450 fathoms. The Hexactinellida are most numerous over continental depths, i.e ., 100 to 200 fathoms; but they extend downwards to over 2500 fathoms and upwards into shallow water (10 to 20 fathoms). The Lithistida are not such deep-water forms as the Hexactinellida, being most numerous from 10 to 150 fathoms. Only one or two species have been dredged from depths greater than 400 fathoms, and none from 1000 fathoms. The Choristida range from shallow water to abyssal depths. A characteristic deep-sea Choristid genus is Thenea, Gray (= Wyville Thompsonia, Wright; Dorvillia, Kent). This is most frequently dredged from depths of from 1000 to 2000 fathoms; but it extends to 2700 fathoms on the one hand and to 100 on the other.

Until about 1876 one of the chief obstacles to the inter pretation of fossil sponges arose from a singular mineral replacement which most of them have undergone, leading to the substitution of calcite for the silica of which their skeletons were originally composed. This change was demonstrated by Zittel (35 ) and Sollas (24 ), and, though it was at first pronounced impossible, owing to objections founded on the chemical nature of silica, it has since become generally recognized. These observers also showed that the fossil sponges do not belong to extinct types, but are assignable to existing orders. Zittel in addition subjected large collections to a careful analysis and marshalled them into order with remarkable success. Since then several palaeontologists have worked at the subject,—Pocta, Dunikowski, and Hinde (7 ), who has published a Catalogue— which is much more than a catalogue—of the sponges preserved in the British Museum. The result of their labours is in general terms as follows. Fossil sponges are chiefly such as from the coarseness or consistency of their skeletons would be capable of preservation in a mineralized state. Thus the majority are Hexactinellida, chiefly Dictyonina; Tetractinellida, chiefly Lithistida; and Calcarea, chiefly Leuconaria. Monaxonid sponges rarely occur; the most ancient is Climacospongia, Hinde, found in Silurian rocks. A very common Halichondroid sponge of this group (Pharetrospongia strahani, Soll.) occurs in the Cambridge greensand; it owes its preservation to the collection of its small oxeate spicules into dense fibres. The Choristida, though not so common as the Lithistids, are commoner than the Monaxonids, particularly in Mesozoic strata.

The distribution of fossil sponges in the stratified systems may be summarized as follows. Calcarea.— Homocoelα, none. Heterocoela, a Syconid, in the Jurassic system. Numerous Leuconaria from the Devonian upwards. Myxospongiae. —None; not fitted for preservation. Hexactinellida.— Lyssacina, from the Lower Cambrian upwards. Dictyonina, commencing in the Silurian; most numerous in the Mesozoic group; still existing. Monaxonida.— Monaxona, from the Silurian upwards. Ceratosa, none; few are fitted for preservation. Tetractinellida.— Choristida, from the Carboniferous upwards; most numerous in the Cretaceous system. Lithistida, from the Silurian upwards; most numerous in the Mesozoic group. In ancient times the Hexactinellids and Lithistids seem not to have been so comparatively uncommon in shallow water as they are at the present day. Thus, in the Lower Jurassic strata of the south-west of England we find Dictyonine Hexactinellids, Lithistids, and Leuconarian Calcarea associated together in a shelly breccia and in company with littoral shells, such as Patella and Trochus. Several Palaeozoic Hexactinellids actually occur in a fine-grained sandstone. Of the Chalk, which is the great mine of fossil sponges, we must speak with caution, owing to the insufficient evidence as to the depth at which it was deposited.

As shown by Protospongia, the phylum of the sponges was in existence in very early Cambrian times, and probably much earlier. Before the end of the Silurian period its main branches had spread themselves out, and, developing fresh shoots since then, they have extended to the present day. Of the offshoots none of higher value than families are known to have become extinct, and of these decayed branches there are very few. The existence in modern seas of the Asconidae, which must surely have branched off very near the base of the stem, is another curious instance of the persistence of simple types, which would thus appear not to be so vastly worse off in the struggle for existence than their more highly organized descendants.

Bibliography.— A fairly complete list of works on sponges published before 1SS2 will be found in Vosmaer’s article “Poriferae,” in Bronn’s Klassen und Ordnungen, vol. ii. D’Arcy Thompson’s Catalogue of Papers on Protozoa and Coelenterata, a still more complete list, extends to 1SS4.

The following is a list of works, including those referred to in the preceding pages :—(1 ) C. Barrois, Embryologie d. quelques Éponges d. l. Manche, Paris, 1876. (3 ) Bowerbank, A Monograph of British Spongiadae, vols, i.-iv., 1864-82 (vol. iv. is posthumous, edited by Dr Norman). (3 ) Carter, a series of papers in the Ann. and Mag. Nat. Hist., from 1847 to the present time (1887). (4 ) J. Clark, On the Spongiae ciliatae as Infusoria flagellata, 1865. (5 ) Grant, Edin. Phil. Journ., 1825. (6 ) Haeckel, Monographie d. Kalkschwämme, 1871. (7 ) Hinde, A Catalogue of the Sponges in the British Museum, 18S3. (8) Id., “On the Beceptaculitιdae, in Quart. Journ. Geol. Soc., xl. 795, 1884. (9 ) Keller, “Studien ii. Organisation u. Entwickelung d. Chalineen,” in Ztschr. f. wiss. Zool., xxxiii., 1879. (10 ) Kent, “Notes on the Embryology of the Sponges,” in Ann. and Mag. Nat. Hist., 1878, ii. 139. (11 ) Von Lendenfeld, “On Aplysillid«." in Ztschr. f. wiss. Zool., xxxviii. (12 ) Id., “A Monograph of Australian Sponges,” in Proc. Linn. Soc., N.S. IVales, vois. ix., x. (other papers by Von Lendenfeld will be found under this reference, and also in the Zool. Anzeiger). (13) Lieberkühn, Developmental History of Spongilla," in Müll. Archiv, 1856. (14 ) Marshall, Genaische Ztschr., xviii., 1885 (translated in Ann. and Mag. Nat. Hist.). (15) Polejaeff, On Sperma and Spermatogenesis in Sycandra raphanus," in Sitz.-Ber. Acad. wiss. Zool., Inst. d. Universität Graz. (16) Id., “Challenger” Report on the Calcarea, 1883. (17 ) Id., Ditto on the Ceratosa, 1884. (18 ) Ridley, on the Zool. Collection of the “Alert,” 1884. (19 ) Schmidt, Sponges of the Adriatic Sea, 1862, with Supplement 1 in 1864, and Supplement 2 in 1866; Sponges of the Coast of Algiers, 1868; Sponge-Fauna of the Atlantic, 1870; Sponges of the Gulf of Mexico, 1879. (20) F. E. Schulze, investigations into the structure and development of sponges, in Ztschr. f. wiss. Zool.,—“ Οn Halisarca," vol. xxviii., 1877; “On Chondrosidse," xxix., 1877; “Οn Aplysinidie," xxx., 1878; “On Metamorphosis of Sycandra raphanus," xxxi., 1878; “On Spongelia," xxxii., 1878; “On Spongidae," ib.; “On Hircinia and Oligoceras," xxxiii., 1879; “On Plakinidae," xxxiv., 1880; “On Corticium candelabrum," xxxv., 1881. (21) Id., “On Soft Parts of Euplectella aspergillum," in Trans. Roy. Soc. Edin., xxix., 1880. (22) Id., Preliminary Report on the “Challenger" Hexactinellida. (23) Id., “On the Relationship of the Sponges to the Choanofiagellatu," in Sitz.-Ber. d. k.-preuss. Akad. f. wiss. Z., Berlin, 1885, translated in Ann. and Mag. Nat. Hist., 1885. (24) Sollas, “On Stauronema," in Ann. and Mag. Nat. Hist., xix., 1877; “On Pharetrospongia," in Quart. Journ. Geol. Soc., xxxiii., 1877; “On Siphonia," ib.∙, “On Protospongia," ib., xxxvi., 1880. (25) Id., “The Sponge-Fauna of

Norway,” in Ann. and Mag. Nat. Hist., 1880-82. (26) Id., “The Flint-Nodules of the Trimmingham Chalk,” ib., vi., 1879. (27 ) Id., “Development of Halisarca lobularis," in Quart. Journ. Micr. Sci., xxiv., 1884. (28) Id., “On Velulina and the Anomocladina," in Proc. R. Irish Acad., iv., 1885. (29 ) Id., “Physical

Characters of Sponge-Spicules,” in Proc. R. Dub. Soc., 1885. (30 ) Vejdovsky,

“The Freshwater Sponges of Bohemia,” in Abh. d. k. Böhm. Akad. d. Wiss., ser. vi., vol. xii., 1883. (31) Vosmaer, On Leucandra aspera (doctor’s diss., Leyden, 188O). (32) Id., “On the Desmacidinidae," in Notes from the Leyden Museum, vol. ii. (33) Sponges of the Willem Barents Expedition, 1884. (34 ) “Porifera,” in Bronn’s Klassen und Ordnungen, vol. ii., 1882, and still in progress. (35) Zittel, studies of fossil sponges, in Abh. d. k. bayer. Akad.,—Hexactinellida, 1877; Lithistida, 1878; Monactinellida and Calcarea, 1878.

Commerce.

When the living matter is removed from a Ceratose sponge a network of elastic horny fibres, the skeleton of the animal, remains behind. This is the sponge of commerce. Of such sponges the softest, finest in texture, and most valued is the Turkey or Levant sponge, Euspongia officinalis, Lin. The other two varieties are the Hippospongia equina, O. Schmidt, and the Zimocca sponge, Euspongia Zimocca, O.S., which is not so soft as the others (see p. 423 above). All three species are found at from 2 to 100 fathoms along the whole Mediterranean coast, including its bays, gulfs, and islands, except the western half of its northern shores as far as Venice and the Balearic Isles, Corsica, Sardinia, and Sicily. Bath sponges occur around the shores of the Bahamas, and less abundantly on the north coast of Cuba. They are of several kinds, one not distinguishable from the fine Levant sponge; others, the “yellow” and “hardhead” varieties, resemble the Zimocca sponge; and of horse sponges there appear to be several varieties, such as the “lamb’s-wool” and the “velvet” sponge (Hippospongia gossypina and H. meandriformis). The fine bath sponge occurs on the shores of Australia (Torres Straits, the west coast, and Port Phillip on the south coast). A sponge eminently adapted for bathing purposes (Coscinoderma lanuginosum, Crtr.; Euspongia mathewsii, Lfd.), but not yet brought into the market, occurs about the South Caroline Islands, where it is actually in use, and at Port Phillip in Australia. The fine bath sponge occurs in the North Pacific, South Atlantic, and Indian Oceans, so that its distribution is world-wide.

The methods employed to get sponges from the bottom of the sea, where they grow attached to rocks, stones, and other objects, depend on the depths from which they are to be brought. In comparatively shallow water they may be loosened and hooked up by a harpoon; at greater depths, down to 30 or 40 fathoms, they are dived for; and at depths of from 50 to 100 fathoms they are dredged with a net. The method of harpooning was the earliest practised, and is still carried on in probably its most primitive form by the Dalmatian fishermen. Small boats are used, manned by a single harpooner with a boy to steer; when, however, the expedition is to extend over night the crew is doubled. The harpoon is a five-pronged fork with a long wooden handle, and if this is not long enough another harpoon is lashed on to it. The Greek fishers use a large boat furnished with two or three smaller ones, from which the actual harpooning is carried on; the crew numbers seven or eight. One of the chief difficulties is to see the bottom distinctly through a troubled surface. The Dalmatian fishers throw a smooth stone dipped in oil a yard or so in front of the boat; the stone scatters drops of oil as it flies and so makes a smooth track for the “lookout.” The Greeks use a zinc-plate cylinder about 1 ½ feet long and 1 foot wide, closed at the lower end by a plate of glass, which is immersed below the surface of the sea; on looking through this the bottom may be clearly seen even in 30 fathoms. This plan is also adopted in the Bahamas, where harpooning carried on after the Greek system gives employment to over 5000 men and boys.

The primitive method of diving with no other apparatus than a slab of stone to serve as a sinker and a cord to communicate with the surface is still practised in the Mediterranean. The diver carries a net round his neck to hold the sponges. On reaching the bottom he hastily snatches up whatever sponge he sees. After staying down as long as he is able—an interval which varies from two to at the most three minutes—he tugs violently at the cord and is rapidly drawn up. On entering the boat from depths of 25 fathoms he quickly recovers from the effects of his plunge after a few powerful respirations; but after working at depths of 30 to 40 fathoms or more he reaches the surface in a swooning state. At the beginning of the season blood usually flows from the mouth and nose after a descent; this is regarded as a symptom of good condition; should it be wanting the diver will scarcely venture a second plunge for the rest of the season. The work is severe, and frequently the diver returns empty-handed to the boat. Diving is usually carried on in the summer months; in winter it is too cold, at all events without a diving-dress. The ordinary diver’s dress with pumping apparatus is largely used by the Greeks. The diving is carried on from a ship manned by eight or nine men, including one, or rarely two, divers. At a depth of from 10 to 15 fathoms the diver can remain under for an hour, at greater depths up to 20 fathoms only a few minutes; the consequences of a longer stay are palsy of the lower extremities, stricture, and other complaints. Dredging is chiefly carried on along the west coast of Asia Minor, principally in winter after the autumn storms have torn up the seaweeds covering the bottom. The mouth of the dredge is 6 yards wide and 1 yard high; the net is made of camel-hair cords of the thickness of a finger, with meshes 4 inches square. It is drawn along the bottom by a tow-line attached to the bowsprit of a sailing vessel or hauled in from the shore.

Prompted by a suggestion made by Oscar Schmidt, that sponges might be artificially propagated from cuttings, the Italian Government supplied funds for experiments to determine the feasibility of cultivating sponges as an industrial pursuit. A station was established on the island of Lesina, off the Dalmatian coast, and experiments were carried on there for six years (1867-72) under the superintendence of Von Buccich. The results were on the whole successful, but all expectations of creating a new source of income for the sponge-fishers of Dalmatia were defeated by the hostility of the fishers themselves.

The details of the method of sponge-farming as practised by Von Buccich are briefly as follows. The selected specimens, which should be obtained in as uninjured a state as possible, are placed on a board moistened with sea water and cut with a knife or fine saw into pieces about 1 inch square, care being taken to preserve the outer skin as intact as possible. The operation is best performed in winter, as exposure to the air is then far less fatal than in summer. The sponge cuttings are then trepanned and skewered on bamboo rods; the rods, each bearing three cuttings, are secured in an upright position between two parallel boards, which are then sunk to the bottom of the sea and weighted with stones. In choosing a spot for the sponge-farm the mouths of rivers and proximity to submarine springs must be avoided; mud in this case, as in that of reef-building corals, is fatal. A favourable situation is a sheltered bay with a rocky bottom overgrown with green seaweed and freshened by gentle waves and currents. So favoured, the cuttings grow to a sponge two or three times their original size in one year, and at the end of five to seven years are large enough for the market. Similar experiments with similar results have more recently been carried on in Florida. The chief drawback to successful sponge-farming would appear to be the long interval which the cultivator has to wait for his first crop.

After the sponge has been taken from the sea, it is exposed to the air till signs of decomposition set in, and then without delay either beaten with a thick stick or trodden by the feet in a stream of flowing water till the skin and other soft tissues are completely removed. If this process is postponed for only a few hours after the sponge has been exposed a whole day to the air it is almost impossible to completely purify it. After cleaning it is hung up in the air to dry, and then with others finally pressed into bales. If not completely dried before packing the sponges “heat,” orange yellow spots appearing on the parts attacked. The only remedy for this is to unpack the bale and remove the affected sponges. The orangecoloured spots produced by this “pest,” or “cholera” as the Levant fishermen term it, must not be confounded with the brownish red colour which many sponges naturally possess, especially near their base. The sponges on reaching the wholesale houses are cut to a symmetrical shape and further cleaned. The light-coloured sponges often seen in chemists’ shops have been bleached by chemical means which impair their durability. Sponges are sold by weight; sand is used as an “adulteration.”

It is difficult to obtain recent statistics as to the extent of the sponge trade; the following table gives a summary of the sponges sold in Trieste, the great European sponge market, in the year 1871 :— [table]

(w. j. s. )