Chapter IX: Introduction: History—description of Halichondria Panicea as an Example of (2)

The structure of the body-wall in Hexactinellida is so constant as to make it possible to give a general description applicable to all members of the group. It is of considerable thickness, but a large part is occupied by empty spaces, for the actual tissue is present in minimum quantity. In the wall the chamber-layer is suspended by trabeculae of soft tissue, between a dermal membrane on the outside and a similar gastral membrane on the inner side (Fig. 89). Thus the water entering the chambers through their numerous pores has first passed through the ostia in the dermal membrane and traversed the subdermal trabecular space; on leaving the chambers it flows through the subgastral trabecular space and the ostia in the gastral membrane, to enter the paragaster and leave the body at the osculum. The trabeculae and the dermal and gastral membranes together constitute the dermal layer. This conclusion is based on comparison with adults of the other groups, for in the absence of embryological knowledge no direct evidence is available. According to {199}the Japanese investigator, Isao Ijima,[234] the dermal and gastral membranes are but expansions of the trabeculae, and the trabeculae themselves are entirely cellular, containing none of the gelatinous basis met with in the dermal layer of all other sponges. There is no surface layer of pinacocytes, the cells forming the trabeculae being all of one type, namely, irregularly branching cells, connected with one another by their branches to form a syncytium. In the trabeculae are found scleroblasts and archaeocytes.

The chambers have a characteristic shape: they are variously described as "thimble-shaped," "tubular," or "Syconate," and they open by wide mouths into the subgastral trabecular space. Their walls have been named the _membrana reticularis_ from the fact that, when preserved with only ordinary precautions, they are seen as a regular network of protoplasmic strands, with square meshes and nuclei at the nodes. This appearance recently found an explanation when Schulze, for the first time, succeeded in preserving the collared cells of Hexactinellids.[235] Schulze was then able to show that the choanocytes are not in contact with one another at their bases, where the nuclei are situated, but communicate with one another by stout protoplasmic strands. The form of the choanocyte can be seen in Fig. 91.

To Schulze's description of the chamber, Ijima has added the important contributions that every mesh in the reticulum functions as a chamber pore or prosopyle; and that porocytes, such as are found in Calcarea, are wanting. This structure of the chamber-walls, the absence of gelatinous basis in the dermal layer, and the slight degree of histological differentiation in {200}the same layer, added to the more obvious character of thimble-shaped chambers, are the chief archaic features of Hexactinellid morphology.

The skeleton which supports the soft parts is, like them, simple and constant in its main features. It is secreted by scleroblasts, which lie in the trabeculae, and is made up of only one kind of spicule and its modifications. This is the hexactine, a spicule which possesses six rays disposed along three rectangular axes. Each ray contains an axial thread, which meets its fellow at the centre of the spicule, where they together form the axial cross. Modifications of the hexactine arise either by reduction or branching, by spinulation or expansion of one or more of the rays. The forms of spicule arising by reduction are termed pentactines, tetractines, and so on, according to the number of the remaining rays. Those rays which are suppressed leave the proximal portion of their axial thread as a remnant marking their former position (Fig. 94). Octactine spicules seem to form an exception to the above statements, but Schulze has shown that they too are but modifications of the hexactine arising by (1) branching of the rays of a hexactine, followed by (2) recombination of the secondary rays (Fig. 92).

The various spicules are named, irrespective of their form, according to their position and corresponding function. The {201}arrangement of the spicules is best realised by means of a diagram (Fig. 93).

_, microscleres in the dermal membrane; _D. Hypoderm[alia]_, more deeply situated dermalia; _Dictyonalia_, parenchymalia which become fused to form the skeletal framework of Dictyonina; _Gastralia Autogastr[alia]_, microscleres in the gastral membrane; _Gastralia Hypogastr[alia]_, more deeply situated gastralia; _Parenchymalia Principalia_, main supporting spicules between the chambers; _P. Comitalia_, slender diactine or triactine spicules accompanying the last; _P. Intermedia_, microscleres between the P. principalia; _Prostalia_, projecting spicules; _P. basalia_, rooting spicules, from the base; _P. marginalia_, defensive spicules, round the oscular rim; _P. pleuralia_, defensive spicules, from the sides. (From Delage and Hérouard, after F. E. Schulze.)]

The deviations from this ground-plan of Hexactinellid structure are few and simple. They are due to folding of the chamber-layer, or to variations in the shape of the chambers, and to increasing fusion of the spicules to form rigid skeletons. A simple condition of the chamber-layer, like that of the young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids, _e.g._ in _Walteria_ of the Pacific Ocean (Fig. 90). Thus is represented in this order the second type of canal system described among Calcarea. More frequently, however, instead of forming a smooth sheet, the chamber-layer grows out into a number of tubular diverticula, the cavities of which are excurrent canals; these determine a corresponding number of incurrent canals which lie between them. In this way there arises a canal system resembling the third type of Calcarea. By still further pouching so as to give secondary diverticula, opening into the first, a complicated canal system is formed, as, for example, in _Euplectella suberea_.

To return to the skeleton, the most complete fusion is attained by the deposit of a continuous sheath of silica round the apposed parallel rays of neighbouring spicules. This may be termed the dictyonine type of union, for it occurs in all those forms originally included under the term Dictyonina, in which the cement is deposited _pari passu_ with the formation of the spicules. In other cases connecting bridges of silica unite the spicules, or there may be a connecting reticulum of siliceous threads, or, again, rays crossing obliquely may be soldered together at the point of contact. These more irregular methods occur in species where the spicules are free at their first formation. Spicules originally free may later be united in a true Dictyonine fashion. The terms LYSSACINA and DICTYONINA are useful to denote respectively: the former all those Hexactinellida in which the spicules are free at their first formation, and the latter those in which the deposit of the cementing layer goes hand in hand with the formation of the spicules. But the terms do not indicate separateness of origin of the groups denoted by them, for there is evidence that Dictyonine types have been derived repeatedly from Lyssacine types, and that in fact every Dictyonine was once a Lyssacine.

The real or natural cleft in the class lies between those genera possessing amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder of the Hexactinellida which bear hexasters (Fig. {203}96). The former set of genera constitute the sub-class Amphidiscophora, the latter the Hexasterophora.

SUB-CLASS 1. AMPHIDISCOPHORA.—_Amphidiscs are present, hexasters absent. A tuft of rooting spicules or basalia is always present. The ciliated chambers deviate more or less from the typical thimble shape, and the membrana reticularis is continuous from chamber to chamber_ (Figs. 94, 95, 97).

SUB-CLASS 2. HEXASTEROPHORA.—_Hexasters are present, amphidiscs absent. The chambers have the typical regular form, and are sharply marked off from one another_ (Figs. 90, 96).

All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora both types of skeleton occur. The subdivision of the Hexasterophora is determined by the presence or absence of uncinate spicules. An "uncinatum" is a diactine spicule, pointed at both ends and bearing barbs all directed towards one end. This method of classification gives us a wholly Dictyonine order, UNCINATARIA, and an order consisting partly of Dictyonine, partly of Lyssacine genera, which may be distinguished as the ANUNCINATARIA. {204}Ova have rarely been found, and sexually produced larvae never; but Ijima has found archaeocyte clusters in abundance, and his evidence is in favour of the view that they give rise asexually to larvae, described by him in this class for the first time (see p. 231).

Both sub-classes are represented in British waters: the Amphidiscophora by _Hyalonema thomsoni_ and _Pheronema carpenteri_; the Hexasterophora by _Euplectella suberea_ and _Asconema setubalense_, and of course possibly by others.

_Hyalonema thomsoni_, one of the glass-rope sponges, was dredged by the _Porcupine_ off the Shetland Islands in water of about 550 fathoms. The spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting tuft is continued right up its axis, to end in a conical projection, which is surrounded by four apertures leading into corresponding compartments of the paragaster.

The crust of Anthozoa of the genus _Epizoanthus_ (p. 406) on the rooting tuft is a constant feature in this as in other species of _Hyalonema_. It contributed to make the sponge a puzzle, which long defied interpretation. The earliest diagnosis the genus received was the "Glass Plant." Then the root tuft was thought to be part of the _Epizoanthus_, which was termed a "most aberrant Alcyonarian with its base inserted in a sponge"; next we hear of the sponge as parasitic {205}on the Sea Anemone. Finally, the root tuft was shown to be proper to the sponge, which was, however, figured upside down, till some Japanese collectors described the natural position, or that in which they were accustomed to find it.

_Pheronema carpenteri_ was found by the _Lightning_ off the north of Scotland in 530 fathoms. The goblet shaped, thick walled body and broad, ill-defined root tuft are shown in Fig. 98, but no figure can do justice to the lustre of its luxuriant prostalia and delicate dermal network with stellate knots at regular intervals. The basalia are two-pronged and anchor-like.

Both the Hexasterophoran genera were dredged off the north of Scotland, and both conform to the Lyssacine type without uncinates. _Euplectella suberea_ is a straight, erect tube, anchored by a tuft {206}of basalia. The upper end of the tube is closed by a sieve plate, the perforations in which are oscula, while the beams contain flagellated chambers, so that the sieve is simply a modified portion of the wall. It is a peculiarity of this as of one or two other allied genera that the lateral walls are perforated by oscula. They are termed parietal gaps, and are regularly arranged along spiral lines encircling the body.

Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that in young specimens oscula are confined to the sieve plate; parietal gaps are secondary formations. The groundwork of the skeleton is a lattice similar to that shown in Fig. 100. The chamber-layer is much folded. Various foreign species of _Euplectella_ afford interesting examples of association with a Decapod Crustacean, _Spongicola venusta_, of which a pair lives in the paragaster of each specimen. The Crustacean is light pink, the female distinguished by a green ovary, which can be seen through the transparent tissues. It is not altogether clear what the prisoner gains, nor what fee, if any, the host exacts.

Ijima relates that the skeleton of _Euplectella_ is in great demand in Japan for marriage ceremonies. He also informs us that the Japanese name means "Together unto old age and unto the same grave," while by a slight alteration it becomes "Lobsters in the same cell," and remarks that the Japanese find this an amusing pun.

The same _Spongicola_ lives in pairs in _Hyalonema sieboldi_. Another case of apparently constant association is that of the Hydroid stocks which inhabit _Walteria_. F. E. Schulze describes _Stephanoscyphus mirabilis_ (see p. 318) in a specimen of _Walteria flemmingi_; the presence of the polyp causes the sponge to grow out into little dome-shaped elevations, each of which shelters one polyp; while in _W. leuckarti_ Ijima finds a similar association in every specimen examined.

{207}FOSSIL HEXACTINELLIDA.

This group has the distinction of including among its Lyssacine members the oldest known sponge, _Protospongia fenestrata_, of Cambrian age (Salter). As preserved it consists of a single layer of quadriradiate, or possibly quinqueradiate spicules, which, arranged as a square meshed lattice, supported the superficial layer of the sponge (Fig. 101). Whether or not the fossil represents the whole of the sponge-skeleton does not appear.[236]

The extraordinary RECEPTACULITIDAE are probably early Lyssacine forms: they are cup- or saucer-shaped fossils, abundant in Silurian and above all in Devonian strata, and have been "assigned in turn to pine cones, Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde[237] brings forward important arguments for retaining them among Hexactinellida. The only elements in the skeleton of the simpler genera, _e.g._ _Ischadites_, are structures comparable to Hexactinellid spicules. The surface of the fossil presents a series of lozenges forming a regular mosaic. Each lozenge is the expanded end of one of the rays of a spicule; it conceals four rays in one plane, tangential to the wall of the cup-shaped fossil, while the sixth ray projects vertically to the wall into the cavity of the cup. In the genus _Receptaculites_ itself there is an inner layer of plates abutting against the inner {208}ends of the sixth rays, and at present problematic. An axial canal is present in each of the rays—the six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current.

The beautiful _Ventriculites_, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of the group were known. In common with a number of fossil Dictyonine species they are distinguished by the perforation of the nodes, a character due to the fact that the siliceous investment which unites the spicules together stops short before reaching the centre of each spicule, and bridges across the rays so as to form a skeleton octahedron. This character is rare in recent Hexactinellids, but, as first pointed out by Carter, it is presented by one or two forms, of which _Aulocystis grayi_ Bwk is best known. The majority of the fossil Hexactinellida belong to the Dictyonine section, a fact attributable to the greater coherence of their skeleton. The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and Cretaceous times.

The OCTACTINELLIDA and HETERACTINELLIDA are two classes created by Hinde[238] to contain certain little-known Devonian and Carboniferous sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie in one plane and 2 are perpendicular to this plane; in the other case, spicules with a number of rays varying from 6 to 30. Bearing in mind the manner in which octactine spicules are known to arise in recent Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed spicules from hexactines by some similar method; while the typical {209}spicule of the Heteractinellida is a euaster. Hence we may refer the Octactinellid fossils to the class Hexactinellida, and the Heteractinellid forms either to the Monaxonida or Tetractinellida.

CLASS III. DEMOSPONGIAE

_Silicispongiae in which triaxonid spicules are absent._

This class has attained the highest level of organisation known among Porifera; the most efficient current-producing apparatus is met with here, so, too, are protective coverings, stout coherent skeletons, and the highest degree of histological differentiation found in the phylum.

Correspondingly it is the most successful group, the majority of existing sponges coming within its boundaries. A few genera and species are exceedingly specialised, for example, _Disyringa dissimilis_ (p. 215). These, however, contribute only a very small contingent to the Demosponge population, those species which are really prolific and abundant being, as we should expect, the less exaggerated types.

CANAL SYSTEM.—With a few exceptions the representatives of the Demospongiae may be said to have taken up the evolution of the canal system at the stage where it was left in _Leucandra aspera_—a stage which the ancestral Demosponges must have reached quite independently of the Calcarea. These commoner members are thus already gifted with the advantages pertaining to a spherical form of ciliated chamber, and so, too, is the Rhagon (Fig. 105), an immature stage noteworthy as the simplest form of Demosponge, and thus the starting-point for the higher types of canal system. The exceptions above alluded to are not without interest: they are the Dendroceratina, of doubtful affinities, (p. 220), which possess small tubular Syconate chambers. They may be regarded either as of independent origin from other Demospongiae, thus making the group polyphyletic, or more simply as representing the ancestral condition, and in this case we must look on the possession of spherical chambers by the Rhagon as a secondary feature. Occupying as it does the important position above indicated, the Rhagon merits a brief description. It is a small discoid or hemispherical body attached by a flat base. It contains a central paragaster, with a single osculum at the free end. Into the paragaster open directly a {210}few spherical flagellated chambers, which lie in the lateral walls of the body. The basal wall of the paragaster, the parts of its lateral walls between the openings of neighbouring chambers, and the entire outer surface of the body are covered with pinacocytes. It is convenient to call the basal part of the sponge from which chambers are absent the hypophare, the upper chamber-bearing part the spongophare. In some of the deeper dermal cells spicules may be already present. In the Rhagon, then, the canal system is of the second type, but all the adult Demosponges have advanced to the third type, and the further evolution in this system is in the direction of improving the mode of communication of the chambers with the canal system. The changes involved go hand in hand with increasing bulk of the dermal layer. A glance at the accompanying figures will show at once the connexion between the phenomena. The increase in the dermal layer (1) greatly reduces the extent of the lumen of the excurrent canals; and (2) results in the intervention of a narrow tube or aphodus between the mouth of each chamber and the excurrent canal. The chamber system is then converted from an "eurypylous" to an "aphodal" type. When the incurrent canal also opens into the chamber by way of narrow tubes, one proper to each chamber and termed "prosodus," the canal system is of the "diplodal" type.

CORTEX.—All the stages in the formation of a cortex are to be seen among the adult members of the group. Certain species (e.g. _Plakina monolopha_, F.E.S.) are destitute even of an ectosome, {211}others have a simple dermal membrane (_Halichondria panicea_, _Tetilla pedifera_) and various others are provided with a cortex, either of simple structure or showing elaboration in one or more particulars. Thus a protective armature of special spicules may be present in the cortex, _e.g._ in _Geodia_, or to a less extent in _Tethya_, or there may be an abundance of contractile elements, and these may be arranged in very definite ways, forming valve-like apparatus that will respond to stimuli.

Everywhere among sponges the goal of the skeleton appears to have been coherence. We have seen how in Calcarea and in Hexactinellida this has been attained by the secretion around the separate elements of a continuous mineral sheath, calcareous in the one case and siliceous in the other. Here we had an excellent instance of the attainment of one end by similar means in two different groups, after their separation from the common stock, and therefore independently. In Demospongiae, on the other hand, the same end—coherence—has been secured by two new methods, each distinct from the former: first the spicules may be united in strands by an organic deposit, spongin; secondly, the spicules may assume irregular shapes and interlock closely with one another, forming dense and stout skeletons. The latter method is that characteristic of the Lithistid Tetractinellida.

CLASSIFICATION.—It is not of great moment which scheme of classification we maintain, seeing that all hitherto proposed are confessedly more or less artificial, and sufficient data for framing a natural one are not yet forthcoming. For convenience, we accept three subdivisions and define them thus:—

I. TETRACTINELLIDA.—Demospongiae possessing tetraxon or triaene spicules
or Lithistid desmas.

II. MONAXONIDA.—Demospongiae possessing monaxon but not tetraxon
spicules.

III. CERATOSA.—Demospongiae in which the main skeleton is formed of
fibres of spongin. The fibres may have a core of sand-grains or of
foreign spicules, but not of spicules proper to the sponge.

But at the same time we admit that some of the Ceratosa are probably descended from some of the families of Monaxonida, so that we should perhaps be justified in separating these families of Monaxonida from the rest, and associating them with the allied families of Ceratosa—a method of classification due to {212}Vosmaer. Again, some Monaxonida approximate to Tetractinellida, and we might, with Vosmaer, unite them under the title Spiculispongiae. This proceeding, though it has the advantage of being at least an attempt to secure a natural classification, involves too much assumption when carried out in detail to be wholly satisfactory.

SUB-CLASS I. TETRACTINELLIDA.[239]

Tetractinellida appear to flourish best in moderate depths from 50 to 200 fathoms, but they are found to be fairly abundant also in shallower water right up to the coast line, and in deep water up to and beyond the 1000 fathom line. Occasionally they lie free on the bottom, but are far more commonly attached; fixation may be direct or by means of rooting spicules; the occurrence of a stalk is rare. There is great variety in the root tuft, which may be a long loose wisp of grapnel-headed spicules, as in many species of _Tetilla_, or a massive tangle, as in _Cinachyra barbata_; in these cases the sponge is merely anchored, so that it rests at the level of the surface of the ooze; in other cases, _e.g._ _Thenea wyvillei_, the root tuft consists of a number of pillars of spicules which raise the sponge above the level of the ooze, into which they descend and there become continuous with a large dense and confused mass of spicules. The parachute-like base of _Tetilla casula_ invites comparison with the "Crinorhiza" forms of some Monaxonids (p. 216).

Two Orders are distinguished thus:—

I. CHORISTIDA.—Tetractinellida with quadriradiate spicules, which are
never articulated together into a rigid network.

II. LITHISTIDA.—Tetractinellida with branching scleres (desmas), which
may or may not be modified tetrad spicules, articulated together to form
a rigid network. Triaene spicules may or may not be present in addition.

ORDER I. CHORISTIDA.

_Plakina monolopha_, from the Adriatic and Mediterranean, furnishes a connecting link between the Rhagon stage and other Tetractinellida. The choanosome is simply folded; there is no distinct ectosome; the chambers are eurypylous. The skeleton {213}consists of microcalthrops and their derivatives. The hypophare is well developed. _Plakina_ thus shows a certain amount of resemblance to _Oscarella_ (p. 196), with which it shares the very remarkable possession of flagellated pinacocytes.

One of the species of _Tetilla, T. pedifera_, continues the series. The folds of its choanosome are more complicated than in _P. monolopha_, and their outer ends are bridged together by a thin layer of ectosome (cf. species of _Sycon_ among Calcarea); the chambers are still eurypylous.

The skeleton reaches a high level: it includes oxeas and triaenes radiately disposed and microscleres (sigmata) scattered throughout the dermal layer. The British _Poecillastra compressa_ from the north of Scotland and Orkney and Shetland is at about the same stage of development, being without cortex and having eurypylous chambers, but it is not so good an example, as the folds of its choanosome are confused.

From _T. pedifera_ we pass to the other species of _Tetilla_ and all the higher genera of Choristida; these possess a cortex not of homologous origin in the various cases, but probably to be classified under one of two heads, typified by _Stelletta_ and _Craniella_ respectively (Fig. 106).

{214}In the Stellettids the cortex arises by the centrifugal growth of a dermal membrane such as that of _Tetilla pedifera_; in _Craniella_ directly from the dermal tissue of the distal ends of the choanosomal folds.

In both cases the end result, after completion of cell differentiation, is a cortex either fibrous throughout or collenchymatous in its outer portion and fibrous in the deeper layers. In the Stellettid type the centrifugal growth of the dermal membrane involves the addition of secondary distal portions to the ends of the inhalant passages. These are the intercortical cavities or canals. Their most specialised form is the "chone." A chone is a passage through the cortex opening to the exterior by one or more ostia, and communicating with the deeper parts of the inhalant system by a single aperture provided with a sphincter (Fig. 106, B).

In the _Craniella_ type the intercortical cavities are parts of the primary inhalant system. They communicate with its deeper parts by sphinctrate apertures. Without any knowledge of the development one would certainly have supposed that the subdermal cavity, pore-sieve and sphinctrate passages of _Craniella_ represented a number of chones, of which the outer portions had become fused (Fig. 106, A).

In both _Craniella_ and _Stelletta_ the chamber system is aphodal, and these genera may fairly be taken as representatives of the average level reached by Tetractinellida. The skeleton is of the radiate type: the type which prevails in the Choristida, but which has an erratic distribution, appearing in some genera of {215}each family but not in others. The genus _Pachymatisma_, of which we have the species _P. johnstonia_ and _P. normani_ in these islands, exemplifies this; it belongs to the highly differentiated family Geodiidae, possesses an elaborate cortex with chones, but its main skeleton is non-radiate.

_Disyringa dissimilis_ is remarkable for the perfection of its symmetry, and for the absence of that multiplication of parts which is so common among sponges. It possesses a single inhalant tube and a single osculum (Fig. 107). Until quite recently it stood alone in the restriction of its inhalant apertures to a single area. Kirkpatrick, however, has now described a sponge—_Spongocardium gilchristi_[240]—from Cape Colony, in which the dermal ostia are concentrated in one sieve-like patch at the opposite pole to the single osculum. _Disyringa_ is still without companions in the possession of an inhalant tube. The concentration of ostia into sieve areas occurs again in _Cinachyra_, each sponge possessing in this case several inhalant areas with or without scattered ostia also.

ORDER II. LITHISTIDA.

The characteristic spicule of Lithistida—the desma—may be a modified calthrop (tetracrepid desma), or it may be produced by the growth of silica over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, _q_), or it may be of the polyaxon type. It is probable that the group is polyphyletic,[241] and that some of its members should remain associated with Tetractinellida, while others should be removed to Monaxonida. Forms with tetracrepid desmas, and those forms with rhabdocrepid desmas which possess triaenes, have Tetractinellid affinities, while forms possessing rhabdocrepid desmas but lacking triaenes, and again those in which the desmas are polyaxon, are probably descendants of Monaxonida.

Owing to the consistency of the skeleton Lithistida are frequently found as fossils. The commonest known example is _Siphonia_.[242] As in the case of so many other fossil sponges the skeleton is often replaced by carbonate of lime, a fact which {216}misled some of the earlier investigators but was established by the researches of Sollas and Zittel.

SUB-CLASS II. MONAXONIDA.[243]

The Monaxonida inhabit for the most part shallow water, but they also extend through deep water into the abysses, thirteen species having been dredged from depths of over 2000 fathoms by the "Challenger" Expedition alone. In some cases, e.g. _Cladorhiza_, _Chondrocladia_, all the species of a genus may live in deep water, while in others the genus, or in others, again, the species, may have a wide bathymetrical range. Thus _Axinella_ spp. occur in shallow water and in various depths down to 2385 fathoms, _Axinella erecta_ ranges from 90 to 1600 fathoms, _Stylocordyla stipitata_ from 7 to 1600, and so on. The symmetry of the deep-water forms contrasts strikingly with the more irregular shape of their shallow-water allies.[244] The shallow-water species are almost always directly attached, some few are stalked; those from deep water have either a long stalk or some special device to save them from sinking in the soft ooze or mud. Thus the deep-sea genus _Trichostemma_ has the form of a low inverted cone, round the base of which a long marginal fringe of spicules projects, continuing the direction of the somal spicules, and so forming a supporting rim. The same form has been independently evolved in _Halicnemia patera_, and an approach to it in _Xenospongia patelliformis_. A similar and more striking case of homoplasy is afforded by the Crinorhiza form, which has been attained in certain species of the deep-sea genera _Chondrocladia_, _Axoniderma_, and _Cladorhiza_; here the sub-globular body is supported by a vertical axis or root, and by a whorl of stout processes radiating outwards and downwards from it, and formed of spicular bundles together with some soft tissue.

There is recognisable in the order Monaxonida a cleft between one set of genera, typically corticate, and suggesting by their structure a relationship, whether of descent or parentage, with the Tetractinellida, and a second set typically non-corticate: these latter are the Halichondrina, the former are the Spintharophora.

{217}ORDER I. HALICHONDRINA.

We have already seen typical examples of the Halichondrina in _Halichondria panicea_ and _Ephydatia fluviatilis_. Within the Halichondrina the development of spongin reaches its maximum among spiculiferous sponges, and accordingly the Ceratosa take their multiple origin here (p. 220). Among Halichondrina spongin co-operates with spicules to form a skeleton in various ways, but always so as to leave some spicules bare or free in the flesh. It may bind the spicules end to end in delicate networks (as in _Reniera_ or _Gellius_), or into strands, sometimes reaching a considerable thickness (as in _Chalina_ and others). In a few cases there appears to be a kind of division of labour between the spicules and spongin, the latter forming the bulk of the fibre, _i.e._ fulfilling the functions of support, while the spicules merely beset its surface as defensive organs, rendering the sponge unfit for food. Fibres formed on this pattern are called plumose, and are typical of Axinellidae. The distinctive fibre of the Ectyoninae is as it were a combination of the Axinellid and Chalinine types: a horny fibre both cored with spicules and beset with them. Spicules besetting the surface of a fibre are termed "echinating." Whenever its origin has been investigated, spongin has proved to be the product of secretion of cells; in the great majority of cases it is poured out at the surface of the cell, and Evans showed,[245] at any rate in one species of _Spongilla_, that the spongin fibres are continuous with a delicate cuticle at the surface of the sponge. In _Reniera_ spp. occurs a curious case of formation of spongin as an intracellular secretion. A number of spherical cells each secrete within themselves a short length of fibre; they then place themselves in rows, so orientated that their contained rods lie end to end in one line. The rods then fuse and make up continuous threads; the cells diminish in breadth, ultimately leaving the fibre free.[246]

ORDER II. SPINTHAROPHORA.

These corticate forms are further characterised by the arrangement of their megascleres, which is usually, like that of most {218}Tetractinellida, radial, or approximating to radial. The microscleres are, when present, some form of aster. The cortex resembles that of Tetractinellida, and v. Lendenfeld has described chones in _Tethya lyncurium_.[247]

The existence of the above points of resemblance between Spintharophora and Tetractinellida suggests a relationship between the two groups as its cause. In judging this possibility the following reflections occur to us. A cortex exists in various independent branches of Tetractinellida. It has in all probability had a different phylogenetic history in each—why not then in these Monaxonida also? Within single genera of Tetractinellida some species are corticate, others not, witness _Tetilla_. The value of a cortex for purposes of classification may easily be overestimated. If we are to uphold the relationship between these two groups, we must base our argument on the conjunction of similar characters in each.

The genus _Proteleia_[248] is interesting for its slender grapnel-like spicules, which project beyond the radially disposed cortical spicules, and simulate true anatriaenes of minute proportions. That they are not anatriaenes is shown by the absence of an axial thread in their cladi. It is not surprising that a form of spicule of such obvious utility as the anatriaene should arise more than once.

Of exceptional interest, on account of their boring habit, are the Clionidae. How the process of boring is effected is not known; the presence of an acid in the tissues was suspected, but has been searched for in vain. The pieces of hard substance removed by the activity of the sponge take their exit through the osculum and have a fixed shape[249] (Fig. 108).

As borers into oyster shells, Clionidae may be reckoned as pests of practical importance, and in some coasts they even devastate the rocks, penetrating to a depth of some feet, and causing them to crumble away.[250]

Sponges, however, as agents in altering the face of the earth do not figure as destroyers merely. On the contrary, it has {219}been calculated[251] that sponge skeletons may give rise with considerable rapidity to beds of flint nodules; in fact, it appears that a period so short as fifty years is sufficient for the formation of a bed of flints out of the skeletons of sponges alone.

_Suberites domuncula_ is well known for its constant symbiosis with the Hermit crab. The young sponge settles on a Whelk or other shell inhabited by a _Pagurus_, and gradually envelops it, becoming very massive, and completely concealing the shell, without however closing its mouth. The aperture of this always remains open to the exterior, however great the growth of the sponge, a tubular passage being left in front of it, which continues the lumen of the shell and maintains its spiral direction. When the crab has grown too big for the shell, it merely advances a little down this passage. The shell is never absorbed, as was once supposed.[252] The crab, besides being provided with a continually growing house, and being thus spared the great dangers attending a shift of lodgings, benefits continually by the concealment and protection afforded by the massive sponge; the latter in return is conveyed to new places by the crab.

_Ficulina ficus_ is sometimes, like _S. domuncula_, found in symbiosis with _Pagurus_, but the constancy of the association is wanting in this case. The sponge has several metamps, one of which, from its fig-like shape, gives it its name.

{220}SUB-CLASS III. CERATOSA.

The Ceratosa are an assemblage of ultimate twigs shorn from the branches of the Monaxonid tree. They are therefore related forms, but many of them are more closely connected with their Monaxonid relatives than with their associates in their own sub-class.

The genera _Aulena_ and _Phoriospongia_, placed by v. Lendenfeld among Ceratosa, by Minchin among Monaxonida, show each in its own way how close is the link between these two sub-classes.

_Aulena_ possesses in its deeper parts a skeleton of areniferous spongin fibres, in fact a typical Ceratose skeleton; but this is continuous with a skeleton in the more superficial parts, which is composed of spongin fibres echinated by spicules proper to the sponge, and precisely comparable to the ectyonine fibres of some Monaxonida.

_Phoriospongia_, as far as its main skeleton is concerned, is a typical Ceratose sponge, with fibres of the areniferous type, but it possesses sigmata free in the flesh.

The sub-class is confined to shallow water, no horny sponge having been dredged from depths greater than 410 fathoms.[253] The greatest number occur at depths between 10 and 26 fathoms.

In the majority of the Ceratosa the skeletal fibres are homogeneous, formed of concentric lamellae of spongin, deposited by a sheath of spongoblasts around a filiform axis. In others, however, the axis attains a considerable diameter, so as to form a kind of pith to the fibre, which is then distinguished as heterogeneous. In one or two cases some of the spongoblasts of a heterogeneous fibre are included in the fibre between the spongin lamellae. _Ianthella_ is the best-known example in which this occurs.

Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished, as their names express, by the nature of the skeleton—net-like, with many anastomoses, in the one; tree-like, without anastomoses between its branches, in the other.

The Dictyoceratina comprise by far the larger number of Ceratosa. They fall into two main families, the Spongidae and Spongelidae, both represented in British waters. The Spongidae {221}are characterised by a granular ground substance and aphodal chamber system; the Spongelidae by a clear ground substance and sac-like eurypylous chambers.

The bath sponge, _Euspongia officinalis_, belongs to the Spongidae. The finest varieties come from the Adriatic, the coarser ones from the Dalmatian and North African coasts of the Mediterranean, from the Grecian Archipelago, from the West Indies, and from Australian seas. The softer species of the genus _Hippospongia_ also form a source of somewhat inferior bath sponges.

Among Dendroceratina, _Darwinella_ is unique and tempts to speculation, in that it possesses isolated spongin elements, resembling in their forms triaxon spicules.

KEY TO BRITISH GENERA OF SPONGES.

{Skeleton calcareous 2
1. {Skeleton siliceous 6
{Skeleton horny, or without free spicules 53
{Skeleton absent 55

2. {Gastral layer continuous 3
{Gastral layer discontinuous, confined to chambers 4

3. {Equiangular triradiate systems present _Clathrina_
{Triradiate systems all alate _Leucosolenia_

4. {Chambers tubular, radially arranged 5
{Chambers spherical, irregularly scattered _Leucandra_

5. {Tufts of oxeate spicules at the ends of the chambers _Sycon_
{Oxeate spicules lying longitudinally in the cortex _Ute_

{All the spicules hexradiate or spicules easily derived from
6. { hexradiate type 7
{Some of the spicules calthrops or triaenes 10
{Megascleres uniaxial 15

7. {Amphidiscs present 8
{Amphidiscs absent 9

{Rooting spicules a well-defined wisp; four apertures lead into
8. { the gastric cavity _Hyalonema thomsoni_
{Rooting tuft diffuse; sponge oval; osculum single
{ _Pheronema carpenteri_

{Sponge tubular, dermal and gastral pinuli absent
9. { _Euplectella suberea_
{Sponge a widely open cup; dermal and gastral pinuli present
{ _Asconema setubalense_

{Tetractine spicule, a calthrop or triaene with short rhabdome;
10. { microsclere aspined microxea _Dercitus bucklandi_
{Triaenes with fully developed rhabdome 11

{222}

11. {Microscleres sigmata _Craniella cranium_
{Sigmata absent, asters present 12

{Microscleres include spirasters _Poecillastra compressa_
12. {Microscleres include sterrasters 14
{Microscleres include euasters: spirasters and sterrasters absent 13

{Two kinds of euaster present _Stelletta_
13. {Microscleres include a euaster and a sanidaster or amphiaster
{ _Stryphnus ponderosus_

14. {Microscleres include microrhabdi _Pachymatisma johnstonia_
{Microscleres include many-rayed euasters _Cydonium milleri_

15. {Some of the microscleres asters 16
{Microscleres absent, or not asters 17

{Skeleton radiate; asters of more than one kind _Tethya_
16. {Sponge encrusting; asters of one kind only _Hymedesmia_
{Skeleton fibrous _Axinella spp._

{Megascleres all diactinal; chelae present _Desmacidon_
17. {Megascleres all diactinal; chelae absent 18
{Some or all of the megascleres monactinal 19

18. {Habitat fresh water{223} 56
{Habitat marine 22

{Megascleres include cladotyles _Acarnus_
{Megascleres include dumb-bell or sausage-shaped spicules forming
{ the main reticulum _Plocamia_
{Microscleres include bipocilli 20
19. {Microscleres include diancistra _Hamacantha_
{Megascleres include forceps _Forcepia_
{Skeleton formed of isolated monactines vertically placed
{ _Hymeraphia_
{None of the above peculiarities present 21

20. {Skeleton fibre not echinated _Iophon_
{Skeleton fibre echinated _Pocillon_

21. {Skeleton with echinating spicules 28
{Skeleton without echinating spicules 30

22. {Spongin abundant 23
{Spongin scanty 25

23. {Fibre not echinated 24
{Fibre echinated _Diplodemia_

24. {Fibre with a single axial series of spicules _Chalina_
{Fibres with numerous spicules arranged polyserially _Pachychalina_

25. {Microscleres absent 26
{Microscleres sigmata and/or toxa 27

26. {Skeleton confused _Halichondria_
{Skeleton reticulate _Reniera_

{Rind and fistulous appendages present; microscleres sigmata
27. { _Oceanapia_
{No rind; skeleton reticulate; microscleres sigmata and/or toxa
{ _Gellius_

{Skeleton confused or formed of bundles of spicules with
{ echinating spined styles 29
28. {Skeleton fibrous or reticulate, or formed of short columns 45
{Skeleton formed of a dense central axis, and columns radiating
{ from it to the surface 52

{Spicules of the ectosome styles _Pytheas_
29. {Spicules of the ectosome oxeas or absent _Clathrissa_
{Main skeleton confused. Special ectosomal skeleton absent
{ _Spanioplon_

{Megascleres of the choanosome not differing from those
30. { of the ectosome 31
{Megascleres of the choanosome differing from those
{ of the ectosome 32

31. {Chelae absent. 33
{Chelae present 44

32. {Trichodragmata present _Tedania_
{Trichodragmata absent 42

{224}

{Skeleton reticulate or fibrous{225} 34
33. {Skeleton radiate or diffuse 37
{Skeleton with radiating fibres forming a reticulum with
{ others crossing them at right angles _Quasillina_

{No microscleres 35
34. {Microscleres sigmata and/or toxa with or without trichodragmata
{ _Desmacella_

35. {Sponge fan- or funnel-shaped 36
{Sponge not fan- or funnel-shaped _Hymeniacidon_

36. {Megascleres slender and twisted _Phakellia_
{Megascleres somewhat stout, not twisted _Tragosia_

37. {Sigmata present, skeleton diffuse _Biemma_
{Sigmata absent 38

38. {Skeleton more or less radiate 39
{Skeleton diffuse; sponge boring _Cliona_

39. {Sponge discoid with marginal fringe _Halicnemia_
{Sponge massive or stipitate without marginal fringe 40

40. {Sponge body prolonged into mammiform projections _Polymastia_
{Sponge body without mammiform projections 41

41. {No microscleres. Megascleres tylostyles with or without styles
{ _Suberites_
{Microscleres centrotylote. Megascleres styles or tylostyles
{ _Ficulina_

42. {Choanosomal megascleres smooth 43
{Choanosomal megascleres spined _Dendoryx_

{Microscleres chelae and sigmata of about the same size
43. { _Lissodendoryx_
{Chelae, if present, smaller than the sigmata _Yvesia_

44. {Isochelae{225} _Esperiopsis_
{Anisochelae _Esperella_

45. {Fibres or columns plumose 46
{Fibres or columns ectyonine 47

46. {Microscleres toxa _Ophlitaspongia_
{Microscleres absent _Axinella_

47. {Skeleton reticulate 48
{Skeleton not reticulate 49

{Microscleres present. Spicules of the fibre core spined _Myxilla_
48. {Microscleres absent. Spicules of the fibre core smooth
{ _Lissomyxilla_

49. {Main skeleton formed of plume-like columns 50
{Main skeleton formed of horny fibres (ectyonine). Special dermal
{ skeleton wanting _Clathria_

{Dermal skeleton contains styles only _Microciona_
50. {Dermal skeleton contains diactine spicules with or without styli
{ 51

{Main skeleton columns with a core of smooth oxeas
51. { _Plumohalichondria_
{Main skeleton columns with a core of spined styles _Stylostichon_

{Central axis contains much spongin. Echinating spined styli
{ present _Raspailia_
52. {Central axis with little or no spongin. Spined styles absent.
{ Pillars radiating from the axis support dermal skeleton
{ _Ciocalypta_

53. {Ground substance between chambers clear; chambers pear-shaped
{ or oval; eurypylous _Spongelia_
{Ground substance granular. Chambers spherical with aphodi 54

54. {Fibres not pithed; sponge fan-shaped _Leiosella_
{Fibres pithed; sponge massive _Aplysina_

55. {Chambers long, tubular, branched _Halisarca_
{Chambers not much longer than broad; not branched _Oscarella_

56. {Amphidiscs present _Ephydatia_
{Amphidiscs absent _Spongilla_

{226}