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.

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 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).