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 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.