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]