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). [Illustration: FIG. 106.—A, _Craniella_ type; B, Stellettid type. _ch_, Chone; _co_, collenchyma; _d.o_, dermal ostia; _fb_, fibrous tissue; _i.c_, intercortical cavity; _sd_, subdermal cavity; _sp_, sphincter. (After Sollas.)] {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). [Illustration: FIG. 107.—_Disyringa dissimilis._ Diagrammatic longitudinal section of the Sponge. × ½. _a_, _b_, _c_, Transverse sections at the levels indicated to show subdivision of the lumina of the excurrent and incurrent tubes; _e.t_, excurrent tube; _i.t_, incurrent tube; _o_, osculum. (After Sollas.)] 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.