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).
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 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 misled some of the earlier investigators but was established by the researches of Sollas and Zittel.