214 



PORIFERA 



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 differentia- 

 tion, is a cortex either fibrous 

 througliout or collencliymatous in 

 its outer portion and fibrous in tlie 

 deeper layers. In the Stellettid 

 type the centrifugal growth of tlie 

 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 inter- 

 cortical 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 be- 

 come fused (Fig. 106, A). 

 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 distriljutiuu, appearing in some genera of 



;. 107. — Disyringa dissimilis. 

 Diagrammatic longitudiual section 

 of the Sponge. x ^. 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 ; ij, incurrent tube ; o, os- 

 culum. (After SoUas.) 



