214 



PORIFERA 



^fe 



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 

 throughout or coUenchymatous 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 Graniellpi 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 



Flo. 



- Disyringa 



of'''tiTs'pmfse''''^xt'"''i''f °° '^'^^ ^^"^^^ certainly have supposed 



Transverse sections at the levels that the subdermal cavity, pore-sieve 



iSet^oft: :xLt1rand and sphinctrate passages of Craniella 



mcurrent tubes ; e.t, excurrent represented a number of chones, of 



tube ; i.t, incurrent tube ; o, os- ^ ■ ^ ,^ , . ■ i i i 



cuium. (After Sollas.) 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 distribution, appearing in some genera of 



