190 



PORIFERA 



meet ; and there results a sheet of dermal tissue perforated 

 by dermal ostia, which open into the inhalant canals, and 

 strengthened by stout spicules running longitudinally. This 

 layer is termed a cortex ; it covers the whole sponge, compacting 

 the radial tubes so that they form, together with the cortex, a 

 secondary wall to the sponge, which is once more a simple 

 sac, but with a complex wall. The cortex may be enormously 

 developed, so as to form more than half the thickness of the 



Fio. 82. — Sycon raphanus. A, Longitudinal 

 section of young decalcified Sponge at a 

 stage somewhat later than that shown in 

 Fig. 81. B, Transverse section of the 

 same through a whorl of tubes, d. Dermal 

 memhratte ; <?, gastral membrane ; H, 

 paragaster ; 5^"*, tetraradiate spicule ; T, 

 radial tube. (After Maas.) 



d.o d.o 



Fig. 83. — Transverse section of the 

 body-wall of Ute.^ passing longi- 

 tudinally through two chambers. 

 a.p, Apopyle ; cl.o, dermal ostium ; 

 fi.ch, flagellated chamber or radial 

 tube ; i.c, inhalant canal ; p, pro- 

 sopyle. (After Bendy.) 



wall (Fig. 84). The chambers taken together are spoken of as 

 the chamber layer. 



We have already alluded to the resemblance between a young 

 Ascon person and a radial tube of Sycon — a comparison which 

 calls to mind the somewhat strange view of certain earlier 

 authors, that the flagellated chambers are really the sponge 

 individuals. If now we suppose each Ascon-like radial tube of 

 Sycon to undergo that same process of growth by which the 



