2IO 



PORIFERA 



CHAP. 



few spherical flagellated chambers, which lie in the lateral 

 walls of the body. The basal wall of the paragaster, the parts 

 of its lateral walls between the openings of neighbouring chambers, 

 and the entire outer surface of the body are covered with pina- 

 cocytes. It is convenient to call the basal part of the sponge 

 from which chambers are absent the hypophare, the upper 

 chamber-bearing part the spongophare. In some of the deeper 

 dermal cells spicules may be already present. In the Ehagon, 

 then, the canal system is of the second type, but all the adult 

 Demosponges have advanced to the third type, and the further 

 evolution in this system is in the direction of improving the mode 

 of communication of the chambers with the canal system. The 



Fig. 104. Diagram of (A) eurypylous and (B) aphodal canal systems, a, Apopyle ; a'. 

 apliodus ; J, excurrent canal ; 1, mcurrent canal ; p, prosopyle ; p', short pro- 

 sodus. (After Sollas.) 



changes involved go hand in hand with increasing bulk of the 

 dermal layer. A glance at the accompanying figures will show at 

 once the connexion between the phenomena. The increase in the 

 dermal layer (1) greatly reduces the extent of the lumen of the 

 excurrent canals ; and (2) results in the intervention of a narrow 

 tube or aphodus between the mouth of each chamber and the 

 excurrent canal. The chamber system is then converted from 

 an " eurypylous " to an " aphodal " type. When the incurrent 

 canal also opens into the chamber by way of narrow tubes, one 

 proper to each chamber and termed " prosodus," the canal system 

 is of the " diplodal " type. 



Cortex. All the stages in the formation of a cortex are to 

 be seen among the adult members of the group. Certain species 

 (e.g. Plakina monolopha, F.E.S.) are destitute even of an ectosome, 



