2IO 



PORIFERA 



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 tlie 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) eiirypylous and (B) apliodal canal systems, a. Apnpyle ; «', 

 apliodus ; JH, excurrent canal ; 7, incurrent canal ; 2'> pi'osopyle ; ^Z, short jiro- 

 sodus. (After Sollas.) 



changes involved go hand in hand with increasing bullc of the 

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

 once the connexion between the phenomena. The increase in tlie 

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

 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 l)y 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 tlie formation of a cortex are to 

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

 (e.g. Plaldna monoloplia, F.E.S.) are destitute even of an ectosome, 



