PORIFERA 



125 



collared, etc. Many are isolated in the jelly, and when they touch they 

 are often not continuous. No nervous system co-ordinates their 

 activities. Even the choanocytes, though the sum of their efforts 

 produces a current, do not keep time in their working. In short, the 

 Porifera are practically colonies of protozoa. Moreover, it would 



Fig. 106. A, Larva (Amphiblastula) of Sycon raphanus. B, The same 

 with flagellated cells invaginating. After Schulze. ca. segmentation cavity; 

 der.c. dermal cells ; fl.c. flagellated cells. C, Section of flagellated chamber of 

 Spongilla lacustris. From Vosmaer. ap. apopyle ; nu. nucleus ; vac. vacuole. 



seem that they took origin from choanoflagellate mastigophora. Now 

 opinion is, as we have seen, not unanimous that the Metazoa arose as 

 colonies of protozoa, and in any case it is unlikely that they sprang 

 from choanoflagellates. Thus the sponges, in spite of certain super- 

 ficial resemblances to the Metazoa, have no real similarity to, and 

 probably no genetic affinity with, that subkingdom. For this reason 



