THE MOST PRIMITIVE METAZOA 57 



(8) A high regenerative capacity does not indicate that an 

 animal is simple. Thus in the coelenterates, the polyps have good 

 regenerative capacity, the medusae do not. In the annelids the 

 Oligochaeta have good regenerative capacity, the Hirudinea do 

 not. In the amphibians the Urodeles regenerate well, the Anura 

 do not. 



(9) There is no evidence that asexual reproduction is necessarily 

 more primitive than sexual reproduction. 



It can be seen that the situation is not as straightforward as the 

 lists might at first sight make it appear. It should also be re- 

 membered that though one might believe that the sponges are 

 more simple than the coelenterates from the point of view of their 

 morphology and life cycles, this is no reason for thinking that they 

 necessarily developed some time before the coelenterates. The 

 sponges may have been a happy afterthought of the Protozoa 

 after they had given rise to the coelenterates ! 



Which group of animals did the sponges come from? There 

 are three different answers to this question. The first derives 

 the sponges from Protozoa such as the choanoflagellates or 

 Volvocinae. The second derives the sponges from a Gastrea type 

 of animal which gave rise to both the coelenterates and the 

 sponges. The gastrula probably came from a colonial protozoan 

 such as Volvox. The third answer derives the sponges from a 

 coelenterate source. Each of these views has something in its 

 favour and something against it. 



Origin from the Protozoa 

 (i) From the Choanoflagellata 



The inner layer of cells in the sponges is composed mainly of 

 Choanocytes. These in many ways resemble the cells of the 

 choanoflagellate protozoa and on this basis it has been suggested 

 that the sponges might be derived from these protozoans. 



In 1880 Saville Kent described a colonial choanoflagellate 

 called Proterospongia (Fig. 18). This consisted of a flat plate of 

 about forty cells. The other cells had the normal choanoflagellate 

 structure whilst the inner cells were amoeboid. Periodically a 

 choanoflagellate cell would withdraw its flagellum and become 

 amoeboid, whilst an amoeboid cell would take up the position and 



