332 THE EVOLUTION OF THE METAZOA 



absolute and that the same differences could be explained 

 rationally. 



As an instructive example I wish to quote here the inter- 

 pretation that was proposed by Marcus (1958). He discusses 

 the evolution of Spongiae in the following paragraph, "The 

 oldest animals, the Zoomastigina, evidently gave origin to 

 sponges and coelenterates, which are connected at their com- 

 mon root (Heider, 1885). Transitory forms between flagellates 

 and sponges do not exist, as Proterospongia is a regenerating 

 fragment of a sponge (Tuzet, 1945). As sponge larvae have 

 ordinary flagellated cells, the Porifera need not have arisen 

 from the Choanoflagellata. In any case, sponges and Cnidaria 

 must have originated together, because both have germ layers. 

 There exist other parallels between sponges and true Metazoa 

 (Eumetazoa). The definitive layers of the Calcarea and simple 

 Tetractinellida are achieved by a process similar to gastrulation 

 by invagination. What previously had been described as in- 

 version of the germ layers, is understood today as multipolar 

 inwandering (Meewis, 1938). Spongillids show an "accelera- 

 tion embryonaire" (Brien and Meewis, 1938) like many limnic 

 Eumetazoa. The alternations of shape in the blastula (pseudo- 

 gastrula, stomoblastula, "plissement" of the coeloblastula of 

 Halisarca) are intelligible as the result of the small space 

 available for embryonic expansion in the body of the parent 

 sponge. "The evolutionary line of sponges was divergent from 

 the beginning and ends blindly." (E. Marcus, 1958:26). 



All these arguments are supported by rather weak facts and 

 they do not prove to be convincing enough. They could be 

 complemented by two additional arguments which were 

 mentioned by Tuzet ; the supposed presence of nerve cells in 

 Spongiae, and the fact that in some Eumetazoa, cells can be 

 found similar to choanocytes. Yet all these objections cannot 

 conceal the basic fact that the Spongiae do not possess an 

 intestine (neither had they secondarily lost such an intestine), and 

 in connection with this they do not have an oral or even less 

 so an anal orifice. It should be mentioned in passing that a 



