4 THE EVOLUTION OF THE METAZOA 



ontogeny of Volvox, is in this connection completely out of 

 place. Volvox, is in fact a real alga in spite of its slight resem- 

 blance to an animal (its spheric form, free swimming ability). 

 The very rare appearance of an organelle in some Eumetazoa 

 which resembles more or less the "collar" of the choanocytes 

 in Spongiae is too insignificant to be used as a proof to support 

 the thesis that Spongiae are really Coelenterata, and it can be 

 explained by the fact that both Parazoa as well as Eumetazoa 

 arise from the ancestors of Flagellata (i.e. the scattered ap- 

 pearance of similar genes). 



Disregarding the fact that the arguments brought forward 

 by Tuzet cannot be considered sound, we must emphasize that 

 the principal mistake in this kind of reasoning lies in the wrong 

 belief that individual morphological or morphogenetic fea- 

 tures and characteristics can represent the special type of orga- 

 nisation of Spongiae; in reality, this is represented by their 

 special "nature," so as to say by the whole spirit of the organi- 

 sation of Spongiae, and by their way of life. Again and again 

 it is necessary to point out that the Spongiae have neither a 

 digestive canal nor an oral opening. To this one could remark 

 that in this respect Spongiae do not represent a unique case ; 

 even among the genuine Eumetazoa there are some in which 

 we can find neither an oral opening nor a digestive canal, e.g. 

 among the extreme endoparasites, or in the strongly aberrant 

 Pogonophora. Nevertheless, there is a basic difference between 

 Spongiae and these cases observed in Eumetazoa. It is beyond 

 any doubt that in the case of the parasitic Eumetazoa and the 

 freely living Pogonophora the absence of the oral opening 

 and of the digestive canal is nothing but a secondary pheno- 

 menon. Spongiae, on the other hand, indubitably have pri- 

 marily neither an oral opening nor a digestive canal, and this 

 is a consequence of their special organisation. The feeding 

 system with its numerous pores, the canal system with layers 

 of choanocytes in chambers separated from each other that 

 serve to maintain the flow of water, the larger aperture for the 

 discharge of surplus water, and many other things, all this is 



