364 THE EVOLUTION OF THE METAZOA 



of Meta2oa (cleavage, the "blastaea" stage) and which could 

 eventually be interpreted as indications; yet there are also 

 other possible interpretations of these same facts which are 

 even more probable. It has already been emphasized that the 

 only satisfactory evidence that can be found in the recent 

 Protozoa point to the evolution of the Spongiae from the 

 Choanoflagellata, while there are no facts available which 

 could be used as evidence of the corresponding evolution of 

 the Eumetazoa. 



In my opinion there are three more or less probable ways 

 ho\^' we can interpret the evolution of the Metazoa from the 

 Flagellata. These three variants are: 



(1) Spongiae Eumetazoa 



/ / 

 Flagellata -> Cihata 



(2) Spongiae 



Flagellata -> Ciliata -> Eumetazoa 



(3) Ciliata 



/ 

 Flagellata -> (blastaea) -> Mesozoa -> Spongiaria -^ 



-> Eumetazoa 



There is no great difference between the first and the third 

 variant. It is only a question of the interpretation of the origin 

 of Spongiae. We have already dealt sufficiently with this pro- 

 blem in our discussions and we have come to the conclusion 

 that the Spongiae represent an isolated group. The diphyletic 

 origin of the Metazoa (the variant No. 2) appears therefore to 

 be more probable than the monophyletic evolution (the variant 

 No. 3). There remain the variants No. 2 and No. 3 only if we 

 disregard the variant No. 1 which seems to us be quite im- 

 probable. Here w^e do not take into account the inessential 

 differences that appear within the frame\\ork of the third 



