146 



THE EVOLUTION OF THE METAZOA 



among the primitive Turbellaria species, a quite simple genital 

 apparatus (Fig. 25) which is similar to the condition found in 

 the Cnidaria. Yet even if there were in recent times no such 

 primitive species of Turbellaria, if they had all died out, we 

 would still be justified in defending the thesis (in view of other 







Fig. 25. An acoeleus turbellarian with its simple genital apparatus 

 (Tetraposthis colymbetes) . (From An der Lahn.) 



circumstances) that the genital organization of Cnidaria had 



been developed out of that which had existed in Turbellaria. 



It is illuminating to observe in this connection the destiny 



of the complicated genital system of Turbellaria as they 



