258 THE EVOLUTION OF THE META7.0A 



have remained strictly individualized, i.e. they have not 

 adopted asexual reproduction. 



There were probably times when tl e Acoela were the only 

 Tarbellaria in existence; at a somewhat later date the Acoela 

 appeared together with the younger (let us call it) eucoelous 

 subtype which had already been equipped with a distinct 

 intestine. There can be no doubt that those recent Turbellaria 

 which stand closest to this younger form are now classified 

 as Rhabdocoela. We will therefore call these hypothetical 

 ancient Turbellaria as "rhabdocoeloid Turbellaria," in order 

 to avoid in this way the accusation that we wdsh to derive 

 one recent form from other recent forms. We will therefore 

 avoid the problem whether these rhabdocoeloid Turbellaria 

 would already be called Rhabdocoela by an expert in Tur- 

 bellaria, or whether he would give some other name to 

 them as, for example, Prorhabdocoela; such a problem is not 

 important. 



Regardless of whether the Turbellaria have been considered 

 as primarily primitive animals— which is certainly a more 

 probable interpretation— or whether they have been viewed 

 as arimals with a secondarily simplified organization which 

 is a result of an eventual retrogression, it has nevertheless 

 been usually thought that various other "lower worms" and 

 not only Platyhelminthes had evolved from the older tur- 

 bellarian forms. A majority of zoologists (let us hope that 

 in the near future they will be followed by the others) consider 

 the Turbellaria to be primarily primitive animals; they can 

 thus easily imagine how ages ago these Turbellaria, together 

 with some Protozoa, were the prevalent animal forms which 

 represent the starting point of the whole subsequent evolu- 

 tion of the eumetazoan animal w orld. It is therefore quite 

 natural that we also try to derive the Cnidaria from these rhab- 

 docoeloid Turbellaria which have shown such an exceptional 

 ability to evolve. At the same time it has been necessary to 

 dethrone the Cnidaria, with Hydra in the front row, from the 

 position they had in the animal system. The Cnidaria have 



