256 THE EVOLUTION OF THE METAZOA 



progressive evolution in the direction of Coelomata. There 

 are many zoologists who still adhere more or less to this 

 interpretation. In our case we should attempt to derive the 

 Cnidaria from the lowest Turbellaria, thus from Acoela. Yet 

 even a slightly closer comparison shows that this cannot be 

 made. Anthopolyps — taking into consideration the solitary 

 forms only— have a much more highly developed organization 

 than the Acoela, even if there are some recent species of Acoela 

 which already possess a complex genital apparatus. 



Among the Turbellaria we must search for forms that had 

 later evolved into Anthozoa among those groups that are 

 already more highly developed than the Acoela. A completely 

 unjustified accusation has been made by Remane (1960) who 

 states that I wish to derive the Cnidaria from the Temnocephala. 

 I have never either mentioned or maintained anything of this 

 kind (cf. Hadzi, 1958 a). On the other hand, I see in the Temno- 

 cephala with their one-sided specialization, an instructive ex- 

 ample, in effect an experiment made by Nature, which 

 shows what can and what must evolve from an animal type 

 when a species that reached a certain level in its organization, 

 abandons the creeping way of life and begins to live as a semi- 

 sessile animal. The Temnocephala are Turbellaria that are 

 closely related to Rhabdocoela; in all probability they had 

 evolved from the latter. Matjasic even considers that they 

 are still similar enough to the latter so that they can still 

 be classified among Rhabdocoela. The Temnocephala had 

 become specialized as epizoic animals and have therefore 

 ceased to creep by means of their ventral surface covered 

 with cilia; they have assumed an erect position and at the 

 same time they have developed a special disc at their aboral 

 end, by means of which they attach themselves to the sub- 

 stratum; their oral orifice has been transplaced to the anterior 

 end of the animal (now to its upper end). An initially solitary 

 pair of tentacle -like excrescences has been polymerized. The 

 ciliation has been reduced due to the fact that they move 

 actively by means of body muscles, and their skin has become 



