NEW INTERPRETATION OF CNIDARIA 273 



A primary small size of an animal type must be strictly 

 distinguished from a secondary diminution (Hadzi, 1956 b). No 

 element which has been mentioned above and which could 

 serve as an indication of a secondary diminution can be found 

 in the Acoela ; they must therefore be considered as primarily 

 small animals, and the simplicity of their structure which is 

 everywhere clearly evident as a primitive characteristic. We 

 will repeat here, in order to emphasize this statement the 

 better, that the Acoela are genuine, perfect, freely living ani- 

 mals which are able to make rapid active movements (certainly 

 in the most primitive way, by means of a pulsation of cilia) and 

 which feed as predatory species; they did not develop all these 

 properties de novo but rather had inherited the same— as we 

 will soon try to prove— from some equally small ancestors. 



The Acoela, as well as all the Turbellaria, are typical bilate- 

 rally symmetric animals. They show no traces of a radial sym- 

 metry, unless we agree with Beklemischev who believes he sees 

 an indication of the radial symmetry in the presence of several 

 pairs of longitudinal nerve trunks. This fact, however, is quite 

 unimportant because it did not lead to the development of a 

 general radial symmetry and because the central nervous 

 system of the whole group of Turbellaria had become con- 

 solidated in the direction of the bilateral symmetry. 



The Origin of the Acoelous Turbellaria 



We have shown that Cnidaria had evolved first as antho- 

 polyps and in all probabihty from the rhabdocoeloid Tur- 

 bellaria, and also that the Acoela are the most primitive sub- 

 group of Turbellaria. Our next problem will be the origin of 

 the Acoela, and the question, "how can the origin of the 

 Acoela be explained?" We are well aware of the fact that in an 

 attempt to answer such a question we are on very uncertain 

 ground, that the only solid facts that can be found are in the 

 recent animal types, and that we cannot avoid making con- 



