22 THE EVOLUTION OF THE METAZOA 



hand, however, attempts have been made to see in the Cteno- 

 phora an initial stage for a higher development. This was sug- 

 gested in connection with the discovery of certain species of 

 Ctenophora which do not live in the plankton as is typical for 

 Ctenophora but rather on the sea bottom, and for this reason 

 have been correspondingly changed, especially in their external 

 forms (Platycfenea). They have been thought in this con- 

 nection as a possible transition to Turbellaria: but the speci- 

 alists have been able to show that these are genuine, though 

 specialized, Ctenophora. In this way, the Ctenophora, too, as 

 the third element of the old group of Coelenterata represent 

 a blind side branch of the animal kingdom: no higher type of 

 animals has been developed out of them. Though there 

 are some connections between Ctenophora and Turbellaria, 

 these connections are in my opinion exactly opposite to 

 those as viewed by the old interpretation. The Ctenophora 

 should be considered to have evolved out of a turbellarian 

 form due to their transition from a life on the bottom of the 

 sea, when they had a pure bilateral symmetry, to a life in the 

 plankton, with a change of their symmetrical structure to that 

 of bisymmetry. A detailed comparison of Ctenophora and 

 Turbellaria brings to light a large number of common charac- 

 teristics and peculiarities, particularly if in connection with the 

 latter we take into consideration the planktonic larvae of 

 Polycladida. The difference, however, can be explained on one 

 hand as a result of the neoteny, and, on the other, as being 

 due to the adoption of a freely moving Ufe in water. This 

 latter specialization makes it particularly impossible to regard 

 the Turbellaria as having evolved from the Ctenophora. A small 

 but welcome contribution which supports my hypothesis of 

 the development of the Ctenophora from the Turbellaria has 

 been the recently proposed theory (Hadzi, 1955) which tries 

 to find the origin of the so-called ciliary rosette organs of 

 Ctenophora in the turbellarian protonephridia. There are pre- 

 sent-day zoologists who have assumed a critical attitude 

 towards all attempts which try to explain the formation of 



