PREVIOUS INTERPRETATIONS OF CNIDARIA 129 



body. During the third stage a brain is formed in the anterior 

 part of the body, at the place where the longitudinal bundles 

 develop a mutual contact; this development is aided by sensory 

 organs that had evolved in the front part of the arimal. 

 Here we are not interested in the subsequent development of 

 the nervous system during which the polymerization (the in- 

 crease of the number of the longitudinal nerve trunks) is 

 followed by an oligomerization (two pairs of longitudinal 

 trunks, a ventral pair, the merging of this pair with the nerve 

 cord). 



Fortunately enough there exist even now species of Turbel- 

 laria that have preserved the primitive conditions of their 

 nervous system. O. Steinbock has given a description of the 

 species Nematoderma hathycola, which has been found in the 

 deep regions of the polar sea, close to Greenland; Steinbock 

 says about this species, "Ein Gehirn in gewohnlichem Sinne, 

 wie wir es bei Turbellarien gewohnt sind anzutreffen, besitzt 

 unser Tier nicht" (Steinbock, 1931). On the other hand there 

 is in Nematoderma a denser and, as it seems, less regularly 

 formed nerve plexus which is situated close under the skin 

 and which shows indications of some thickenings or of conden- 

 sation in the longitudinal direction. Hanstrom is inclined 

 to attribute the main role in the formation of the brain to the 

 emergence of sensory organs. We are convinced, however, 

 that the influence of the function of the muscle system during 

 the active forward movement, with the anterior end orientated 

 forwards, has been at least as important. It is even possible 

 that the static organ, the light receptors, and even more 

 probably the special chemoreceptors have been developed only 

 after the Anlage of the brain had been founded. 



If we stick obstinately to the thesis of the evolution of 

 Cnidaria in the direction Hydrozoa-Scyphozoa-Anthozoa and 

 to the belief that Cnidaria have nothing in common with 

 Turbellaria, we are forced to suppose that the nervous system 

 of Eumetazoa has developed at least twice, thus diphyletically, 

 and so to speak, out of nothing. The opinion cannot be taken 



