124 evolution: the ages and tomorrow 



the starfish. Nissen reviews the Hterature and finds much ex- 

 perimental work on the righting reaction, but no conclusive 

 results. Psychologists have tried to teach the use of nonpre- 

 ferred arms over preferred arms in the righting reaction, and 

 have probably introduced local tissue changes instead of 

 learning, although this change may be learning^ in a certain 

 sense. In view of the peculiar nature of the nervous system 

 of the starfish this sort of test would not seem to be suitable. 

 Certainly the echinoderm is higher in its over-all structure 

 and nervous system than the coelenterate, and the latter 

 does definitely show learning capacity. 



With the advent of the lower worms, flatworm and 

 roundworm, the process of real centralization is under way. 

 Here at last we have animals with a "head end," developing 

 with this end forward and evolving sense organs and mouth 

 and brain. In the flat^^^orm we have a truly basic nervous 

 system: a ganglion at the anterior end, a single "brain," and 

 two longitudinal nerve cords running posteriorly and giv- 

 ing off lateral branches. It is a bilaterally symmetrical plan, 

 which was followed later by all the higher animals, includ- 

 ing man. Flatworms have shown definite capacity for learn- 

 ing. Nissen in his summary of the literature in the Handbook 

 of Experijnental Psychology reports one series of experi- 

 ments with these animals. They were stimulated to locomo- 

 tion by light, being exposed to cycles of 5 minutes light fol- 

 lowed by 30 minutes of darkness. Each time they started 

 forward they were stopped by a tactile stimulus applied to 

 the head end. After 25 such treatments they did not move 

 forward during the periods of light. Extirpation of the head 

 ganglion removed this conditioned inhibition of the crawl- 

 ing response to light. 



In the annelid worms (the earthworm is a somewhat 

 modified example) there is a two-part brain: one part in 

 front of the mouth and the other just back of it, the two be- 

 ing joined by a heavy nerve ring. A large double nerve 

 trunk connects these "brains" with the posterior part of the 



