THE EVOLUTION OF THE BRAIN IO5 



The second evolutionary stage in the nervous system is 

 that seen in most parts of the sea anemone's body and in 

 coral animals and jellyfishes. In this type a nerve cell 

 intervenes betv^een the primitive receptive neurone and the 

 muscles and represents what may be called a primitive 

 motor neurone. This motor element transmits the impulse 

 received from the sensory neurone to the many muscle 

 fibers with which it is connected. Such a type of nervous 

 system, since it is composed of two kinds of neurones, has 

 been called a dineuronic system. 



From a dineuronic system it is an easy step to a system in 

 which beside the sensory and the motor neurones there are 

 intercalated internuncial neurones. Indeed, polyneuronic 

 systems are found in the worms, the crabs, the insects, the 

 snails, and all higher animals including man. 



When representative animals possessing this kind of 

 nervous system are examined they are seen to exhibit two 

 important phases in the evolution of the parts concerned. 

 These phases pertain first of all to the composition of the 

 nervous system so far as the three types of neurones are 

 involved and, secondly, to the location of the system in the 

 animal. 



The cellular composition of the polyneuronic systems 

 varies in different animals. In the worms and other like 

 forms the central nervous organs are composed predomi- 

 nantly of sensory and motor neurones with only a moderate 

 number of internuncial elements present. In higher animals 

 such as the crabs and insects the internuncial neurones show 

 a larger increase -than the sensory and motor elements. 

 This increase of the internuncials becomes excessive in the 

 vertebrates till in man the brain, as already stated, is almost 

 exclusively internuncial. Moreover those parts of the human 

 brain which are most important to man, the hemispheres 

 proper, are entirely internuncial. 



The second important feature of the polyneuronic systems 

 relates to their location. In the sea anemones and jellyfishes 

 the nervous system is for the most part a thickened portion 

 of the outer skin and thus is in reality a part of the outer 

 covering of the animal. This condition is also realized in 

 some worms though in the majority of these animals the 



