NERVE TRUNK-PATHWAYS 



617 



(Kerkut, 1954-55). The primitiveness of the central coordmative control 

 at this stage may best be illustrated by the analogy used by von Uexkiill 

 (1897) : when a dog runs the animal uses its legs ; when a sea-urchin runs 

 the spines move the animal. 



As evolution proceeds, we shall see that nerve-nets with directive conducting 

 trunks get progressively less important in worms, Arthropods and Molhiscs ; but it 

 is of interest that the most primitive Proto-chordates, the Balanoglossids (Hemi- 

 chordata), have a comparable non-integrated system of dorsal and ventral nerve-cords 

 with collar-connections associated with a peripheral nerve-net (Bullock, 1940). It 

 will be remembered ^ that these worm-like burrowing creatures are without eyes. It is 

 of interest that this primitive type of nervous system is an indication of the great 

 phylogenetic age of the emergence of the chordate stock^. 



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269 



111 



THE GANGLIONIC NERVOUS SYSTEM 



When bilateral symmetry was gained (as in worms) a further great 

 advance in neural economy became j^ossible. for now the sensory organs, the 

 food-and-danger predictors gathered preferentially at the anterior end of 

 the animal, led to a concentration of nerve-elements here also, thus inaugu- 



1 p. 227. 2 p_ 233. 



