chap xii.] NERVOUS SYSTEM. 403 



lowly and little differentiated representative of a large 

 group of animals, in the higher members of which con- 

 centration is exceedingly well marked. (See page 411.) 



With the exception, then, that in Peripatus and; 

 Proneomenia, the anterior end of the nerve cords is 

 enlarged into a cerebral mass, we should appear- 

 to be able to see no essential difference between them 

 and a Craspedote Medusa, save in the fact that the 

 Medusa has a complete nerve ring. In so far, how- 

 ever, as there is in both the Arthropod and Mollusc 

 just named, a commissure at the hinder end of the 

 body which connects the right and left cords with one 

 another, it is clear that the nerve system, if not a 

 ring, is at any rate a closed system ; that, in other 

 words, it may be compared to a ring drawn out length- 

 wise (Balfour). If this comparison be a just one we 

 are soon able to explain the reason why the anterior 

 end of a Nemertine or Arthropod or a Mollusc is better 

 developed than the rest of the nerve cord, for these 

 animals are all bilaterally, in place of being circularly 

 or radially, symmetrical ; and it follows, therefore, that 

 they do not advance in any direction indiscriminately, 

 as does a jelly-fish, but that there is one end which 

 is always directed forwards, and which first comes into 

 contact with friends, foes, or food. It is at that end, 

 naturally, that sense organs are first and best deve- 

 loped, and it is at that end, therefore, that the central 

 portion of the nervous system comes to be largest and 

 most highly developed. 



In connection with this, the discovery by Kleinen- 

 berg of a nervous ring in the larvae of certain Annelids 

 is of great significance ; for though the adult poly- 

 chsetous worm is bilaterally symmetrical, and has a 

 central nervous system of the same character, the 

 larva has a rounded head-disc. 



After the disappearance of the diffused or plexi- 

 forui arrangement of the nefVe fibres the system may 



