564 COMPARATIVE ANATOMY chap. 



epithelial in position. The finer structure of the epithelium cannot be 

 dealt with here. 



III. The Nervous System (Figs. 456 and 457). 



The facts of fundamental importance will be treated of first. 



The whole nervous system, with the single exception of a part 

 situated in the collar, lies In the body epithelium. 



Below the surface of the whole of the body epithelium, there is 

 an uninterrupted layer of nerve fibres, a close continuous nerve 

 plexus. 



The principal or main portions of the nervous system are 

 merely local thiekening-s of this network ; and are the two longi- 

 tudinal nerve cords, one mediodorsal and the other medioventral, 

 which run throughout the whole length of the trunk. 



At the boundary between the collar and the trunk, the 

 network of nerve fibres thickens into a nerve ring, which forms a 

 more specialised connection between the dorsal and ventral nerve 

 cords. 



The dorsal cord is produced anteriorly as far as to the base of 

 the proboscis, where it divides into two diverging branches, which 

 encircle that base. This circular thick;ening of the epithelial nerve 

 plexus, however, is not sharply circumscribed. 



The collar portion of the dorsal cord leaves its epithelial position, 

 and runs longitudinally through the coelom of the collar above the 

 buccal cavity. 



Special. — Besides smaller ganglion cells, others of remarkable size, so-called 

 giant ganglion cells, sometimes occur in the thickenings of the nerve plexus, especi- 

 ally in the collar region. Near the nerve cords and fibres the body epithelium has 

 few or no glands, and above the cords it is thickened. Further, all along these 

 cords, i.e. in the dorsal and ventral middle lines, and especially in the latter, the 

 body wall appears to have sunk in. 



The cord in the collar, which may best be regarded as the central portion of the 

 nervous system, forms the so-called dorsal nerve cord of the collar. This lies in 

 the median line in the ccelom of the collar above the pharynx, and consists of several 

 parts. Tlierc are two perihsemal tubes, which clasp between them the collar 

 portion of the dorsal blood vessel ; the nerve substance itself lies upon these, or 

 in a channel foimed by them. The nerve cord of the collar is a thick, almost 

 cylindrical band, the dorsal part of which consists of cells, which, however, are not 

 nerve cells but may be glandular ; the ventral part, that turned to the intestine, 

 consists of nerve tissue, and is a direct prolongation of the dorsal nerve cord of the 

 trunk. Anteriorly, i.e. at the anterior end of the collar, this band divides, one 

 portion running into the nerve tissue of the base of the proboscis, and the other 

 into the epithelial nerve tissue of the circular collar ridge which surrounds this base. 



In the genus Ptycliodcra, the nerve cord of the collar is connected with the 

 epithelium of the dorsal body wall in the median plane by means of a varying 

 number of epithelial tubes, the so-called roots of the nerve cord. Of these, only 



