NERVOUS SYSTEM OF VERMES. 



149 



Similar nerves are given off from the oesophageal ring to the 

 alimentary canal. 



The concrescence of two separate structures in the ventral chord 

 of the Gephyrea is the reverse of the permanent separation of the 

 two halves of the ventral chord, which obtains in other divisions of 

 the Annulata. It would not, however, be safe to regard these stages 

 as lower ones, until observation shall have shown, which it has not done 

 yet, whether or no they are preceded by an earlier stage, like that 

 in the Gephyrea. The connection of two separate ventral chords by 

 means of transverse commissures would be more easily explicable if 

 the ventral chord were previously single. 



The nervous system of Sagitta has a peculiar character. Lateral 

 commissures from the cerebral ganglion in the head, pass backwards 

 and downwards to the ventral surface of the body, and pass into a 

 large ventral ganglion, which lies just below the 

 integument, aud gives off peripheral nerves to all 

 sides. 



§ 119. 



A higher grade of differentiation is seen in the 

 nervous system of the Hirudinea and Annelides. 

 The cerebral ganglia are connected by commissures 

 with a ventral chord, and so far these groups re- 

 semble the Gephyrea. In many Annelids the two 

 halves of the ventral chord are homogeneous, and 

 only indicate their metameric character by giving 

 off nerves. In most, however, there are central- 

 form-elements regularly distributed along it. The 

 ventral chord then appears to be broken up into 

 separate ganglia, which are connected with one 

 another by longitudinal commissures. Each gan- 

 glion, again, is broken up more or less regularly 

 into two halves, which are connected together by 

 transverse commissures. The two ventral chords 

 then form a chain of ventral ganglia (Fig. 64.). 

 In many Hirudinea the longitudinal chords of the 

 ventral medulla are, during the early stages, sepa- 

 rated from one another. Later on they are placed 

 very close to one another, and almost seem to be 

 a single chord. In this case, therefore, the separa- 

 tion of the chords must be regarded as the more 

 primitive condition. The longitudinal chords are 

 still closer in the Scoleina, and in the Nereidas, 

 Amphinomidas, and Euniceas among the Chasto- 

 poda; but in all these cases there is not a real fusion, but only 

 a close approximation, which appears to be still closer on account 

 of the connective tissue investing the two nerve-chords. 



In the tubicolous Annelids the ganglia -bearing longitudinal 

 trunks are separate ; in the Serpulida) especially, the lateral portions 



Fig. 64. Anterior 

 portion of the ner- 

 vous system of 

 Capitella capi- 

 lata. g Cerebral 

 ganglion. o Optic 

 nerves, c (Esopha- 

 geal commissure. 

 b Ventral chord, 

 with two ganglia. 

 n Nerves passing off 

 from them (after 

 Claparede). 



