NERVOUS . SYSTEM OF VEEMES. 



147 



wards consist of a dorsal and a ventral trunk, wliicb. pass along the 

 corresponding median lines. In addition to these, two chords arise 

 from the ventral portion of the oesophageal ring, and, converging 

 posteriorly, unite to foi'm a mass of ganglionic cells (G. cephali- 

 cum). The median nerves run along the whole length of the body. 

 Both send fibres into the matrix of the integument. It is clear tliat 

 this arrangement is a modification, speaking generally, of the simple 

 conditions of the nervous system of other Vermes; but it is so 

 peculiar that any special comparison is altogether impossible. 



The same holds for the nervous system of the Acanthocephali. 

 A small " ganglion " placed at the base of the sheath of the pro- 

 boscis gives off branches an- 

 teriorly as well as posteriorly. 

 Its relation to the doi'sal cen- 

 tral organ of other Vermes is 

 obscure, as it is placed between 

 the bundles of the ventral re- 

 tractors of the sheath of the 

 proboscis, 



§ 117. 



In the second form of the 

 nervous system two longitu- 

 dinal trunks are predominant ; 

 these arise from the cerebral 

 ganglia, and pass backwards. 

 This arrangement is first seen 

 in the Nemertina, and is di- 

 rectly related to what obtains 

 in the Turbellaria, in which 

 there are often two greatly 

 ■ developed longitudinal nerves 

 passing backwards. The size 

 of these two peripheral longi- 

 tudinal trunks is dependent 

 on the length of the body. 

 As there are ganglionic cells 

 in them, they are not exclu- 

 sively peripheral organs. The 

 cerebrum, too, in the Nemer- 

 tina is more largely developed, 

 for several large segments can 

 be made out in each of the 

 two ganglia. The commis- 

 sure between the two halves 

 is traversed by the organ 

 which we have already de- 

 scribed above as the proboscis 



Fig. 63. Head of a Nemertine (Omma- 

 toplea alba). g Central nervous system. 

 n Lateral trunks, o Eje-spot. ^jp'^p" Pro- 

 boscis, ps Its sheath, i Enteron. e Lateral 

 oi'gau. d Dorsal vascular trunk. I Lateral 

 vascular trunk (after Carm. M'Intosh). 



Although in most of them the 



longitudinal trunks (Fig. Go, ■);) ruu exactly along the lateral edge of 



