. Elements of Comparative Anatomy. 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 i
. Elements of Comparative Anatomy. 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 â 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'
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