Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 NEEVOUS SYSTEM OF MOLLUSCA. 345 extending over tlie oesopliagus, passes along tlie sides of tlie body in- ternally to the branchice, and forms tliepallial nerve-trunk (i^l). There is also a group of four small ganglia below the oesophagus (sublingual ganglia). The two trunks of the pedal nerves are thicker than the commissures which pass to them from the nervous band; they must consequently be regarded as central parts of the system. They seem to be the longitudinal trunks of a 'ventral me
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 NEEVOUS SYSTEM OF MOLLUSCA. 345 extending over tlie oesopliagus, passes along tlie sides of tlie body in- ternally to the branchice, and forms tliepallial nerve-trunk (i^l). There is also a group of four small ganglia below the oesophagus (sublingual ganglia). The two trunks of the pedal nerves are thicker than the commissures which pass to them from the nervous band; they must consequently be regarded as central parts of the system. They seem to be the longitudinal trunks of a 'ventral medulla/' in which the ganglionic cells are not definitely distributed into distinct groups any more than they are in the Gephyrea. The structure of this chord re- quires, however, to be further investigated. The principal point in the arrangement of the nervous system in the Placophora is that we may recognise in it just the same relation of parts as in the Solenogastx'es, and specially in Chfetoderma (§ 121). § 267. The relatively feeble development of the cerebi'al ganglia in the Lamellibranchiata is due to the absence of a head, and its related sensory organs. These ganglia (Fig. 181, a) are often placed so much to the side that there is a long commissure be- tween them (Lucina, Pano- paea, Anodonta, Unio, Mytilus, Area, Cardium, Pholas, etc.). A few smaller branches only are given oif in addition to the large vis- ceral commissures. There are two pedal gang-lia in- stead of the pedal nerve- trunks, the nerves from which are distributed in the ventral portion of the body, and especially in the foot. They are placed at the root of the foot, and are some- times deeply imbedded in it. The commissural chords vary in length in propor- tion to the development of the foot, aiid the distance between it and the anterior parts of the body. When the foot is feebly developed, or when it is produced very far forwards, the cerebral and pedal ganglia may be markedly approximated (Solen, M
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