NEKVOUS SYSTEM OF MOLLUSCA. 



315 



extending over the oesophagus, passes along the sides of the body in- 

 ternally to the branch ias, and forms the pallia! nerve-trunk (pi). 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 baud ; 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 distiuct 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 

 Solenogastres, and specially in Chaetoderma (§ 121). 



§ 207. 



The relatively feeble development of the cerebral 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- 

 pasa, Anodonta, Unio, 

 Mytilus, Area, Cardium, 

 Pholas, etc.). A few smaller 

 branches only are given off 

 in addition to the large vis- 

 ceral commissures. There 

 are two pedal ganglia 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, and the distance 

 between it and the anterior 

 parts of the body. When 

 the foot is feebly developed, 

 or when it is produced very 

 Ear forwards, the cerebral 

 and pedal ganglia may be markedly approximated (Solon, Mactra). 

 They may indeed come to be directly approximated, as in Pecten 

 (Fig. 182, c), where the smaller pedal ganglia are placed between 



Fig. 181. Nervous system of Cytherea 

 Chioue. a Snpra-cesophageal (cerebral) gan- 

 glion, b Pedal ganglia, c Visceral ganglion. 

 d Ganglia of the respiratory tabes (siphonal 

 ganglia), ma Anterior, mp Posterior adductor 

 of the shell, p Foot, t Edge of mantle, t' Nerve 

 to edge of mantle, br Branchiae, i Enteric 

 canal, h Liver, r Hind-gat. tr Respiratory 

 siphon, ta Cloacal siphon (after Duvernoy). 



