KEEVOUS SYSTEM OF MOLLUSCA. 



U7 



other baud, the same nerves in Chastodernia (§ 121) are connected 

 with a terminal ganglion. 



The nervous system of the Scaphopoda is almost exactly similar to 

 that of the Lamellibranchiata. 



Duvernoy, Sur le systeme nerv. des Moll, acephales. Mem. Acad, des Sc. Paris. 

 T. XXIII. 



§ 268. 



Owing to the distinctness of the head, and the development in it 

 of several sensory organs, which are often very highly differentiated, 

 the nervous system of the Gastropoda is distinguished broadly from 

 that of the preceding divisions by the larger size of its cerebral 

 ganglia. These are connected with 

 the ventral parts of the system by 

 a commissure ; in the lowest of the 

 Prosobranchiata — the Zeugobran- 

 chia — the nervous system has many 

 points of similarity with that of the 

 Placophora. For instance, a rudi- 

 ment of a ventral ganglionic chain 

 can be made out in Fissurella 

 (Fig. 183) and Haliotis, inasmuch 

 as the nerves from the pedal ganglia 

 — pedal nerves — are united by 

 transverse commissures. The pallial 

 nerves are not given off directly 

 from the cerebral ganglia, but 

 run below the pedal nerves, and 

 appear to arise from a nervous 

 mass common to themselves and 

 these latter. The epipodium is 

 supplied by the branches of the 

 pallial nerves (pT) . Double commis- 

 sures pass from the cerebral ganglia 

 to the ganglionic mass, which gives 

 off these nerves ; one of them is 

 connected with the pedal nerve- 

 trunks, and the other with the 

 pallial nerves, that is, with the vis- 

 ceral nerves given off from the same 

 point. In the rest of the Proso- 

 branchiata there are no indications 



whatever of any ventral ganglionic chain. There are pedal ganglia 

 in which the ganglionic elements, distributed along the nerve-trunks 

 of Haliotis and Fissurella, are distinctly concentrated. At the same 

 time, the relations of these nerves must be more exactly investigated. 

 A ganglion seems to be given off from the secondary pedal ganglia, 

 and to pass laterally into the commissures, and so to be as much 



Fig. 183. Nervous system of Fissu- 

 rella. C Cerebral ganglia, cs Their 

 commissure, p Pedal nerves, pi Pallial 

 nerves. A Visceral ganglion. B Buc- 

 cal ganglia (after H. v. Jhering) . 



