142 



COMPARATIVE ANATOMY 



CHAP. 



of the supra-uisophageal mass, and represents the visceral commissure, in which is 



found a small visceral ganglion. 



Among the Nudibranchia the two bnccal ganglia are always found on the 



posterior and lower wall of the pharynx. They are connected with each other by a 



buccal commissure, and with the brain by two cerebrobuccal connectives, in whose 



course accessory ganglia may be found. 



The whole peri-cesophageal complex of ganglia is in the Nudibranchia enclosed in 



a capsule of connective tissue. 



III. Pulmonata (Fig. 123). The central nervous system here possesses all the 



typical ganglia of the Gastropoda. These, grouped 

 together as in so many Opisthobranchia and many 

 Prosobranchia, immediately behind tin-, pharyngeal 

 bulb, form the peri-OMOphageal complex, into which 

 even the parietal and visceral ganglia have been 

 drawn. The cerebral ganglia lie close to each other 

 dorsally, and all the other ganglia, which are also 

 .lose together, lie ventrally. The cerebropedal and 

 oerebroplenn] connectives are consequently always 

 easily distinguished. In Testacella they are even of 

 some length, in adaptation, no doubt, to the special 

 shape and the great development of the pharyngeal 

 bulb. All other connectives and commissures, on the 

 contrary, are much shortened, so that the ganglia 

 connected by them lie close together. A visceral 

 ganglion is always found, and usually also in each 

 pleurovisceral connective a parietal ganglion. When 

 an osphradium is present (Basommatophora) it is 

 innervated from the parietal ganglion of the same 

 side. In Pulmonata with a dextral twist, the osphra- 

 dium lies on the right, and in those with a sinistral 

 twist on the left ; in the former the right parietal 

 ganglion is the larger, and in the latter the left. 

 The smaller parietal ganglion may also fuse with 

 the neighbouring pleural ganglion. Lobes are often 

 formed in the cerebral ganglia, in which certain 

 visceral groups of nerves have their origin. The pedal com- 

 missure is often double. Buccal ganglia are always 



found. They lie posteriorly on the pharynx below the oesophagus, and are joined to 



one another by the buccal commissure and to the cerebral ganglia by cerebrobuccal 



connectives. 



C. Scaphopoda. 



The nervous system of the Scaphopoda (Fig. 101, p. 113) is 

 symmetrical ; the visceral connectives are not crossed. The two 

 cerebral ganglia lie very near one another in front of (or, if the 

 intestine is regarded as horizontal, above) the gullet over the snout ; 

 the two pedal ganglia, close to one another, lie on the anterior side 

 of the foot, more or less at its centre, and are joined to the cerebral 

 ganglia by two long cerebropedal connectives. The two pleural 

 ganglia lie close to and above the cerebral ganglia, so that the 

 cerebropleural connective is very short. The pleuropedal connective 



l'i<;. i-j:5. Central portion of the 

 Nervous System of Helix pomatia 

 (after Bohmig and Leuckart), some- 

 what diagrammatic, the ganglia 

 being in reality less ilistinct. 1, 

 Buccal ganglion ; 2, optic nerve with 

 thickened root (3) arising from the 

 cerebral ganglion (4) ; 5, pedal ; 6, 

 pleural ; 7, parietal ; 

 ganglion. 



