142 



COMPARATIVE ANATOMY 



of the supra-cesophageal mass, and represents the visceral commissure, in whicli is 

 found a small visceral ganglion. 



Among the Nudibranchia the two buccal 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 cerebrobnccal connectives, in whose 

 course accessory ganglia may be found. 



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

 a capsule of connective tissue. 



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

 typical ganglia of the Gastropoda. These, grouped 

 together as in so many Opisthobranchia and many 

 Prosohranchia, immediately behind the pharyngeal 

 bulb, form the peri-cesophageal 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 

 close together, lie ventrally. The cerebrojiedal and 

 cerebropleural connectives are consequently always 

 easily distinguished. In Testacella they are even of 

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

 sliape 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 {Baso7iimatophora) it is 

 innervated from the parietal ganglion of the same 

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

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

 (after Bohmig and Leuckart), some- twist on the left ; in the former the right parietal 

 ^vhat aiaKiainniatic, the ganglia ganglion is the larger, and in the latter the left, 

 being in reality less distinct. 1, ™, ,, • , , •,■ , ^ 



Buccal ganglion; 2, optic nerve with ^he .smaller parietal ganglion may also fuse with 

 thickened root (3) arising from the the neighbouring pleural ganglion. Lobes are often 

 cerebral ganglion (4) ; 5, pedal ; (3, formed in the cerebral ganglia, in which certain 

 pleural ; 7, parietal ; 8, visceral gj-otips of nerves have their origin. The pedal corn- 

 gang ion. niissure 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 cerebrobnccal 

 connectives. 



Fig. 123.— Central portion of the 

 Nervous System of Helix pomatia 



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 tlie 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 



