548 LECTURE XXU. 



the complex tongue, which are so many accessory parts appended to 

 the simple opening of the gullet, with which the alimentary canal 

 commences in the bivalves. The additional ganglia in question are 

 placed below the pharynx, and are brought into communication with 

 the sentient centres by a filament continued downwards and forwards 

 from each of these ganglia : they also inter-communicate by a loop 

 which forms a third azygos rudimentary ganglion beneath the 

 oesophagus completing an anterior ring corresponding to that which 

 is formed by means of pedial ganglion posteriorly. 



The ganglions corresponding to the pedial pair in the BullcBa ap- 

 pear not to be joined together by a transverse band, but to be con- 

 nected only with the branchial ganglion, and through them with the 

 cerebral ones. The three are placed so close together, that Cuvier 

 describes them as forming one mass. There are two pharyngeal 

 ganglia formed upon filaments descending from the cerebral ganglions. 

 The labial ganglions, which are developed in addition to the pedial 

 ganglions, originate from the latter in the Sulla lignaria and are 

 connected, through them, with the cerebral ring. 



In the Haliotis, the superior or oesophageal part of the oesophageal 

 circle is still a simple commissural chord. The sides of the circle are 

 formed by a double chord, which unite below in a single branchio- 

 pedial ganglion ; from which the visceral, as well as the branchial and 

 muscular, nerves radiate. The cerebral ganglia are distinct and 

 connected by a commissure in Janthina, Turbo, Lymneus, and 

 Planorbis. The sub-oesophageal mass in the Limnceus stagnalis is 

 of an orange-colour, and consists of seven ganglions united together 

 by a loose cellular tissue. 



In the Doris and Onchidium the cerebral, pedial, and branchial 

 ganglions have coalesced into one annular mass, which, however, is 

 chiefly super-ocsophageal in its position, united below, by a slender 

 chord passing across the under parts of the oesophagus. Two small 

 nerves are given off, which descend and form two small pharyngeal 

 ganglia, which, according to Cuvier, are united together. In the 

 Doris Solea the quadripartite character of this large mass is however 

 obvious. The olfactory ganglia are sessile in front of the cerebrals. 

 This is the general character of the nervous centres in other Nudi- 

 branchiate and Apneustal Gastropods. The optic ganglia are at the 

 back part of the cerebral, on which the acoustic capsules are sessile. 

 The pharyngeal ganglia complete a small ring about the beginning 

 of the oesophagus, and the pedial, or chief sub-oesophageal masses, 

 with their intervening commissure, form the second and chief ring. 

 The branchial ganglia are behind the cerebral ; they supply the 

 skin of the back and the gills : beneath and sessile on their front 



