134 COMPARATIVE ANATOMY chap. 



snout. Even when the auditory organs are found in close proximity 

 to the pedal ganglia, or in close contact with them, they receive their 

 nerves from the cerebral and not from the pedal ganglia. 



2. The pedal ganglia supply nerves to the musculature of the foot, 

 and occasionally to the columellar muscle also {Patella). 



3. The pleural ganglia send nerves chiefly to the mantle, the 

 columellar muscle, and the body walls lying behind the head. 



4. The parietal ganglia innervate the ctenidia and osphradium, 

 and also send some nerves to the mantle. 



5. The visceral ganglia supply nerves to the viscera. The con- 

 nectives and commissures also may give ofi' nerves which belong to the 

 areas innervated by the neighbouring ganglia. 



6. The buccal ganglia, which will be described below, innervate 

 the muscles of the pharynx, the salivary glands, the oesophagus, the 

 anterior aorta, etc. 



A comparison of the typical nervous system of tlie Gastropoda with that of the 

 Amphincura reveals the following homologies : — 



1. The cerebral ganglia of the Gastropoda correspond with the cesophageal ring 

 of Chiton, with tlie exception of the central portion of its lower half ; and further 

 with the cerebral ganglia of the Solenogastres. 



2. The pedal ganglia of the Gastropoda answer to the pedal cords in the Am- 

 2)hineura, concentrated each into a single ganglion. The arrangement in the Dioto- 

 cardia, which are the more primitive Prosobranchia, is very interesting in this con- 

 nection ; in the Diotocardia the pedal ganglia are continued posteriorly as two 

 true pedal cords, which, like those of the Amphineura, are connected by transverse 

 commissures. 



It is more diHieult to compare the pleural, parietal, and visceral ganglia of the 

 Gastropoda with nerves found in the Amphincura. The most satisfactory view 

 seems to lie that this whole complex of ganglia, together with its connectives, corre- 

 sponds with the pleurovisceral cords of Chiton. The areas of innervation coincide, 

 these being the mantle, ctenidia, osphradia {Chiton?), and viscera. 



3. If tliis last assumption is correct, the pleural ganglion must be supposed to 

 have arisen by the concentration into one ganglion of that part of the pleurovisceral 

 cord of Chiton which contains the pallial ganglionic cells, this concentration having 

 taken place at the anterior end of the cord, where it leaves the esophageal ring. If, 

 then, the two component portions of each side of the ring, the cerebropedal and the 

 pleural, move further apart, and at the same time the cerebral and pedal ganglia of the 

 ring become more individualised as ganglia, a double cerebropedal connective comes 

 into existence on each side. One of these connectives shows no ganglion in its course, 

 and is the true cerebropedal connective of the Gastropoda. The second, however, has 

 the pleural ganglion in its course, and from this latter spring the visceral cords ; this 

 second connective is thus divided into a cerebropleural and a pleuropedal connective. 



4. Chiton has numerous gills on each side, each of which receives two nerves 

 from the pleurovisceral cord near it. The Gastropoda have at the most two gills, 

 one on the right and one on the left. In correspondence with this reduction, the 

 ganglionic cells of the pleurovisceral cords belonging to the branchial nerves of 

 Chiton have become concentrated on each side into a single ganglion belonging to the 

 single gill. The ])arietal ganglion is thus accounted for. That portion of each 

 pleurovisceral cord which lies between the pleural and the parietal ganglia becomes 

 the pleuroparietal connective, which consists of fibres only without ganglion cells. 



