HI 



lie chiefly on tlie side nearer the gill filaments. The 

 branchial nerve gives oii: laterally, from its ventral 

 surface, small nerves, at frequent intervals along its 

 whole length. These pass towards the base of the gill 

 filaments at the sides of the afferent branchial vessel, until 

 the two longitudinal ctenidial muscles are reached. They 

 are continued between the muscles and the epidermis to a 

 point about level with the middle of the muscle bundles, 

 where they bear a considerable number of ganglion cells. 

 From the ganglia thus formed, nerves arise, which extend 

 in almost a continuous sheet to the bases of the filaments, 

 down each of which a delicate branch passes. Thus, the 

 gills have a very thorough nerve innervation, and are 

 probably very sensitive, though specially differentiated 

 sense cells have not been seen. 



The Pallia! System is supplied chiefly by the visceral 

 ganglia. It is the most extensive in the body, and this is 

 to be expected considering the array of sense organs with 

 which the mantle is supplied, together with its important 

 muscular system. It consists of a large nerve which runs 

 parallel with the mantle edge, just interior to the pallial 

 artery. This, the circumpallial nerve (fig. 26, N. c.) is 

 physiologically a ganglion. It is well supplied with 

 ganglion cells, and is thickest in the middle of its course. 

 Anteriorly and posteriorly the nerves become very fine, 

 and eventually reaching the hinge line the nerves of both 

 valves become continuous, so that the circumpallial nerve 

 is one continuous cord, which is much attenuated in two 

 places at the hinge line, anterior and posterior, respec- 

 tively. From the circumpallial nerve branches arise to 

 innervate the eyes and tentacles (fig. 4, N. c.) ; the optic 

 nerves will be described in the chapter on the eye. Each 

 of the long extensible outer tentacles is supplied with a 

 nerve which runs up to the centre and gives branches to 



