SEA-FISHERIES LABORATORY. 419 
lie chiefly on the side nearer the gill filaments. The 
branchial nerve gives off 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 Pallial System is supphed 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 supplhed, together with its important 
muscular system. It consists of a large nerve which runs 
parallel with the mantle edge, just interior to the palhal 
wer) Vhis, the circumpallial nerve (ig. 26, V.c.) 1s 
physiologically a ganglion. It is well supplhed 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 circumpallal nerve branches arise to 
innervate the eyes and tentacles (fig. 4, V.c.); the optic 
nerves will be described in the chapter on the eye. Hach 
of the long extensible outer tentacles is supplied with a 
nerve which runs up to the centre and gives branches to 
