The visceral ganglion of Pecten, etc. 
5 
from the dorsal surface of the lateral lobes, but quite near the margins, 
so that they almost appear to arise from the sides. In the angle between 
the cerebro-visceral connective and the branchial nerve, but situated 
more superficially than either is a perfectly spherical accessory ganglion 
(fig. 1 acc.g). It has the usuai cortex of ganglion cells and is pigmented. 
This ganghon was not found at ali by Rawitz; and I, owing to an error 
in following its Innervation area, referred to it as the osphradial ganglion 
in a previous paper (1909). This however is incorrect. 
Three nerves enter this ganglion. One brauch connects it with the 
visceral ganglion, the nerve fibres entering with those of the ventral 
side of the connective. Another nerve brauch (fig. 1 hr'') leaves the 
accessory ganghon and enters the branchial nerve. The third and 
largest (fig. 1 vi.n) passes anteriorly and towards the surface and 
enters the visceral mass (the gonad region) where it divides into 
smaller branches. 
It is probable that these ganglia correspond to the accessory ganglia 
occurring in Dreissensia and some other lamellibranchs. 
The branchial nerve (fig. 1 hr.n) is concerned mainly with the Inner- 
vation of the gills, but not far from the visceral ganglion, it receives some 
fibres from another nerve (fig. 1 os.h.n) which is concerned in the Inner- 
vation of the osphradium and significantly enough after this, it gives 
off some small branches (fig. 1 os') which rise to the epithelium and also 
take part in the Innervation of the osphradium. It will be shown later 
that the branchial nerve is almost always in connection with the osphra- 
dium in the lamelhbranchiata. The nerve which arises from the ventral 
surface of the ganghon is perhaps the most extraordinary of ali. I have 
termed it the osphradio-branchial, because though its fibres have not 
been traced to the gills, a brauch from this nerve enters the branchial nerve 
as above described. The nerve arises at about the middle of the lateral 
groove and then crosses over the unpigmented lateral lobe. Some little 
distance from the ganglion it divides and the branches are slightly 
different on the two sides. On the left side the nerve splits, one 
brauch entering the branchial nerve and another approaching the 
osphradial epithelium where it divides into several smaller nerves 
which innervate this organ (fig. 1 os"). On the right side the os- 
phradio-branchial nerve receives a nerve (fig. 1 os.p) which arises in a 
pecuhar position posteriorly and joins it at about the point where the 
branchial nerve brauch is given off. This difference between the two 
sides is of minor importance, for the posterior nerve arises on both sides 
of the ganghon at the posterior ends of the lateral lobes and crosses over 
