14 
W. J. Dakin 
and leave it by a peripheral nerve. A third group of fibres have been 
lost either because they break up into the elementary fibrillae of the 
neuropil, or because of the difficulty of foUowing them through the latter. 
Just before entering the ganglion some of the fibres of the cerebro-vis- 
ceral connective pass directly into the branchial nerve (textfig. Wi). 
The most ventrally situated fibres in the connective pass towards the 
middle line and the anterior edge of the ventro-central lobe. Others 
can be traced to large ganghon cells situated near the dorsal wall of the 
ganglion at the point of entrance of the connective (textfig. mg), and as 
axones, directly to the large ganglion cells of the anterior end of the dorso- 
central lobes (textfig. m^). A number of fibres from the cerebro-visceral 
connectives pass along the same side of the ganglion and leave it by way 
of the palliai nerves (^2). Another thin bündle of the connecti vefibres 
can be traced through the ganglion and out by the osphradio-branchial 
nerve {n^). Finally some of the fibres can be traced as axones to the 
ganglion cells situated in the lateral regions and a certain proportion 
pass through the ganglion to the opposite side from their point of entry, 
where by analogy with other lamellibranchs, they probably are in direct 
communication with ganglion cells. 
The palliai nerves as mentioned above contain some nerve fibres 
from the cerebro-visceral connectives. They also contain fibres (text- 
fig. mg) which are axones of large, probably motor cells, situated in the 
dorsocentral lobes. 
They contain further a number of fibres (textfig. 0) which arise in the 
lateral lobes in a way to be described later and which are probably sensory. 
The osphradio-branchial nerve contains some fibres from the cerebro- 
visceral connectives, but most arise in the visceral ganglion and of these, 
some can be traced from a point in the ventro-central lobe near the ventral 
surface and somewhat posterior, whilst others are soon lost in the neuro- 
pil. No fibres of the osphradio-branchial nerves entered the lateral lobes, 
as far as could be made out from my sections. 
The most definite fibres in the neuropil are the thick axones of the 
muscle nerves (textfig. m-^^m^ which leave the ganglion on the dorsal 
surface. These are motor fibres and it is interesting to trace them to 
the large ganglion cells of the pigmented dorso-central lobes. The muscle 
nerves contain axones from cells situated even at the anterior ends of these 
lobes, and they pass in two well defined tracts posteriorly through the 
ganglion. At about the middle of the ganglion, there is a very definite 
Crossing of the fibres, so that the left muscle nerves contain some fibres 
from the right side of the ganglion and vice versa. 
