The visceral ganglion of Pecten, ctc. 
29 
already pointed out that Pelseneer does not mention the branchial 
nerves at ali in his first account and states that no fibres from the vis- 
ceral ganglion pass to the osphradium. His figure in the second paper 
is perfectly correct as far as it goes. The two visceral gangHa lie touching 
one another in their usuai position on the posterior adductor muscle. 
The cerebro-visceral connectives enter the anterior ends of the ganglia 
and are somewhat deeply situated. The branchial nerves arise laterally 
not far from the anterior ends and pass immediately towards the sur- 
face. The osphradia are to be found above the branchial nerves. There 
is, as Pelseneer correctly observed a very definite bundle of fibres 
from the cerebro-visceral connective which passes round the visceral 
ganglion from the point where the connective enters, to the point of 
origin of the branchial nerve. Since these fibres reach the branchial nervo 
where it has turned towards the epithelium, it almost appears as if the 
fibres from the cerebro-visceral connective had taken a direct course 
for the osphradial epithelium situated nearest the visceral ganglion. 
There are ganglion cells situated along the ventral surface of the bran- 
chial nerves for a considerable distance from the ganglion, from which 
processes pass to the osphradial epithelium (fig. 15). It will be seen 
therefore that as in other cases the branchial nerve is the path by means 
of which the osphradial fibres reach the sense organ. It has not yet been 
shown however whether the fibres from the osphradial ganglion cells 
enter the visceral ganglion exclusively or the cerebro-visceral connec- 
tive, though from my sections I should be inclined to believe that both 
visceral and cerebral gangha are concerned. 
S pi sul a solida. This being a representative of one of the 
allied genera to Mactra is interesting for purposes of comparison. The 
conditions observed resemble Mactra very closely. The visceral ganglia 
are fused forming a single ganglion situated on the posterior adductor. 
The branchial nerves (fig. 21 hr.n) are the most superficial and leave the 
ganglion at the anterior lateral horns. They turn very soon almost at 
right angles to pass towards the branchial axis. The cerebro-visceral 
connectives (fig. 21 c.v.con) are situated more deeply and pass anteriorly, 
and the posterior palliai nerves (p.pall) are the only other very pro- 
minent nerves present. The osphradia are situated again, above the 
branchial nerves from which arise the innervating fibres. The most 
proximal of these arise at the point of Insertion of the branchial nerves 
into the visceral ganglion. Just however as the cerebro-visceral connec- 
tive reaches the ganghon certain of its fibres remain in a way outside, 
that is to say, they do not pass into the interior of the ganglion but 
