30 
W. J. Dakiu 
remain outside the cortex of ganglion cells though within the sheath 
of the ganghon. In this way they pass round and enter the branchial 
nerve at its base. There is at present no evidence to show that these 
fibres innervate the osphradium to the exclusion of fibres from the vis- 
ceral ganglion which form the majority in the branchial nerve. Probably 
in all forms certain fibres from the cerebro-visceral connectives pass 
through the visceral ganglion and leave by means of branchial and palliai 
nerves. In Mactra and Spisula the fibres pass more directly but it would 
not be possible to say which fibres innervated the osphradium unless 
one fibre could be followed continuously from the epithelium to the gang- 
lion cell and from thence to its destination. So far this has not been 
done, but no fibres leave the cerebro-visceral connective and pass c 1 e a r 
from the branchial nerve and its ganglion cells, to the os- 
phradial epithelium. 
Venus Casina. The two visceral ganglia are fused together 
mà situated in the usuai position on the posterior adductor muscles 
The cerebro-visceral connectives (fig. 19 c.v.con), the branchial nerve. 
(hr.n) and the two posterior palliai nerves {p. pali) are again the most 
important nerves leaving the visceral ganglion. The branchial nerves 
leave the latter at the anterior ends and lie directly above the cerebro- 
visceral connectives for the first part of their course. The osphradial 
epithelium is to be found over the branchial nerves and the two osphradia 
are separate. The cells are not distinguished by greater depth so much 
as is usually the case, but the cuticular non cihated margin is distinct 
and there are often yellow pigment granules present. The osphradial 
fibres rise from the ganglion cells on the ventral surface of the branchial 
nerve. Like Spisula though not so distinct, a certain number of the 
nerve fibres of the cerebro-visceral connective remain outside the cortex 
of ganglion cells of the visceral ganglion, but at the same time within 
the sheath and contour of the latter. In this way they pass directly 
into the branchial nerve. 
There are more scattered nerve cells to be found in the cerebro- 
visceral connectives of Venus than in most of the species examined but 
there is a very distinct difference between these nerves and the branchial, 
with their ganglion cell cortex. The conditions described above are re- 
produced almost exactly in Tapes pullastra which also belongs to the 
family Veneridae, so that further description is unnecessary. 
C a r diu m e dui e. The osphradia have not been referred to 
before in this genus. They lie in the usuai position above the branchial 
nerves but are not to be detected without the aid of sections. The two 
