The visceral ganglion of Pecten, etc. 
33 
Speeles figured by him. In P. dadylus however the fibres of the branchial 
nerve run together up to the visceral ganglion and there, as in some other 
species, certain fibres can be traced through the ganglion to the cerebro- 
visceral connectives. There is a thick cortical layer of ganglion cells on 
the ventral surface of the branchial nerve and from these cells fibres 
pass to the osphradium which lies immediately above the nerve (fig. 25). 
It is easy to see that the nerve endings in the osphradium are in 
connection with these ganglion cells, and also easy to see that processes 
of the latter pass into the branchial nerve. Jt is however difficult to 
follow the complete course of these fibres in the latter, and it is only 
possible to say as before that the branchial nerve is composed of nerve 
fibres from visceral ganglion and cerebro-visceral connectives. As the 
nerve endings are comparatively distinct when sections are stained with 
the more ordinary methods, I append a more detailed description of 
the osphradium of this species. List who is the only other author to de- 
scribe the histology of the osphradium in the lameUibranchiata concludes 
by stating that the cells of the osphradial area are to be distinguished 
frpm the other cells of the epithelium of the ventral body wall, by a com- 
plete absence of cilia, a distinct marginai cuticle, and an increased depth. 
This is usually the case but in some species of which Pholas is one, the 
last of these features is not present. The absence of ciHa and the distinct 
cuticle or rather modified outer edge of the cells, are very characteristic. 
The osphradial epithelium consists of a single layer of very compact 
cells, without vacuoles. These cells are about twice as deep as they are 
broad and the cuticular margin which is clear and does not readily stain, 
is about Ve depth of the cells. The surrounding cells are more vacuo- 
lated and about twice the breadth of the osphradial cells. The cytoplasm 
has a great affinity for stains and appears deeply coloured after methyl 
blue eosin or iron haematoxylin. The cells contain numerous small gra- 
nules which stain intensively with the latter stain. This unfortunately 
renders the nerve endings much more difficult to trace by reason of the 
dark background. 
The nucleus is situated at about the centre of the celi, if anything, 
rather nearer the surface. It is ovai in shape, but almost spherical, and 
contains chromatin granules and a small nucleolus. The cuticular margin, 
as stated above, remains usually clear, and appears crossed by delicate 
fibrils running perpendicularly to the surface. These were after some 
difficulty found to be endings of nerve fibrillae. The most proximal 
nerve fibres innervating the osphradium arise from ganglion cells on 
the margin of the visceral ganglion. The figure (fig. 25) illustrates the 
Mittheilungen a. d. Zool. Station zu Neapel. Bd. 20,1. 3 
