The visceral ganglion of Pecten, etc. 
25 
This organ is much more highly developed in certain of the gastero- 
poda, than in the lameUibranchiata. In some of the former it resembles 
in structure a gill and was actually described as a rudimentary or ac- 
cessory gill. It is absent In certain forms — some terrestrial Strep- 
toneura and in terrestrial Pulmonates — and when absent its place 
appears to be taken by a rhinophoric ganglion. In the lameUibran- 
chiata the most obvious osphradium, that of Arca, is only visible to the 
naked eye as a pigmented streak, slightly raised above the general surface. 
Usually however the osphradium is invisible to the unaided eye 
and sections are necessary for its recognition. These reveal the presence 
of a patch of modified epithelial cells situated above the branchial nerve 
in dose proximity to the visceral ganglion, and apparently this 
organ is universally present, though it varies somewhat as regards 
development. Very little detailed work has been pubHshed on the struc- 
ture and Innervation of this organ and it was considered as innervated 
by the visceral ganglion. In 1889 however a short paper by Pelseneer 
appeared on the Innervation of the osphradium in Madra and this is 
usually referred to as showing that the sense organ is innervated by the 
cerebral ganglion. The paper was not illustrated by figures and Pel- 
seneer States in it that sections passing through the cephalic extremity 
of the visceral ganglion show, (a) the cerebro-visceral connective, (b) the 
visceral ganglion, (c) the osphradium. Certain fibres from the first 
named nerve pass direct to the osphradium, whilst at the same time 
it is very definitely stated that no fibres from the visceral ganglion termi- 
nate in the osphradium. These Statements are definite enough, but 
in 1891 Pelseneer published another reference to the osphradium, 
with figures of Mactra, Mya, and Pholas and this paper is in the main 
correct. Here however the osphradial nerves are figured as passing to 
the branchial nerve which is stated to traverse the osphradium. I can 
find no evidence in the paper of any actual fibril being traced from the 
cerebro-visceral connective to its end between the cells of the osphradial 
epithelium. As a matter of fact the histology of this organ is not con- 
sidered except that the epithelium is said to be made up of «cellules 
élevées et ciliéesw. The epithelial cells of the osphradium are, however, 
characterised by absence of cilia. List (1902) investigated in great detail 
the osphradium in the Mytilidae and came to the following conclusion. 
The osphradium is innervated by nerves which spring from the visceral 
ganglion and these nerves and their lateral branches contain numerous 
ganglion cells whose fibrillar processes penetrate the osphradial epithe- 
lium. With regard to Pelseneer's view List states that no confirma- 
