150 Prof. G. J. Allman on the Anatomy of Acton. 
pharyngeal ganglia, are placed upon the cesophagus just as this 
organ emerges from the buccal mass; but the means by which 
they are connected with the other parts of the nervous system, I 
have not succeeded in detecting. 
The great supra-cesophageal pair seems to be chiefly destmed 
for the organs of sense. The optic nerves, Pl. VI. p, arise from 
this pair, and large nerves, ¢ 7, are sent off from it to the tenta- 
cula and lips and the parts immediately surrounding the mouth. 
Some nerves going to the generative organs would also seem 
traceable to the same source, while the digestive and other sy- 
stems would appear to derive their nerves, s, from the remainder 
of the cesophageal mass. 
Vision.—The organs of vision are placed at a short distance 
behind the tentacula, and to the naked eye present the appear- 
ance of minute black points ; with the assistance of the micro- 
scope, the followmg parts may without difficulty be detected in 
them. The first thing which strikes us is a pigmental body, 
Pl. VII. fig. 2 a, of a somewhat pyriform figure, furnished poste- 
riorly with a prolongation, at one side of which the optic nerve, 
b, may be seen entering. Whether this body contains a cavity 
with a retinal expansion I cannot assert as the result of observa- 
tion, though all analogy would lead us to suppose such to be the 
case. 
The pigment is of a deep black colour, and is doubtless in- 
tended to effect the absorption of all rays of light which may 
happen to fall on it. A crystalline lens, c, of a nearly spherical 
figure and powerfully refractive, is imbedded in the anterior ex- 
tremity of the pigmental body, and surrounding the whole appa- 
ratus is a transparent capsule, d, perforated posteriorly by the 
optic nerve, and apparently performing the function of a cornea 
anteriorly. 
Hearing.—The organs detected by Siebold in several mollusca, 
and supposed by this anatomist to be destined for the function of 
hearing, are demonstrable without much difficulty in Acton. 
They look like appendages to the cerebral system, and present 
the appearance of nearly spherical bodies, Pl. VII. fig. 1 gg, with 
a transparent refractive nucleus, and supported upon a club- 
shaped peduncle. They would seem to consist of two spheres 
concentric with the nucleus, which become apparent when a short 
exposure has caused some degree of collapse in the bodies from a 
slight evaporation of their contents. The nucleus, which corre- 
sponds with the otolites observed by Siebold, is single, and did 
not present the peculiar oscillatory motion manifested by the oto- 
lites of other mollusca. The peduncles arise between the first 
and second pair of ganglia, but with which of these they are more 
intimately connected I have not been able to satisfy myself. 
Touch.—Though the sense of touch is without doubt univer- 
ak 20 ‘as neil 
