LAWSON, ON LIMAX MAXIMUS. 29 
Organ of Vision—The eyes are two in number, and are 
situated, one at the extremity of each superior tentacle, and 
are recognisable as a pair of black spots within the membranes 
by which they are surrounded. By delicate manipulation 
the eye, together with the tentacular and optic nerve, may be 
separated from the surrounding darkly stained connective 
tissue, and then is seen the origin of the nerve of sense from 
the extremity of the tentacular branch. The latter, just 
before it terminates in the end of the tentacle, expands into 
five or six short, thick, and unequal divisions, thus exhibiting 
a palmate end, the fingers being arranged in such a position 
as would be assumed by those of the human hand, when 
grasping a large ball, and from the centre of the palm, so 
constituted, a very fine nervous thread travels to the eyeball, 
the intervening distance being exceedingly short. This at- 
tenuate nerve having reached the eye, apparently enters the 
posterior part of the sphere, but, so far as I could observe, it 
becomes blended with the membrane of the ball, which is a 
connective-tissue structure, and after the choroidal pigment 
has been completely removed, the two tissues, those of the 
optic nerve and sclerotic, appear, not only continuous, but 
identical in structure, and this peculiarity, although at first 
sight anomalous, is at once appreciable by an appeal to in- 
vestigations, of my friend, Prof. Beale,* into the structure and 
homologies of connective tissue. Indeed, I very much doubt 
that any structure resembling a retina, has ever been observed 
in pulmonates, and this idea is borne out by the fact that, 
Von Siebold, in speaking of the organs of vision generally, 
among cephalopora, writes, “The internal surface of the 
choroid is covered by a whitish pellicle, which is wndoubtedly 
the retina ;”? adding afterwards, ‘“ Kihn affirms that he has 
seen this white pellicle in Paludina,” as if he hesitated to 
accept the onus probandi himself. I confess I am sceptical 
as to its existence, having never observed the faintest trace of 
it myself. The sclerotic membrane forms a more or less 
spherical sac, which is quite transparent at a point opposite 
the apparent penetration of the nerve; internally this sac is 
lined with exceedingly fine, black, granular pigment, which, so 
far as I could observe, is not enclosed in cells, but is bedded 
in the inner wall of the sclerotic, and for the most part is 
disposed in regular lines, long and short alternately, which 
assume the horizontal position. The eyeball owes its globu- 
lar form to being filled with a thick, tenacious, perfectly 
transparent, vitreous humour ; this is very well observed by 
* See ‘Quart. Journ. of Micros. Science’ for Oct., 1861; ‘ Med.-Chir. 
Review,’ Oct., 1862; ‘ Archives of Medicine ;’ &c. 
