38 Moll 
NOLLUSCA. 
eyes occur in which the retina is fully developed, though the pigment is 
continuous across the pupil. (1) The external surface is covered with 
pigmented epithelium up to the cornea, where it suddenly becomes much 
thinner ; (2) the cornea consists of columnar cells ; (3) the iris of larger 
cells filled with pigment ; (4) the stalk is of loose tissue and has enor- 
mous blood spaces, with erector and depressor muscles, which are replaced 
anteriorly by fibres forming a hyaline pseudocornea ; the lens consists of 
* a group of mesoderm cells suspended in a large blood sinus. The poste- 
rior portion of the organ is a thick concave disc enclosed in a mem- 
branous ommateal sac, the cells within which form a closed vesicle with 
obliterated central cavity. (1) The anterior wall consists of : (a) an 
outer ganglionic layer; (b) an inner ganglionic layer; ( c ) the retino- 
phora ; (d) rods containing the retinidia. 2. The posterior wall is com- 
posed of : (n) an outer vitreous network ; (5, c) an argentea made up of 
two modified layers of cells ; ( d ) a red tapetum. The optic nerve 
divides into two branches : one enters the eye axially, the fibres forming 
the axial nerve-fibres of the retinophorae ; the other ascends towards the 
shell side of the retina, its fibres uniting with the ganglionic layers. The 
development of the eye in Pecten is by transitory cups, probably homo- 
logous with the invaginated eyes of Area. Other species are also investi- 
gated. In the theoretical part of the work the Molluscan ommatidium is 
defined as consisting of a doublo cell, the retinophora, with two nuclei 
and an axial nerve-fibre. Four types of eyes are recognised — diffuse, 
invaginate, facetted, and pseudo-lenticulate — and the theory is broached 
that eyes have originated from “ heliophags,” or organs for the absorp- 
tion of solar energy. This memoir has been criticised in the Quart. J. 
Micr. Sci. xxvii, p. 285 ; the author’s reply will be found in Zool. Anz. 
x, pp. 256-261, 1887. 
The “ eye ” of Pecten is probably a phosphorescent organ ; Sharp. 
In Pecten jacobceus the thin peripheral margin of the layer of p ; gment 
cells passes into the retina, the sensitive region of the optic vesicle being 
turned away from the light. This difference from other Mollusca may 
be accounted for by the lens in the latter being a secretion formed with- 
in the optic vesicle, whilst in Pecten , as in the Vertebrata , it is of cellular 
origin, and developed externally to the vesicle ; Butschlt (2). 
The retina of Helix contains both pigmented and colourless cells ; the 
latter are flask-shaped, and their contents do not stain, but harden to a clear 
gray mass in osmic acid ; they are not regularly polygonal, but interca- 
lated between the others, in all forms from a polygon to a star. The 
pigmented cells have a clear central space and a homogeneous cell-body, 
very different from that of the others ; the centre is highly refractive ; 
Carriers. 
Onchidium chameleon has small papillae, and no eyes, while 0. 
ddmelii has three on each papilla ; the eyes multiply by division, semi- 
detached examples not being rare. The lens consists of one cell, 
and a sphincter muscle for accommodation is present. Below the ganglion- 
cells of the optic nerve are others which contain highly refractive plano- 
convex bodies, and below these again other elongated hexagonal cells 
surrounded by pigment-cells. In a space at the bottom of each hexagonal 
