In most land and fresh-water Mollusca the eye may be regarded, roughly speaking, as a ball connected by an exceedingly fine thread (the optic nerve) with a nerve centre (the cerebral ganglion). In Paludina this ball is elliptic, in Planorbis and Neritina it is drawn out at the back into a conical or pear shape. In Helix (Fig. [87]) there is a structureless membrane, surrounding the whole eye, a lens, and a retina, the latter consisting of a nervous layer, a cellular layer, and a layer of rods containing pigment, this innermost layer (that nearest the lens) being of the thickness of half the whole retina.
Fig. 87.—Eye of Helix pomatia L., retracted within the tentacle; c, cornea; ep, epithelial layer; l, lens; op.n, optic nerve; r, retina. (After Simroth.)
Comparing the eyes of different Gasteropoda together, we find that they represent stages in a general course of development. Thus in Patella the eye is scarcely more than an invagination or depression in the integument, which is lined with pigmented and retinal cells. The next upward stage occurs in Trochus, where the depression becomes deeper and bladder-shaped, and is filled with a gelatinous or crystalline mass, but still is open at the top, and therefore permits the eye to be bathed in water. Then, as in Turbo, the bladder becomes closed by a thin epithelial layer, which finally, as in some Murex, becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. [88]), which probably corresponds with the ‘vitreous humour’ of other types.
Fig. 88.—Eyes of Gasteropoda, showing arrest of development at successive stages: A, Patella; B, Trochus; C, Turbo; D, Murex; ep, epidermis; l, lens; op.n, optic nerve; r, retina; v.h, vitreous humour. (After Hilger.)
In Nautilus the eye is of a very simple type. It consists of a cup-shaped depression, with a small opening which is not quite closed by the integument. The retina consists of cells which line the interior of the depression, and which communicate directly with the branches of the optic nerve, there being no iris or lens. This type of eye, it will be observed, corresponds exactly with that which occurs in Patella. It appears also to correspond to a stage in the development of eyes in the Dibranchiata (e.g. Octopus, Sepia, Loligo). Lankester has shown[285] that in Loligo the eye first appears as a ridge, enclosing an oval area in the integument. By degrees the walls of this area close in, and eventually join, enclosing the retinal cells within the chamber in which the lens is afterwards developed (Fig. [89]). It thus appears that in some cases the development of the eye is arrested at a point which in other cases only forms a temporary stage towards a higher type of organisation.
Fig. 89.—Three stages in the development of the eye of Loligo; r, r, ridge, enclosing p.o.c, primitive optic chamber; or, orifice between the closing ridges; s.o.c, secondary optic chamber; ci, ci, ciliary body; l, rudimentary lens; R, retina. (After Lankester.)
