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. [Illustration: 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. [Illustration: 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.)] [Illustration: FIG. 90.--Eye in =A=, _Loligo_; =B=, _Helix_ or _Limax_; =C=, _Nautilus_: _a.o.c_, anterior optic chamber; _c_, cornea; _int_, integument; _ir_, iris; _l_, lens; _l´_, external portion of lens; _op.n_, optic nerve; _op.g_, optic ganglion; _p.o.c_, posterior optic chamber; _r_, retina. (After Grenacher.)]