ts of an optic
nerve. Such is the structure of the eye of the limpet (_Patella_), and
in such a simple eye we obtain the clearest demonstration of the fact
that the retina of the Molluscan cephalic eye, like that of the
Arthropod cephalic eye and unlike that of the vertebrate myelonic eye,
is essentially a modified area of the general epiderm, and that the
sensitiveness of its cells to the action of light and their relation
to nerve-filaments is only a specialization and intensifying of a
property common to the whole epiderm of the surface of the body. What,
however, strikes us as especially remarkable is that the simple form
of a pit, which in _Patella_ serves to accumulate a secretion which
acts as a refractive body, should in _Nautilus_ be glorified and
raised to the dignity of an efficient optical apparatus. In all other
Mollusca, starting as we may suppose from the follicular or pit-like
condition, the eye has proceeded to acquire the form of a _closed_
sac, the cavity of the closed vesicle being then filled partially or
completely by a refractive body (lens) secreted by its walls (fig. 14,
B). This is the condition attained in most Gastropoda. It presents a
striking contrast to the simple Arthropod eye, where, in consequence
of the existence of a dense exterior cuticle, the eye does not form a
vesicle, and the lens is always part of that cuticle.

[Illustration: FIG. 14.--Diagrams of Sections of the Eyes of Mollusca.

A, _Nautilus_ (and _Patella_).

B, Gastropod (_Limax_ or _Helix_).

C, Dibranchiate Cephalopod (Oigopsid).

Pal, Eyelid (outermost fold).

Co, Cornea (second fold).

Ir, Iris (third fold).

Int 1,2,3,4, Different parts of the integument.

l, Deep portion of the lens.

l^1, Outer portion of the lens

Co.ep, Ciliary body.

R, Retina.

N.op, Optic nerve.

G.op, Optic ganglion.

x, Inner layer of the retina.

N.S., Nervous stratum of the retina. (From Balfour, after
Grenacher.)]

The development of _Nautilus_ is still entirely unknown. Dr Arthur
Willey, during his sojourn in the East Indies, made special efforts to
obtain fertilized eggs, both by offering rewards to the native
fishermen and collectors and by keeping the living adults in
captivity, but without success.

_Phylogeny and Classification._--As _Nautilus_ is the only living
genus of the Tetrabranchiata