ily detected gastric ganglion.

[Illustration: FIG. 33.--Horizontal section of the eye of _Sepia_
(Myopsid). (From Gegenbaur, after Hensen.)

KK, Cephalic cartilages (see fig. 8).

C, Cornea (closed).

L, Lens.

ci, Ciliary body.

Ri, Internal layer of the retina.

Re, External layer of the retina.

p, Pigment between these.

o, Optic nerve.

go, Optic ganglion.

k and k', Capsular cartilage.

ik, Cartilage of the iris.

w, White body.

ae, Argentine integument.]

In the Dibranchiate division of the Cephalopoda the greatest
elaboration of the dioptric apparatus of the eye is attained, so that
we have in this class the extremes of the two lines of development of
the Molluscan eye, those two lines being the punctigerous and the
lentigerous. The structure of the Dibranchiate's eye is shown in
section in fig. 14, C, and in fig. 33, and its development in figs. 34
and 37. The open sac which forms the retina of the young Dibranchiate
closes up, and constitutes the posterior chamber of the eye, or
primitive optic vesicle (fig. 37, A, poc). The lens forms as a
structureless growth, secreted by both the internal and external
surfaces of the front wall of the optic vesicle (fig. 37, B, l). The
integument around the primitive optic vesicle which has sunk below the
surface now rises up and forms firstly nearest the axis of the eye the
iridian folds (if in B, fig. 37; ik in fig. 33; Ir in fig. 14), and
then secondly an outer circular fold grows up like a wall and
completely closes over the iridian folds and the axis of the primitive
vesicle (fig. 33, C). This covering is transparent, and is the cornea.
In the oceanic Decapoda the cornea does not completely close, but
leaves a central aperture traversed by the optic axis. These forms are
termed Oigopsidae by C. d'Orbigny, whilst the Decapoda with closed
cornea are termed Myopsidae. In the Octopoda the cornea is closed, and
there is yet another fold thrown over the eye. The skin surrounding
the cornea presents a free circular margin, and can be drawn over the
surface of the cornea by a sphincter muscle. It thus acts as an
adjustable diaphragm, exactly similar in movement to the iris of
Vertebrates. _Sepia_ and allied Decapods have a horizontal lower
eyelid, that is to say, only one-half of the sphincter-like fold of
integument is movable. The statocysts