elemnitidae_, &c., which have a small
chambered shell resembling that of _Nautilus_ with or without the
addition of plate-like and cylindrical accessory developments (fig. 17,
A, C, fig. 19).

The pair of cephalic eyes are highly-developed vesicles with a
refractive lens (fig. 33), cornea and lid-folds,--the vesicle being in
the embryo, an open sac like that of _Nautilus_ (fig. 34). Osphradia are
not present, but cephalic olfactory organs are recognized. One or two
pairs of large salivary glands with long ducts are present. An ink-sac
formed as a diverticulum of the rectum and opening near the anus is
present in all Dibranchiata (fig. 25, t), and has been detected even in
the fossil _Belemnitidae._ Branchial hearts are developed on the two
branchial afferent blood-vessels (fig. 28, _vc'_, _vi_).

[Illustration: FIG. 16.--Decapodous Cephalopods.

A, _Cheiroteuthis Veranyi_, d'Orb. (from the Mediterranean).

B, _Thysanoteuthis rhombus_, Troschel (from Messina).

C, _Loligopsis cyclura_, Fer. and d'Orb. (from the Atlantic Ocean).]

[Illustration: FIG. 17.--Internal Shells of Cephalopoda.

A, _Conoteutliis dupiniana_, d'Orb. (from the Neocomian of France).

B, Shell _Sepia orbigniana_. Fer. (Mediterranean).

C, Shell of _Spirulirostra Bellardii_, d'Orb. (from the Miocene of
Turin). The specimen is cut so as to show in section the chambered
shell and the laminated "guard" deposited upon its surface.

D, Shell of _Splrula laevis_, Gray (New Zealand).]

In the Dibranchiata the shell shows various stages of degeneration,
culminating in its complete disappearance in _Octopus_. As in other
Mollusca, there is a tendency in Cephalopods for the mantle to extend
over the outside of the shell from its edges, and when these secondary
mantle-folds entirely cover the shell and meet or fuse together the
shell is surrounded by the mantle both externally and internally, and is
said to be internal, though it remains always a cuticular structure
external to the epidermis. This procebs is generally accompanied by a
reduction of the size of the shell in comparison with that of the body,
so that the relations of the two are gradually reversed, the body
outgrows its house and instead of the mantle being enclosed by the
shell, the shell is enclosed by the mantle. The earliest stage of this
process is shown in the recent _Spirula,_ though it is perhaps not
impossible that in some of the later fossil Ammonoids the shell