in
_Loliginidac_ (fig. 21) and _Sepiolidae_. Lastly, in the Octopoda the
shell is represented only by small chitinous rudiments to which the
retractor muscles of the head and funnel are attached; these are paired
in _Octopus_, unpaired in other cases as in _Cirrhoteuthis_.

[Illustration: FIG. 20. FIG. 21. FIG. 20.--The calcareous internal shell
of _Sepia officinalis, _the so-called cuttle-bone, a, Lateral expansion;
b, anterior cancellated region; c, laminated region, the laminae
enclosing air.]

[Illustration: FIG. 21.--The horny internal shell or gladius or pen of
_Lohgo_.]

The early appearance of the sac of the mantle in which the shell is
enclosed has led to an erroneous identification of this sac with the
primitive shell-sac or shell-gland of the Molluscan embryo. The first
appearance of the shell-sac in Dibranchiata is shown in figs. 35, 36.
Its formation as an open upgrowth of the centro-dorsal area, and the
fact that it appears and disappears without closing in _Argonauta_ and
_Octopus_, was demonstrated by E. Ray Lankester.

[Illustration: FIG. 22.--The Argonaut in life. (After Lacaze-Duthiers)
Tr. Float: Br.a, anterior arms: Br p, posterior arms: V, the expanded
portion of them, once called the sails; B, the beak; C, the shell; En,
the Funnel.]

In _Argonauta_ (the paper nautilus) the female only possesses a shell,
in which the body is contained; but this is not homologous with the
true shell in other cases; it is a structure _sui generis_ secreted by
the expanded arms of the dorsal pair which are closely applied to it on
either side (fig. 22).

[Illustration: FIG. 23.--Head and circumoral processes of the
fore-foot of _Onychoteuthis_ (from Owen).

a, Neck.

b, Eye.

c, The eight short arms.

d, Long prehensile arms, the clavate extremities of which are
provided with suckers at e, and with a double row of hooks beyond at
f. The temporary conjunction of the arms by means of the suckers
enables them to act in combination.]

[Illustration: FIG. 24.--Male of _Ocythoe catenulata_, Steenstrup
(_Octopus carena_, Ver.), showing the hectocotylized arm. (From
Gegenbaur.)

t^1, t^2, t^3, t^4, The first, second, third and fourth arms or
processes of the fore-foot.

h, The third arm of the right side hectocotylized.

x, The apical sac of the hectocotylized arm.

y, The filament which issues from the sac when development is
complete.