ry Tube._--The principal differences from _Nautilus_ are the
following:--the mandibles are similar in shape, but are chitinous, not
calcified. In the radula there are three teeth on each side of the
median tooth in each row, except in _Gonatus_, in which there are only
two lateral teeth, and the _Cirrhoteuthidae_, in which the radula has
entirely disappeared. In front of the radula is the so-called tongue,
a fleshy projection corresponding to the sub-radular organ of other
Mollusca.

[Illustration: FIG. 27.--Alimentary canal of _Loligo sagittata_ (from
Gegenbaur). The buccal mass is omitted.

oe, Oesophagus.

v, The stomach opened longitudinally.

x, Probe passed through the pylorus.

c, Commencement of the caecum.

e, Its spiral portion.

i, Intestine.

a, Ink-bag.

b, Its opening into the rectum.]

In most of the Dibranchiata there are two pairs of salivary glands. In
the Decapoda the ducts of the posterior pair unite into a median duct
which opens on the surface of the sub-radular organ. The anterior pair
is but slightly developed except in the Oigopsida. In the Octopoda
there are also two pairs, but the posterior pair, except in
_Cirrhoteuthis_ where they are absent, are large and displaced
backwards, being situated near the oesophageal proventriculus.
Connected with the intestine immediately beyond the pylorus is a
thin-walled caecum, spherical in _Rossia_ and _Leachia_, elongated in
_Loligo_, but usually coiled into a spiral (fig. 27). The hepatic
ducts open into the caecum. The liver is developed as a paired gland,
more or less fused into one in the adult, but the ducts are always
paired. The ducts are covered by a number of glandular follicles
forming what is called the pancreas.

The ink-sac, absent in _Nautilus_, is a rectal caecum developed from
its dorsal wall. It is present in all Dibranchiata except _Octopus
arcticus, O. piscatorum_ and _Cirrhoteuthis_. It consists of a deeper
part or gland proper and a reservoir. It extends to the posterior
extremity of the body in _Sepia_, but in _Octopoda_ is usually
embedded in the surface of the liver. The pigment of the secretion is
melanin, and its function is to produce a dense opacity in the water,
which conceals the animal.

_Vascular System_ (fig. 28).--The ventricle lies in the pericardial
cavity, except in Octopoda where this cavity is much reduced