mes containing the ovary or
testis--the viscero-pericardial sac--which opens to the exterior
either directly or through the renal organs. It has no connexion with
the vascular system. The renal organs are always paired sacs, the
walls of which invest the branchial afferent vessels (figs. 28, 29).
They open each by a pore into the viscero-pericardial sac, except in
_Nautilus_. The anal aperture is median and raised on a papilla. Jaws
(fig. 6, e) and a radula (fig. 9) are well developed. The jaws have
the form of powerful beaks, either horny or calcified (_Nautilus_),
and are capable of inflicting severe wounds.

Cerebral, pleural and pedal ganglia are present, but the connectives
are shortened and the ganglia concentrated and fused in the cephalic
region. Large special ganglia (optic, stellate and supra-buccal) are
developed. Sense-organs are highly developed; the eye exhibits a very
special elaboration of structure in the Dibranchiata, and a remarkable
archaic form in the nautilus. Otocysts are present in all. The typical
osphradium is not present, except in _Nautilus_, but other organs are
present in the cephalic region, to which an olfactory function is
ascribed both in _Nautilus_ and in the other Cephalopoda.

Hermaphroditism is unknown in Cephalopoda; male and female individuals
always being differentiated. The genital aperture and duct is
sometimes single, when it is the left; sometimes the typical pair is
developed right and left of the anus. The males of nearly all
Cephalopoda have been shown to be characterized by a peculiar
modification of the arm-like processes or lobes of the fore-foot,
connected with the copulative function. The term hectocotylization is
applied to this modification (see figs. 6, 24). Elaborate
spermatophores or sperm-ropes are formed by all Cephalopoda, and very
usually the female possesses special capsule-forming and nidamental
glands for providing envelopes to the eggs (fig. 4, g.n.). The egg is
large, and the development is much modified by the presence of an
excessive amount of food-material diffused in the protoplasm of the
egg-cell. Trochosphere and veliger stages of development are
consequently not recognizable.

The Cephalopoda are divisible into two orders, Tetrabranchiata and
Dibranchiata, the names of which (due to Sir R. Owen) describe the
number of gill-plumes present; but in fact there are several characters,