are situated ventrally between
the pedal and visceral ganglia, and are entirely enclosed in the
cranial cartilage. The cavity of each is continued into a small blind
process which is the remnant of the embryonic connexion of the vesicle
with the external surface. The sensory epithelium is at the anterior
end of the vesicle forming a macula acustica, and in the cavity is a
single otolith, partly calcareous and partly organic except in
_Eledone_, in which it is entirely organic. The nerve arises from the
cerebral ganglion on each side and passes through the pedal ganglion.

There is no branchial osphradium in the Dibranchiata corresponding to
that of _Nautilus_, but the olfactory organ or rhinophore near the eye
is present. In _Sepia_ and the majority of the Dibranchiata it is a
simple pit, in some of the Oigopsida it is a projection which may be
stalked.

[Illustration: FIG. 34.--Diagrams of sections showing the early stage
of development of the eye of _Loligo_ when it is, like the permanent
eye of _Nautilus_ and of _Patella_, an open sac. (From Lankester.)

A, First appearance of the eye as a ring-like upgrowth.

B, Ingrowth of the ring-like wall so as to form a sac, the primitive
optic vesicle of _Loligo_.]

_Reproduction and Development._--The modification of one or a pair of
the arms in the male for purposes of copulation has already been
described. In many genera the sexes differ from one another in other
characters also. As a rule the males are more slender or smaller than
the females. The maximum degree of sexual dimorphism occurs in
_Argonauta_ among the Octopods; in this genus the female may be
fifteen times as large as the male, and the peculiar modification of
the dorsal arms for the secretion of the shell occurs in the female
only, no shell being formed in the male. In most cases the females are
much more numerous than the males, but the opposite relation appears
to exist in those Octopoda in which the hectocotylus is autotomous,
for as many as four hectocotyli have been found in the pallial cavity
of a single female. When the hectocotylus is not detached it is
usually inserted into the pallial cavity of the female so as to
deposit the spermatophores in or near the aperture of the oviduct, but
in _Sepia_ and _Loligo_ they are merely deposited on the ventral lobes
of the buccal membrane.

The eggs are laid shortly after copul