swims away with the detached
arm lodged beneath her mantle-skirt. There, in a way which is not
understood, the fertilization of the eggs is effected. Specimens of
the female _Ocythoe_ with the detached arm adherent were examined by
Cuvier, who mistook the arm for a parasitic worm and gave to it the
name _Hectocotylus_. Accordingly, the correspondingly modified arms of
other Cephalopoda are said to be hectocotylized. J.J.S. Steenstrup has
determined the hectocotylized condition of one or other of the arms in
a number of male Dibranchs as follows:--in all, excepting _Argonauta_
and _Ocythoe_ and _Tremoctopus_, the modification of the arm is
slight, consisting in a small enlargement of part or the whole of the
arm, and the obliteration of some of its suckers; in _Octopus_ and
_Eledone_ the third right arm is hectocotylized; in _Rossia_ and
_Sepiola_ the fourth left arm is hectocotylized along its whole
length, and the fourth right arm also in the middle only; in _Sepia_
the fourth left arm is modified at its base only; in _Sepioteuthis_,
the same at its apex; in _Loligo_, the same also at its apex; in
_Loliolus_, the same along its whole length; in _Ommatostrephes_,
_Onychoteuthis_ and _Loligopsis_ no hectocotylized arm has hitherto
been observed. Thus, speaking generally, it is one or both of the
fourth pair of short arms which are modified in the Decapoda, of the
third pair in the Octopoda. In the pallial cavity are situated one
pair of gills in the Dibranchiata (fig. 25), attached dorsally along
the whole of their afferent borders. On each side of the branchia is a
series of lamellae, least in number in the Octopoda. Each lamella is
transversely folded, and the folds are in turn folded, so that the
respiratory surface is increased. On the somatic wall of the pallial
cavity, between and ventral to the gills, are the following apertures:
the anus and opening of the ink-sac, close together in the median
line; a pair of apertures of the renal sacs, on either side of the
median line; external to the renal orifice, on the left side, the
genital aperture in _Cirrhoteuthidae_ and Myopsida. In other Octopoda,
and in nearly all the Oigopsida among the Decapoda, the genital ducts
are paired in the female, but only the left is developed in the male.
The funnel forms a complete tube in the Dibranchiata, and in the
majority of the Decapoda, as in _Nautilus_, it is pr