320 THE, CEPHALOPODA 
aperture of the shell are wider in the male than in the female. 
The maximum of sexual dimorphism is found in Argonauta, in 
which genus the males are much smaller than the females: the 
latter may attain to fifteen times the length of the other sex, and 
they have an external shell and the characteristic enlargement of 
the dorsal arms (Fig. 301, 1V), both of which features are absent in 
the males. Generally speaking, the males are also distinguished by 
the phenomenon of hectocotylisation, which consists in a curious 
modification for copulatory purposes of a part of the pedal circum- 
oral crown (see p. 323). 
It has been shown that the majority of the Cephalopoda are 
hyperpolygynous, that is to say, the males are less numerous than 
the females: thus in some species of Loligo the males are to the 
females as 15: 100, in Octopus as 25: 100, and in the six specimens 
of Spirula hitherto examined only one was a male. Nautilus pom- 
pilius, on the other hand, is hyperpolyandrous, but in WV. macromphalus 
more females have been found than males. Again, in those Octopoda 
in which the hectocotylus is autotomous, the males appear to be 
more numerous, for as many as four hectocotyli have been found 
in the pallial cavity of a single female. 
The ovary or testis of the Cephalopoda is single and median ; 
it is situated near the aboral extremity of the body in the coelom, 
and is, in fact, nothing more than a projection from the wall of the 
latter cavity (Fig. 252, gg). The gonaducts open into the coelomic 
cavity, without being directly continuous with the gonad (Figs. 278, 
o.d, and 286, V, Il); they bear accessory glands on their course 
(Figs. 284 and 286, I, VI, VII), and their external apertures are 
on the somatic wall of the pallial cavity (Figs. 275, pe, l.sp; 276, 
r.ov, lov). The male duct has no copulatory organs at its extremity, 
but in the Dibranchia a single arm (or two arms in Spirula and 
Idiosepion) and in Nautilus a part of the circumoral crown is modified 
for the purpose of fertilisation: this modification is temporary and 
periodic in the Dibranchia, permanent in Nautilus. 
The females of nearly all the Oigopsida (Thysanoteuthidae, 
Ommatostrephidae, Onychoteuthidae, Gonatidae, etc.), and of the 
Octopoda with the exception of the Cirrhoteuthidae, are the only 
members of the Cephalopoda that preserve the primitive number 
of two functional and symmetrical gonaducts. In them the two 
oviducts originate near the same point in the genital capsule of 
the coelom (Fig. 278), and their external orifices are more deeply 
(aborally) situated in the pallial cavity in those forms in which 
the hectocotylus of the male is caducous. In Nautilus there is 
only a single functional gonaduct, situated on the right side, but | 
its left homologue is always present in the form of a rudimentary 
duct known as the “ pyriform appendage” (Lankester and Bourne), 
which is provided with an external orifice (Fig. 284, Pyr) but has 
