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, IV), 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 

 liyperpolygynous, 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 N. macrompfialus 

 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, II) ; 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, 

 T.WJ l.ov). 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 whicli 

 the hectocotylus of the male is caducous. In Nautilus there ia 

 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 



