20 THE MOLLUSCA 
branchia. In the forms with separate sexes there is often a definite 
sexual dimorphism, which is exhibited not only by the presence of 
a copulatory organ (Cephalopods and the majority of Gastropods), 
but also in the breadth and even in the greater size of the 
females (Fig. 7, f). It has been shown that in the Cephalopods 
hyperpolygyny is the rule, and in certain Aflantae and American 
Unionidae, hyperpolyandry. The gonads are primitively paired 
and developed from the coelomic wall, but they are only in direct 
communication with the remainder of the coelom (the pericardium) 
in the Aplacophora (Figs. 5°, d; 30, C) and the Cephalopoda (Fig. 
5%, 6). In the former case the genital products fall into the peri- 
cardium and are carried to the exterior by the renal ducts, which thus 
act as gonaducts. In other cases the genital products may be dis- 
charged into the reno-pericardial duct (Zrochus, Fig. 55 ; Solenomya), 
and are thence expelled through the kidneys, or they may be 
discharged directly into the kidneys, more or less close to the 
external renal orifice (various archaic Lamellibranchs, the majority 
of Rhipidoglossa, Scaphopods). Otherwise, in all groups, the gonads 
open to the exterior by their proper pores, which are nearly 
always adjacent to the renal openings ; they may, however, be more 
or less removed to a distance from the latter, and in certain 
hermaphrodite forms (Pulmonates and Nudibranchs) the renal 
orifice is near the anus and the genital aperture is secondarily 
separated from it and shifted towards the penis. 
Accessory glands are often developed on the course 
of the genital duct, especially in the female. 
The male and female elements are formed from 
the epithelium of the gonad: each oogonium gives 
rise to a single ovum with its polar bodies, while 
™ each spermatogonium gives rise to several sperma- 
tozoa. The eggs of Cephalopods, of the majority 
Diie Seine of Polyplacophora, and of the Lamellibranch 
The male in sity on Pseudokellya (Fig. 220) are invested by a continuous 
the shell-aperture of Cellular follicle. In hermaphrodite Molluscs the 
Palen male,” Spermatozoa ripen before the ova; the herma- 
phroditism is therefore protandric. The herma- 
phroditism also is not self-sufficient, and the ova of one individual 
must normally be fertilised by the spermatozoa of another individual. 
The ‘ parthenogenesis” observed in hermaphrodite Pulmonata 
(Arion, Limnaea), which have been isolated from the time of their 
birth, is possibly due to an abnormal autofertilisation. 
“Progenesis” has only been observed in one Gymnosome 
(Clione), in which the larval characters are preserved for a long time. 
Copulation only takes place in such Gastropods, whether monoecious 
or dioecious, as are provided with a penis, and in the Cephalopoda. 
In several members of the latter class the copulatory organ, or 
Fia. 7. 
