20 THE MOLLUSC A 



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, /). It has been shown that in the Cephalopods 

 hyperpolygyny is the rule, and in certain Atlantae 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 W *, d 30, C) and the Cephalopoda (Fig. 

 5 6t '*, b). 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 (Trockus, Fig. 55 ; Solen&mya), 

 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 Khipidoglossa, 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 

 MMi puiuduia * Polyplacophora, and of the Lamellibranch 

 The male in situ on Pseudokellya (Fig. 220) are invested by a continuous 

 the S&4I3& Sr cellular follicle. In hermaphrodite Molluscs the 

 fJniaieTwt'maiie 2 ' f ' 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 



