=A.G=, albumen gland. =Ac.G=, accessory gland. =F.O=, female orifice. =Gl.E=, glandular enlargement. =H.D=, hermaphrodite duct. =H.G=, hermaphrodite gland. =Li=, liver. =M.O=, male orifice. =P=, penis sac. =Pr=, prostate. =R.M=, retractor muscle of penis. =Sp=, spermatheca. =V.D=, vas deferens.] Most of the _Opisthobranchiata_, but not all, have separate sexual orifices. Numerous variations from the type just described will be found to occur, particularly in the direction of the development of accessory glands, which are sometimes very large, and whose precise purpose has in many cases not been satisfactorily determined. _Pelecypoda._--In the dioecious Pelecypoda, which form the great majority, the reproductive system is simple, and closely parallel in both sexes. It consists of a pair of gonads, which are either ovaries or testes, and a pair of oviducts or sperm-ducts which lead to a genital aperture. The gonads are usually placed symmetrically at the sides or base of the visceral mass. The oviduct is short, and the genital aperture is usually within the branchial chamber, thus securing the fertilisation of the ova by the spermatozoa, which are carried into the branchial chamber with the water which passes through the afferent siphon. Hermaphrodite Pelecypoda are rare, the sexes being usually separate. The following are assured instances: _Pecten glaber_, _P. jacobaeus_, _P. maximus_, _Ostrea edulis_, _Cardium norvegicum_, _Pisidium pusillum_, _Cyclas cornea_, _Pandora rostrata_, _Aspergillum dichotomum_, and perhaps _Clavagella_. The greater number of these have only a single genital gland (gonad) on each side, with a single efferent duct from each, but part of the gland is male and part female, _e.g._ in the Pectens above mentioned. _Pandora_ and _Aspergillum_ have two distinct glands, respectively male and female, on each side, each of the two glands possessing its separate duct, and the two ducts from each side eventually opening near one another. It appears probable that the _Septibranchiata_ (_Cuspidaria_, _Poromya_, _Lyonsiella_, etc.) must also be added to the number of hermaphrodite Pelecypoda which have separate male and female glands. It is worthy of remark that all the hermaphrodite Pelecypoda belong to forms decidedly specialised, while forms distinctly primitive, such as _Nucula_, _Solenomya_, _Arca_, and _Trigonia_ are all dioecious. In Gasteropoda similarly, the least specialised forms (the _Amphineura_, with the exception of the _Neomeniidae_, and the _Rhipidoglossa_) are dioecious. It is possible therefore that in the ancestors of the Mollusca the separation of the sexes had already become the normal type of things, and that hermaphroditism in the group is, to a certain extent, a sign or accompaniment of specialisation.[262]