(_b_) The channels which provide for the passage of the seminal products; namely, the _oviduct_ in the female, the _vas deferens_ or sperm-duct in the male. (_c_) The external generative organs. [Illustration: FIG. 49.--Generative and other organs of _Littorina obtusata_ L., _female_. =A=, anus. =Br=, branchia. =Buc=, buccal mass. =H=, heart. =Hep=, hepatic duct. =I=, continuation of oesophagus. =Ki=, kidney. =Li=, liver. =M=, muscle of attachment. =O´=, female orifice. =Od=, oviduct. =Oes=, oesophagus. =Ov=, ovary. =Ra=, radula. =St=, stomach. =U=, uterus. (After Souleyet.)] [Illustration: FIG. 50.--Generative and other organs of _Littorina obtusata_ L., _male_. =A=, anus. =Br=, branchia. =H=, heart. =I=, intestine. =Li=, liver. =M=, muscle of attachment. =Pe=, penis. =Te=, testis. =VD=, vas deferens. (After Souleyet.)] =Dioecious Mollusca.=--The common _Littorina obtusata_ will serve as a typical instance of a dioecious prosobranchiate, exhibiting the simplest form of organs. In the female the _ovary_, a lobe-shaped body, is embedded in the liver. An _oviduct_ with many convolutions conveys the ova into the _uterus_, an oblong chamber which consists simply of a dilatation of the oviduct. The ova descend into the uterus, which is sometimes furnished with a seminal pouch. In this seminal pouch, or above it, in the oviduct, the ova come into contact with the spermatozoa. The lower part of the uterus secretes a gelatinous medium (or capsule, as the case may be) in which the fertilised ova become enclosed previous to exclusion. In position the oviduct abuts on the kidney, while the uterus is in close proximity to the rectum, and the female external orifice is found close to the anus, within the branchial cavity. The male organs of _Littorina_ are more simple. The _testis_ is lodged, like the ovary, in the liver; the _vas deferens_ is, like the oviduct, convoluted, and eventually traverses the right side of the neck, emerging near the right tentacle, and terminating in the _penis_ or external copulative organ (Fig. 50). This system prevails, with but slight modifications in detail, throughout the prosobranchiate Gasteropoda. The most important modification is the passage of the seminal products in certain cases (many of the Diotocardia) through the right kidney, with which the oviduct and vas deferens always stand in close relation. The same arrangement occurs in the Scaphoda and some Pelecypoda. The penis varies greatly in form and size. In the Strombidae (see Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in _Littorina_ it is somewhat spinulose at one side; in _Paludina_ a portion of it is lodged in the right tentacle, which becomes atrophied and much more obtuse than the tentacle on the left side.