(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.

Spermatozoa.—The shape of the spermatozoa and of the ova in Mollusca is of the usual type. In Paludina Ampullaria, and certain species of Murex two types of spermatozoa occur, one hair-like, the other worm-like, three times as long as the former, and not tapering at one end. The former type alone take part in fertilisation, and penetrate the ovum. It has been suggested that these worm-like spermatozoa are a kind of incipient ova, and indicate a possible stage in commencing hermaphroditism. And, since the nearest allies of the Prosobranchiata (in which these types occur) are hermaphrodite (i.e. the Opisthobranchiata and Pulmonata), it is not unreasonable to suppose that the Prosobranchiata should show some tendency towards hermaphroditism in their genital glands.[256]

Cephalopoda.—The special characteristic of the reproductive organs in female Cephalopoda is the development of various glands, some of considerable size, in connexion with the ovary and oviduct. Sepia, Loligo, and Sepiola are furnished with two large nidamental glands, which open into the mantle cavity independently of the oviduct. Their purpose is to produce a viscid mucus, which envelops the ova at the moment of their emission and eventually hardens into the egg-capsules. A pair of accessory nidamental glands occur in Sepia, as well as a pair of smaller glands situated on the oviduct itself.

In many of the male Cephalopoda the vas deferens is long and dilated at its outer end into a glandular reservoir, within which are formed the spermatophores, or narrow cylindrical packets which contain a very large number of spermatozoa. When charged, the spermatophores pass into what is known as Needham’s sac, where they remain until required for use. These spermatophores are a very characteristic part of the reproductive arrangements in the Cephalopoda. The male of Sepia has been noticed to deposit them, during union, upon the buccal membrane of the female. During the emission of the ova by the female, the spermatophores, apparently through the agency of a kind of spring contained at one end, burst, and scatter the spermatozoa over the ova.

The Hectocotylus Arm.—Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called hectocotylus of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement.