It appears most probable that the dart is employed as an adjunct to the sexual act. Besides the fact of the position of the dart sac anatomically, we find that the darts are extruded and become embedded in the flesh just before or during the act of copulation. It may be regarded, then, as an organ whose punctures induce excitement preparatory to sexual union. It only occurs in well-grown specimens. When once it begins to form, it grows very rapidly, perhaps not more than a week being required for its entire formation.

Fig. 54.—Darts of British land snails: A, Hyalinia excavata Bean; B, Helix hortensis Müll.; C, Helix aspersa Müll. (After Ashford.)

The dart is almost confined to Helicidae, a certain number of exceptions being known which border on Helix. Hyalinia nitida and excavata are the only British species, not Helices, which are known to possess it. It has not been noticed to occur in the slugs, except in the N. American genus Tebennophorus. About one-third of the British Helices are destitute of the dart.[260] H. rufescens possesses a double bilobed sac, but only two darts, which lie in the lower lobes. It does not use the darts, and could not do so, from the relative sizes of dart and sac; it has often been watched when uniting, but the use of the darts has never been observed. From this it has been inferred that the darts are degenerate weapons of defence, and that they were in fact at one time much stronger organs and more often used.[261] This theory, however, does not seem consistent with the whole circumstances of the occurrence, position, and present use of the darts.

Hermaphrodite Mollusca.—(b) Digonopora.—As an example of the Digonopora or hermaphrodite Mollusca with separate generative apertures for the male and female organs, we may take the common Limnaea stagnalis (Fig. [55]). It will be seen from the figure that the relative positions of the hermaphrodite gland and duct, and of the albumen gland, are the same as in Helix. When the oviduct parts company from the vas deferens, it becomes furnished with several accessory glands, one of which (Gl.E.) probably serves as a reservoir for the ova, and answers more or less to a uterus. The tube leading to the spermatheca is short, and there is no divergent caecum. The female orifice lies near to the external opening of the branchial cavity. The vas deferens, which is very long, is furnished with a large prostate gland. The penis sac is greatly dilated, and there is no flagellum. The male orifice is behind the right tentacle, slightly in advance of the female orifice (compare Fig. [102]).

Fig. 55.—Genitalia of Limnaea stagnalis L. (from a dissection by F. B. Stead), × 2.

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.