It is believed that in the Tetrabranchiate Cephalopoda (Nautilus) a union of the four inner ventral arms may correspond functionally to the hectocotylising of the arm in the Dibranchiates.

Hermaphrodite Mollusca.—(a) Monogonopora.—The reproductive system in the hermaphrodite Mollusca is far more complicated than in the dioecious, from the union of the male and female organs in the same individual. As a type of the Monogonopora, in which a single orifice serves for both male and female organs, may be taken the common garden snail (Helix aspersa), the accompanying figure of which is drawn from two specimens found in the act of union (Fig. [53]).

Fig. 53.—Genitalia of Helix aspersa Müller, drawn from two individuals in the act of union, from a dissection by F. B. Stead.

Beginning from the inside and proceeding outwards we have firstly the hermaphrodite gland or ovo-testis (H.G.), a yellowish white mass of irregular shape, embedded in the liver (L.) and forming part of its spiral but not reaching quite to the apex. Within this gland are developed the ova and spermatozoa. The former are rather large round cells, produced within the outer wall of the gland, while the spermatozoa, which are produced in the more central part, are thread-like bodies, generally aggregated in small bundles. From the hermaphrodite gland the ova and spermatozoa pass through the upper part of the hermaphrodite duct (H.D.), which is always more or less convoluted. Below the convoluted portion, the duct opens into the albumen gland (A.G.), a large linguiform mass of tissue which becomes dilated at the time of pairing, and secretes a thick viscid fluid which probably serves to envelop the ova. Up to this point both the male and female elements follow the same course, but on their exit from the albumen gland they diverge. The hermaphrodite duct becomes greatly enlarged, and is partially divided by a kind of septum into a male and female portion. These run parallel to one another, the larger or female portion (H.DF.), through which the ova pass (and which is sometimes termed the uterus) being dilated into a number of puckered folds, while the smaller or male portion (H.DM.) is comparatively narrow, and not dilated. At their anterior end, the two portions of the duct separate completely from one another, the female portion being then termed the oviduct (OV.) and the male portion the vas deferens (V.D.).

Following first the oviduct, we find that it soon widens into the vagina (V.), which is furnished with a pair of mucous glands (M.G.), one on each side. These are much branched, and resemble little bunches of whitish seaweed. A little above the mucous glands a long tube diverges from the vagina, which is furnished with a produced coecum (C.) and a pouch, the spermatheca (SP.) at the extreme end. In this pouch, and in the duct leading to it, is stored the spermatophore received in union with another snail. Just below the mucous glands the vagina is joined by the dart sac (D.S.), which is more fully described below. Finally, at its lower end the vagina unites with the penis sac at a point just posterior to the common orifice.

Returning now to the male organs, we find that the vas deferens is the continuation of the male portion of the hermaphrodite duct, after its final separation from the female portion. It passes under the retractor muscle of the upper right tentacle, which has been cut away in the specimen figured, to dissect it out. Just before the vas deferens widens into the penis sac, it branches off into a long and tapering tube, the flagellum, in which the spermatozoa are stored and become massed together in the long packet known as the spermatophore. The penis sac (P.S.) is the continuation of the vas deferens beyond the point at which the flagellum diverges. It joins the vagina at its extreme anterior end, uniting with it to form the common genital aperture, which cannot be exactly represented in the figure. The penis itself lies in the interior of the penis sac, and is a rather long muscular tube which is protruded during union, but at other times remains retracted within the sac.

In the Helicidae generally, the form of the generative organs varies with each separate species, sometimes merely as regards the size of the different parts, at others in the direction of greater simplicity or complication. The mucous glands may be absent, and the flagellum greatly reduced in size, or absent altogether.

The Dart Sac.—A remarkable part of the reproductive system in many of the true Helicidae is the so-called dart, Liebespfeil, or telum veneris. It consists of ‘a straight, or curved, sometimes slightly twisted tubular shaft of carbonate of lime, tapering to a fine point above, and enlarging gradually, more often somewhat abruptly, to the base.’ The sides of the shaft are sometimes furnished with two or more blades; these are apparently not for cutting purposes, but simply to brace the stem. The dart is contained in a dart sac, which is attached as a sort of pocket to the vagina, at no great distance from its orifice. There are four different forms of sac. It may be single or double, and each of these divisions may be bilobed, each lobe containing one dart at a time. In Helix aspersa the dart is about 5/16 in. in length, and ⅛ in. in breadth at its base (see Fig. [54]).