Accessory glands sometimes open upon it. The oviduct in the lower part of its course usually dilates into a uterus, the walls of which secrete albumen, or more rarely there is a specialised albumen gland, e.g. Paludina, Natantia.

1 Professor Ray Lankester states ('Mollusca,' Encyclopaedia Brit. (ed. ix.) xvi. p. 645) that the Zygobranchia retain the archaic character of the absence of special genital ducts. 'The generative products escape by the larger nephridium,' i. e. the right. And Wegmann, in his monograph on Haliotis, maintains the same view; (A. Z. Expt. (2), iii. 1884, p. 340). On the other hand, Haller states explicitly that he has found a separate genital duct in both Fissurella and Haliotis (M. J. xi. 1886, p. pp. 10, 14). As to Patella, Harvey Gibson agrees in his lately published memoir with the statement of Professor Lankester. The Zygobranchia have no accessory sexual glands.

The vagina is always short, and there is in some instances an appended receptaculum seminis, e.g. Paludina, Natantia. The genital opening is usually at some distance from the anus. In hermaphrodite forms there is an hermaphrodite duct to the ovo-testis, which may remain outwardly undivided as far as the aperture, but has a partially divided cavity, as in Aplysia and most other Opisthobranchia Palliata. It usually divides, sooner or later, into separate male and female ducts. A ciliated furrow leads from the genital aperture to a hollow evaginable penis on the right side of the head in Aplysia and a few others. When the ducts are separate, the vas deferens ends in an evaginable penis, and has commonly an appended prostatic (?) gland. A vesicula seminalis is often present. The oviduct has a uterine section, an attached receptaculum seminis, and invariably an albuminiparous gland. The two apertures, male and female, open as a rule, except in aquatic Pulmonata, into a sexual cloaca, which lies either near the anus, or on the right side of the body, and in all Pulmonata far forwards, close to the head.

Some Pulmonata and certain species of Doris possess a dart, attached in the former to the female, in the latter to the male, duct.

Two forms of spermatozoa are found in the same individual in various Azygobranchia, and of these one form only appears to fertilise the ovum in Paludina. In the hermaphrodite groups copulation may or may not be reciprocal. The ovum is small, and is usually devoid of a vitelline membrane. It is surrounded by a quantity of albumen, the surface of which hardens. In some of the Pulmonata there is a calcareous egg-shell, e.g. Testacella, Helix. In some Azygobranchia, e. g. Paludina, and some Pulmonata (e. g. Clausilia, Pupa), the ova develope within the uterus, but in most cases they are laid, either singly or in masses, contained sometimes in a fixed hard cocoon with terminal aperture, as in many Azygobranchia. One ovum only frequently developes in this case, the others serving as food material. The egg-masses are usually fixed to some object, except in the terrestrial Pulmonata. They are carried about attached to a float-like structure formed by the foot in Ianthina. Segmentation is unequal, but the degree of inequality depends upon the amount of food-yolk present.

The hypoblast cells are sometimes of very large size, and two yolk cells persist for a long time in Aplysia. When the blastomeres are all relatively small, the gastrula is invaginate, e.g. Paludina; when the hypoblast cells are large, it is formed by overgrowth. There is a Trochosphere and Veliger stage. The velum is said to be absent in some Pulmonata. It usually becomes bilobed or multilobed, and is large in size in Natantia, and persists to a greater or less extent in the Opisthobranch Tergipes and the Pulmonate Limnaeus.

Most Gastropoda Anisopleura are marine; the basommatophorous Pulmonata, and some Azygobranchia, e. g. Paludina, Neritina, are freshwater; some of the last-named group are also found in brackish water, e. g. Littorina, whilst others, such as Cyclostoma, are, like the stylommato-phorous Pultnonata, terrestrial. A few are parasitic: Entoconcha mirabilis inhabits the perivisceral cavity of a Synapta (Holothurioided); in the genus Eulima, which lives upon Holothurioidea, some species are ecto-, others endo-parasitic. Stybifer is ecto- or pseudo-parasitically attached to Holo-thurioidea, Echinoidea, and Asteroidea. Some are entirely pelagic, as the Natantia and Phyllirhoe. The majority are reptant, but of these some few can swim, as does, e.g. Oliva, by means of the epipodia. The majority are vegetable feeders, the siphonostomatous Azygobranchia carnivorous. The Anisopleura appear in the primary rocks, the existing genera Patella, Turbo, Natica, and Pletirotomaria in the Lower Silurian. Though the number of fossil forms is great, the extinct genera are not numerous.