SLUG. The name generally given to the members of a group of land molluscs related to the land snails but lacking an external shell. They include the families Limacidae and Arionidae of the Pulmonate Gastropoda. There are several other groups of less familiar Pulmonates (e.g., the Daudebardiidae, Veronicelli dae and Rathousiidae) which may be called "slugs." The name is indeed extended to all shell-less elongate Gastropods such as the "sea-slugs" (Nudibranchia : Oncidiidae).
The common slugs (e.g., the black slug, Anion ater and Agrio limax agrestis, the field slug) are placed in the sub-order Stylom matophora of the Pulmonata. The shell is wholly internal and may be absent. The mantle is seen as a shield-shaped projection at the anterior end. In the living animal a small contractile ori fice is seen on the right-hand edge of the mantle. This is the pneumostome or respiratory orifice through which air is drawn into the lung-like mantle-cavity. In Lintax the shell is persistent, but it has no spiral apex; in Anion it is very degenerate and is reduced to a few calcareous granules. In the tropical Veronicelli
dae and Rathousiidae it is entirely absent. Most of the land slugs have two pairs of tentacles ; the eyes are borne on the extremities of the posterior pair and the anterior pair carries the tactile organs. A typical land slug such as Anion ater inhabits woods, fields, gardens, hedgerows and wilder places. It is more or less omnivo rous and Ellis states that it feeds on green leaves, fruit, fungi and dead animal and vegetable matter. The sea-slugs are principally represented by a number of completely naked genera of Nudi branchiata Gastropods. The most striking of them are the Eolids, which are small, often brightly coloured animals. They are found in shallow waters and in rocky pools. (For the peculiar function of the cnidosacs of these animals, see GASTROPODA.) Certain of the Oncidiidae are remarkable for the numerous eyes scattered over the dorsal surface which are stated to be a special adaptation in relation to the attacks of a littoral fish (Periophthalmus) which preys upon them. (G. C. R.)