Snails and slugs possess a head which bears unjointed tentacles, and also eyes, which, in many cases, are borne on the tips of these. The foot serves for creeping, and is flattened in a sole-like way on its under side. The mantle is generally small, and limited to the dorsal side of the animal. The usually spiral shell covers therefore only a small part of the body, but the rest of the body can also be mostly or entirely drawn back into it. Snails possess jaws, but the chief organ used in attacking the plant structures is the swollen tongue, covered by a rasping plate (radula), on the surface of which regular rows of teeth are arranged. The rest of the tongue consists of muscle, and two small cartilages are also found within it. The teeth stick up when the tongue is protruded, and the whole apparatus forms a kind of rasp, by the backward and forward movements of which the food is reduced to small fragments. Marine snails breathe by gills, which are found in the mantle-cavity, while most of the freshwater and all the land forms (“Pulmonata”) breathe by means of the inner wall of the mantle-cavity, which thus serves as a lung. Some members of the group are hermaphrodite, while the sexes are distinct in others; the first is the case in all terrestrial forms. Such snails pair, however, mutually fertilizing each other. They are not able to fertilize themselves. It is only among land snails and slugs that injurious kinds are found. Species of the genus _Helix_, _e.g._ the =Edible= or =Roman Snail= (_H. pomatia_) and the =Field Snail= (_H. nemoralis_), may do harm in fruit-culture; =Slugs= are very injurious to agriculture (_Limax_ and _Arion_); in Limax, the respiratory opening is situated behind the middle of the right margin of the mantle, while the genital opening lies behind the right feeler; in Arion, the respiratory opening is situated on the front edge of the mantle, and the genital opening immediately in front of it. Any kind of snail or slug may be harmful, especially if it attacks young plants. Some kinds, however, live chiefly in woods, feeding upon toadstools and dung, perhaps also on bark and weeds (_e.g._ dandelion). This is especially true of the larger kinds, but these too, if they multiply very rapidly, may effect much damage in gardens and fields. The commonest slugs are— The =Common Black Slug= (_Arion ater_).—About four inches long, and generally black. The =Grey Field Slug= (_Limax agrestis_, Fig. 144).—Four-fifths of an inch long, small and smooth, with dark tentacles, grey back, often a reddish sheen, and bright grey or white under side. The =Large Slug= (_Limax maximus_).—Up to six inches long, with black flecks on the hump-like mantle, and black streaks on the rest of the body.