The Crustacea breathe by gills, and are therefore suited to an aquatic life. A few species, however, live in damp earth, or in places where the air is damp (wood lice). Crustacea have two pairs of antennæ, and a large number of appendages arranged in a characteristic way, but differing very much in shape in the different groups; skin usually hard and thick. Lobsters, crayfish, crabs, wood lice, and the small sand-hoppers, water-fleas, etc., belong to the Crustacea. No Crustacean is harmful agriculturally. =Third Sub-Kingdom: VERMES= (WORMS). Worms[3] are bilaterally symmetrical animals, in which the body is enclosed in a “dermo-muscular tube.” Under the delicate epidermis there is found a layer, which does not, as in the higher animals, consist exclusively of dermis, but is partly composed of muscle-fibres, which form a distinct coat internally. Worms are able to move by contracting the various components of the dermo-muscular tube thus formed. In some worms limbs assist in the movements, but in others this is not so; in any case, however, the limbs play a relatively subordinate part. These limbs, which are only present in the bristle-worms, are small, always unjointed, foot-stumps, which bear bristles. Leeches and some other worms possess suckers by which they can attach themselves, and move by alternately contracting and extending their bodies. Footnote 3: In ordinary language one understands by “worms” elongated, cylindrical animals; and several insect larvæ (_e.g._ “wireworms,” p. 103) have the name applied to them. But to zoologists all “worm-shaped” animals are not worms, nor have all worms an elongated cylindrical shape (liver-fluke). There are segmented and unsegmented worms (thread worms, liver-flukes); the degree of segmentation is also very various. In many tapeworms each joint may be regarded as an individual animal. The joints of other segmented worms (earthworms) do not become detached, like those of tapeworms, for the purpose of reproduction. The nervous system is absent in the lowest worms; in the higher worms ganglia always constitute its central parts, and in annelids these are arranged in pairs on the ventral side of the body, and united by nerve fibres into a ventral cord, which is connected in front with a nerve ring surrounding the gut and thickened above into cerebral ganglia. In many worms (_e.g._ tapeworms and flukes) the nervous system is of much simpler structure. The sub-kingdom of worms contains a very large number of forms. It is only necessary to mention here representatives of three classes: Segmented worms (_Annelida_), round worms (_Nematelminthes_), and flat worms (_Platyelminthes_). CLASS: =ANNELIDA= (SEGMENTED WORMS).