The general nature of an Isopod may be readily understood by the examination of the common woodlouse that abounds in gardens and damp places almost everywhere, and the reader will probably remember having seen similar creatures crawling over the rocks on the sea shore. The body is generally depressed or flattened, but convex above, and is composed of seven segments, each segment bearing a pair of legs which terminate in a pointed claw, while the posterior appendages are modified into flat, leaf-like organs of respiration. [Illustration: FIG. 199.--MARINE ISOPOD 1. _Sphæroma serratum._ 2. _Limnoria lignorum._ 3. _Ligia oceanica._ 4. _Nesæa bidentata._ 5. _Oniscoda maculosa_] When engaged in ‘shrimping’ one frequently meets with shrimps or prawns that are disfigured by a tumourous swelling on the side of the body, and if the swelling be opened a little parasite will be dislodged. This parasite is an Isopod (_Bopyrus_), the appendages of which are imperfectly developed. The female is very much larger than the male, and, as is usual with parasitic creatures, the greater part of the body-cavity is occupied by the well-developed organs of reproduction. There are several other parasitic isopods, some of which live on the bodies of fishes, and are popularly known as fish-lice, but these are not so likely to come in the way of the sea-side naturalist as the more typical forms that crawl about on the rocks and among the weeds of the coast. A few of the latter are shown in the accompanying illustration, including the Sea Pill-ball (_Nesæa bidentata_), common on the rocky coasts of the south-west, and distinguished by the two sharp projections on the last segment; the Serrated Pill-ball (_Sphæroma serratum_), very common on most rocky shores, and characterised by the fine sawlike teeth on the outer edge of the outer plates of the ‘tail’; the Great Sea-slater (_Ligia oceanica_), also an abundant species; the Spotted Hog Louse (_Oniscoda maculosa_) that lives among the tufted sea weeds; and the Boring Pill-ball (_Limnoria lignorum_) that bores into the woodwork of piers and jetties, often doing considerable damage. [Illustration: FIG. 200.--MARINE AMPHIPODS 1. The spined sea screw (_Dexamine spinosa_). 2. _Westwoodia cœcula._ 3. _Tetromatus typicus._ 4. The sandhopper (_Orchestia littorea_). 5. _Montagua monoculoides._ 6. _Iphimedia obesa._ All enlarged] The above and other isopods feed on various animal and vegetable substances, some species being quite omnivorous in habit. Most of them are eagerly devoured by birds and fishes.