Isopoda

Isopoda (Gr., 'equal-footed'), an order of higher Crustaceans in the division with unstalked eyes. The body is usually flattened, and the first pair of abdominal legs form a lid overlapping the others, which generally bear gills. They are mostly marine, but the wood-lice are terrestrial; they live mainly on decaying animal matter, but many are parasitic. The genus Tanais seems ancestral and primitive; Asellus is very common, both in fresh and salt water; the gribble (Limnoria) bores into wharf-posts and ship-sides; Idotea includes the largest forms with adaptive colours and sometimes phosphorescence; the family Bopyridæ are parasitic on other Crustaceans, and have very small males; the family Ægidæ includes many 'fish-lice'; some of the parasitic Cymothoidæ are first male and then female. The Oniscidæ are terrestrial, and feed on decaying vegetable matter; they are familiarly known as 'wood-lice,' 'sow-bugs,' 'pill-bugs,' 'slaters,' of which Oniscus, Porcellio, and Armadillo are common genera. See CRUSTACEA, FISH-LICE, WOOD-LICE.