ISOP'ODA (Neo-Lat. nom. pl., from Gk. Coos, isos, equal + was, pour, foot). An order of nialacostracous crustaceans of the section Ar throstraca, mostly aquatic—some marine, some inhabitants of tresh waters—but some terres trial, inhabiting damp places, as the wood-louse and the like. They are easily recognized by the fact that the body is flattened dorso-ventrally, and many of them—e.g. the csowbug' or `pillhug' (Porcellio)—have the habit of rolling up into a ball with the head tucked safely inward. The first segment of the thorax is fused with the head, but the remaining seven are free, and bear limb-like appendages without gills. In females the basal joints of several of these appendages bear lamellae, which form a brood-pouch for the eggs. There is never any carapace. The maxil lipcds, of which there is only a single pair, usu ally fuse to form a sort of lower lip. The ab dominal appendages are biramose. and serve for swimming and breathing; the most anterior pair are usually thick, and form an operculum which serves to protect the more delicate appendages behind. The heart is situated chiefly in the ab domen, hut extends forward a short distance into the thorax. While most of the species lead a free life, some of the marine forms (Cyrnothoa, etc.) are parasitic on fishes or on other crusta ceans, or bore into wood, etc. See illustration under Gninta.E.
One group (Bopyridte) are parasitic, living under the carapace of various shrimps. The fe
males of Bopyrus palcrmonetieola (Packard) are many times larger than the males, and are much degenerated, the head being without eyes and appendages; they retain their position on their host by means of the sharp, hook-like legs around the edge of the body. The male in general ap pearance shows but slight modifications and is about one-fifth as large as the female, and is lodg0 partly out of sight under the ventral plates of its consort.
Some of these forms are notably degenerate. The Isopoda are a comparatively small group, and are generally small individuals, few species reaching a length of one inch, except in the colos sal deep-sea Bathynontus yiyantetts, dredged in the Caribbean Sea from a depth of nearly a mile. The colors are usually dull, blackish, gray, or brown hut some of the marine forms are highly colored, red or brown, according to the hue of the seaweed they rest on.
Fossil isopods, though of little geological im portance, are known from rocks as early as the Devonian and Carboniferous; also from the Jurassic limestones of Bavaria and other Mesozoic formations. Most of them have some superficial resemblance to the marine Sphteroma, and to the terrestrial wood-lice of modern time. See CRUSTACEA.