Isopoda

Isopoda, an order of Crustacea (q.v.) belonging to the Malacostraca, and forming with the Amphipoda (q.v.) the group Arthrostraca (q.v.). The three main characters of the sides are that the body is somewhat compressed from above downwards, that there are branchiae (or gills) on the limbs of the abdomen, and that the head is distinct from the segment bearing the first pair of limbs. The order has been divided according to the mode of life into the Natatorial, Sedentary, and Cursorial groups. The first has a broadened abdomen, which acts as a swimming tail: most of these are marine, and some are parasitic upon fish. The Cursorial group is the one which is best known generally, as it includes the common wood-lice (Oniscus), notable from their habit of rolling up into a ball when threatened with danger. Many of the parasitic Isopods illustrate the phenomenon of degeneration, for they have lost the use of their eyes and antennae (feelers): some, such as Cryptoniscus, have also lost all trace of the abdominal limbs, and even of segmentation.