ENTOMOSTRACA, one of the two great sub-classes of cnistacea (the other is Malacos traca), including minute forms; the °water fleas, having a horny shell of many pieces, a well-developed cephalo-thorax, mandible and three pairs of maxillae, five pairs of thoracic feet but no abdominal feet and no gills, breath ing instead by specialized organs. They have a great variety of shapes and of means of locomotion. The young is a nauplius and de veloped by numerous molts. The group in .cludes many thousands of species, divided into four orders,— Phyllopoda, Ostracoda, Copepoda and Cirripedia (barnacles). They abound in stagnant fresh waters and also in the sea, and furnish an immense quantity of subsistence for fishes that are used for human food. They exist and increase in innumerable millions. The descendants of a single.cyclops may in one year number over 4,000,000,000. At one time they render the surface of the sea-water phosphor escent by their vast luminous congregations. At
another time the Atlantic Ocean is colored red over a space of hundreds of square miles by the assembly of these minute creatures, attract ing multitudes of fishes, even of whales, which feed upon them. On the other hand, some forms are equally injurious as parasites. Those belong chiefly to the copepod group — siphon °stomata, having mouths fitted for suction. Some are commensal, entering the gill-sac or digestive cavity and feeding upon the food, not upon the tissue, of the host. Some attach themselves long enough to suck the blood of their victim and then pass on, while others enter the body as permanent residents and embed themselves in the tissue. Thus they are the pests of starfish, jellyfish, worms, ascidians, fishes and whales. See BARNACLE; COPEPODA ; CRUS