ENTOMOS1RACA (Gr. insect-shells), a term introduced by Muller, and adopted by Latreille, envier, and other naturalists, to designate the second of their two great divi sions of crustaceans (q.v.). The number of species of E. is very great. They are all of small size, except the king-crabs (iimutus), which in many respects differ from all the rest, and have recently been formed by seine naturalists into a sub-class of crustaceans by themselves. Many of them are minute, and exist in great numbers both in fresh and salt water, particularly in stagnant or nearly stagnant fresh water, affording to many kinds of fishes their principal food. They differ very much in general form; the num ber of organs of locomotion is also very various—in some very few, in some more than 100—usually adapted for swimming only, and attached to the abdominal as well as to the thoracic segments; but there never is a fin-like expansion of the tail, as in some of the malacostracous crustaceans. The antenna; of some are, however, used as organs of locomotion. Some of the E. have mouths fitted for mastication, and some for suction. Not a few are parasitic. The heart has the form of a long vessel. One or two nervous knots or globules supply the place of a brain. The organs of respiration are in certain species attached to some of the organs of locomotion, in the form of hairs, often grouped into beards, combs, or tufts, or blade-like expansions of the anterior legs are subser lent to the purpose of respiration: in others, no special organs of respiration are known to exist. The eyes are sometimes confluent, so as to form a single mass—one eye—in the
front of the head. The name E. has been given to these creatures in consequence of most of the species having shells of one or two pieces, rather horny than calcareous, and of very slender consistence, generally almost membranous and transparent. In very many, the shell consists of two valves, capable of being completely closed, but which, at the pleasure of the little animal, can also be opened so as to permit the antennae and feet to be stretched out.
The study of the smaller crustaceans has recently been prosecuted with great assiduity and success, by Milne-Edwards and others; and in consequence of the great differences existing among them, new classifications have been proposed, and the name E. has by some been restricted to those which have a mouth formed for mastication, but no special organs of respiration, forming a section which is subdivided into two orders, ostrapoda and copepoda, the former having a bivalve shell or shield, the latter destitute of it.—But the name E. is still commonly employed in its former Wider sense.