EN'TOMOS'TRACA (Neo-Lnl. nom. pl., from f:k. rprepov, •laomon, insect + fkrrptenw,ostrokon, shell). (me of the two SIIIWInS.:eS Of crus t :weans Many of them are minute, and in great numbers I mu 11 ill fresh and salt water. particularly in stagnant. or nearly stagnant fresh water. affording to many kinds of fishes t I • it prineipal fend. They differ much in general form; the number of organs of locomo i is also ye rine ie stem. few; in sonic, More loO—u-mally adapted for swimming only, and attached to the posterior as well as to the anterior segments; but there never is a fin-like expansion of the tail, as in some of the inalacos tracous crustaceans. The body is divisible into two parts, a head and a trunk, but the latter shows no differentiation into thorax and abdo men. The antenna are generally well developed, and are often used, especially the second pair, as organs of locomotion. Some of the Entomos traca have mouths fitted for mastication, and some for suction. Not a few are parasitic. The heart has the form of a long vessel. 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 subservient to the purpose of respira tion; in others, no special organs of respiration are known to exist. The nervous system, like that of most arthropods (q.v.), consists of a
brain or supra-resophagral ganglion and a more or less elongated double ventral cord connected with it by a commissure on each side of the esophagus, and provided with six or seven pairs of ganglia. In most entomostracans, however, the nervous system is more concentrated, some times to such an extent that it consists of a single ganglionic mass, through which the esoph agus passes. The eyes are of two distinct sorts; nearly all the species have a median un paired eye, sometimes well developed and some times greatly reduced. .lany forms also have a pair of lateral eyes, which are sometimes stalked. The name Entomostraca has been given to these creatures in consequence of most of the species having shells of many pieces, rather horny than calcareous, and very delicate, gener ally almost membranous and transparent. In many, the shell consists of two valves, including more or less of the body, capable of being com pletely closed, but which, at the pleasure of the animal, can also be opened so as to permit the antenna and feet to be stretched out.
The Entomostraca comprise many thousand species, which are readily grouped in four great orders, according to the arrangement and struc ture of the shell and appendages; PitYLLororet; OSTRACODA COPEPODA CIRRIPEDIA