Small Crustaceans having the entire body enclosed in a shell or carapace, which is composed of two valves united along the back by a membrane. The branchiae are attached to the posterior jaws, and there are only two or three pairs of feet, which subserve locomotion, but are not adapted for swimming. A distinct heart is sometimes present (Cypridina), but is more usually wanting (Cypris and Cythere).

Little is known, of the development of the Ostracoda, but the young of Cypris are said to be "shell-bearing Nauplius forms" (Claus), possessing only the three anterior pairs of limbs, but protected by a bivalve shell. As in other Nauplii, the third pair of limbs, though now locomotive, are ultimately transformed into the mandibles. They pass through several stages, with complete moults, before arriving at sexual maturity. The young of Cythere, on the other hand, have at birth the two pairs of antennae and two pairs of jaws, with three pairs of rudimentary abdominal limbs.

The order includes the Cyprides (fig. 145, a), which are of almost universal occurrence in fresh water. The common

Fig. 145.   Fresh water Entomostraca. a Cypris tris striata: b Daphnia pulex; c Cyclops quadricornis.

Fig. 145. - Fresh-water Entomostraca. a Cypris tris-striata: b Daphnia pulex; c Cyclops quadricornis.

Cypris is completely protected from its enemies by a bivalve carapace, which it can open and shut at will, and out of which it can protrude its feet. The closure of the carapace is effected by means of an adductor muscle. Locomotion is mainly effected by means of a pair of caudal appendages. The Cypris is extremely prolific, and a single impregnation appears to last the female for its entire lifetime. Young females, produced in this way, are also capable for some generations of producing fresh individuals without the influence of a male (parthenogenesis).

The marine Ostracoda are mostly shallow-water forms, but there are deep-sea types which attain a comparatively gigantic size (nearly an inch in length).