Diagnosis
Small crustaceans with a bivalved carapace or shell, resembling a clamm shell, enclosing an unsegmented body with five to seven paired appendages.
The shell shape is highly variable and its surface is smooth to ornate. As the animal grows, the shell is shed, growing again after each moult. Valves of the shell connected dorsally by a ligament. Shell penetrated by pores, bearing setae of various kinds. The shell is closed by muscles which leave attachment scars on the inner surface of the valves. First antenna (or antennule) and second antenna (or antenna) well developed. The maxilla (or fourth limb, maxillula, or first maxilla) is a variable feeding limb. Fifth limb (or maxilla, or first thoracic leg) generally with a large epipodial plate. Furca either anterior or posterior to the anus. Digestive system consisting of mouth and gut.
Most species have an anterodorsal median eye, which sometimes receives light through a translucent spot in the shell. Other species have paired compound lateral eyes.
There is an open circulatory system; a heart may be present or absent. Respiration is by diffusion through the thin cuticle that covers the entire body.
Sexes usually separate; fertilization internal, accomplished by a complicated copulatory apparatus.
Embryos are brooded in the posterodorsal section of the shell, or the eggs released into the water. There are four to nine instar larvae with bivalve shells, thus resembling the adults.
Most ostracods live burrowed in the substrate or are epibenthic crawlers, others are planktonic.