OSTRACODA, a sub-class of the Crustacea (q.v.) compris ing minute forms found in fresh water and in the sea and dis tinguished by having the body and limbs completely enclosed in a bivalve shell. They are remarkable for having a smaller num ber of appendages than any other Crustacea, there being not more than four pairs and sometimes only two pairs of limbs behind the mandibles. The antennules and antennae are used for swimming or creeping. The mandibles have a large palp, often biramous and sometimes leg-like. The remaining limbs are very varied in form but some of them are usually leg-like and used in locomotion.
The breeding habits of the os tracods present several features of interest. Parthenogenesis is common, at least among the fresh water species, in many of which males are rarely seen; while in some species they have not yet been discovered. A colony of a species of Cypris has been kept in an aquarium for more than 3o years and during the whole of that time no males made their appearance, the colony reproduc ing exclusively by parthenogene sis. In some Ostracoda the thread like spermatozoa are not only relatively but absolutely larger than those of any other animals.
In one species which, when adult, is no more than .6 millimetres in length the spermatozoa are from 5 to 7 millimetres long.
Most species live on or near the bottom, creeping among weeds or burrowing in mud, but some marine species are planktonic.
One of the latter, Gigantocypris, is the largest member of the group, reaching a length of 23 millimetres, but most species are much smaller and some do not exceed half a millimetre in length.
Numerous fossil Ostracoda have been described from all geo logical formations from the old est to the most recent, but the affinities of many of them are obscure.
The recent Ostracoda are gen erally grouped in two orders. The Myodocopa are exclusively ma rine and can usually be recog nized by the presence of a notch in the margin of the shell near the anterior end. They generally have paired compound eyes and a heart. The Podocopa include both marine and freshwater species. There is no notch in the shell, no paired eyes and no heart. (W. T. C. )