Ostracods (formally called Ostracoda) take their name from the Greek 'ostrakon', which means 'a shell', and refers to the bi-valved carapace that is characteristic of these tiny crustaceans, which resemble water fleas. They had evolved by the early Cambrian, about 545 million years ago, and are found commonly as fossils. Ostracods are still living today in all aquatic habitats from the deep sea to small temporary ponds.

Ostracods are small animals belonging to the phylum Crustacea. Most are between 0.5 and 1.5 millimetres long, but a few (e.g. Gigantocypris, right) grow to about 25 millimetres.

Unlike most crustaceans, ostracods are not segmented, so that the head and body merge into one. They usually have seven pairs of limbs,or appendages, which are adapted for locomotion (swimming or crawling), grasping, cleaning the carapace, feeding, or as sensory organs. Some ostracods have eyes, others are blind, and all have setae (minute hairs) which protude through the pores and are used for sensory purposes.

Gigantocypris with embryos

Fresh water ostracod Darwinula with left valve removed to show animal's body

Some ostracods are bioluminescent; in other words they glow in the dark. It is said that during the Second World War, Japanese soldiers and sailors would keep cultures of these ostracods in bowls so that they could use the light to read their map and instruments.

Podocopids vary considerably in shape, have an arched dorsal margin and a complex hinge. Their adductor muscle scars are often arranged in a simple vertical row of four. Most living ostracods belong to this group.