Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species (13,000 of which are extant taxa) have been identified,grouped into several orders. Ostracods are small crustaceans, typically around 1 millimetre (0.04 in) in size, but varying from 0.2 millimetres (0.008 in) to 30 mm (1.2 in) in the case of Gigantocypris. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on gross morphology, but the group may not be monophyletic; their molecular phylogeny remains ambiguous. Ecologically, marine ostracods can be part of the zooplankton or (most commonly) they are part of the benthos, living on or inside the upper layer of the sea floor. Many ostracods, especially the Podocopida, are also found in fresh water and terrestrial species of Mesocypris are known from humid forest soils of South Africa, Australia, New Zealand and Tasmania. They have a wide range of diets, and the group includes carnivores, herbivores, scavengers, and filter feeders.