Ostracods have a long fossil history, being known from the Cambrian, and have undergone extensive radiations, especially since the early Mesozoic. Including fossil forms, the subclass Ostracoda can be divided into six orders, of which three, the Palaeocopida, Myodocopida, and Podocopida, contain recent species. The Palaeocopida contains one modern family, the Punciidae, known from the seas around New Zealand. Myodocopids are predominantly swimmers; one suborder lives in the benthic boundary layer just above the seabed, while the other suborder has radiated into the pelagic zone of the sea. Podocopids are generally smaller than other ostracods and, for the most part, live as epibenthos. Within the Podocopida, there is a tendency for reduction of appendage segments or rami, and from turgor appendages to ones with more exoskeletal integrity and strength. Podocopid carapaces also tend to be more highly ornamented with ridges and spines than those seen in the other orders. The origin of the Ostracoda has long been a mystery. They are unlike all other crustacean groups in their body organization, having retained some larval features such as feeding with antennae as well as with other mouthparts. In addition, the nauplius of ostracods is unique in possessing carapace folds from the earliest stage. The paucity of appendages on the os-tracod body has resulted in considerable confusion regarding the homologies of some of the post-cephalic legs. A recent study by Tsukagoshi and Parker (2000) suggests that ostracods have the 5-6-5 or 5-7-4 trunk segmentation pattern of other maxillopodans.
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