Ostracods are most notable in having their entire body enclosed within the carapace folds. As a result, there are many reductions in body segments and appendages. The carapace fold originates at the posterior margin of the larval head shield and extends anteriorly, laterally, and posteriorly to cover the body. Dorsally, the carapace fold is divided by a hinge and flexible cuticle. A system of adductor muscles is used to close the "valves," as the two parts of the carapace fold are called, and hydrostatic pressure applied at the top of the mandible serves to open the valves. The two valves are asymmetrical, with one valve fitting inside the other. Because the valves are often heavily calcified, ostracod valves are common features of the fossil record, have been studied extensively, and a specialized terminology has been developed to describe the fine details. Ostracods have typical crustacean head appendages: antennules, antennae, mandibles, maxillules, and maxillae. In contrast to many other crustacean groups, however, ostracods use their antennae, and sometimes their antennules, for locomotion. As a consequence, these appendages are short and robust. The antennules are uniramous and composed of five to eight segments. The antennae are biramous, with one ramus, the exopod, often reduced in the podocopids. Ostracod mandibles have a coxal gnathobase, with the remainder of the appendage developed into a palp, which is often biramous. Posterior to the mandible is the maxillule, which is quite variable in morphology. The protopod usually has a set of setose endites, and from the pro-topod arises a short endopod used in manipulating food particles and a dorsally directed setose vibratory plate. It is not
known with certainty whether this vibratory plate is an exopod or an epipod. The trunk limbs have had various names applied to them. Posterior to the maxillule is the maxilla, but this limb has usually been called a maxilliped, or first trunk limb, in response to the degree to which the limb looks like the maxillule or the next two pairs of trunk limbs, and is used for feeding or locomotion. If used for feeding, the endopod is shortened and forwardly directed and the vibratory plate is a smaller version of that on the maxillule. If used for locomotion, the maxillae are changed considerably in shape, with the endopod directed posteriorly and the vibratory plate reduced to a simple seta. Of course there are many examples of intermediate forms for this appendage. The next two pairs of trunk limbs are the thora-copods, and they are also quite variable in design. Sometimes there is also variation between right and left limbs as well as sexual dimorphism. In some cases, these limbs are absent.
Myodocopids have transformed the second thoracic limb into a long multiarticulate vermiform appendage capable of extending into the dorsal space between the body and the hinge area of the valves, presumably as a cleaning device. In the most recently evolved podocopids, these trunk limbs look like typical crustacean walking legs and have often been termed pereopods. According to recent research, there are several limbless trunk segments following the two pairs of tho-racopods, and the body typically ends in a pair of caudal rami. In most ostracods, the penes of the male are quite large relative to the remainder of the body. In higher podocopids, the penial lobes can be as much as one-third of the body length and are very complicated, highly chitinous structures.
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