Cumaceans have a large carapace that extends backward and ventrally to cover the first three thoracic somites. The anterior aspect of the head usually bears a single middorsal eye, but some species have two eyes, each member of the pair being located anterolaterally. The mandibles are usually of the basic, generalized feeding type with strong molar and incisor, but the molar occasionally is modified into a long styli-form process. Because the first three thoracic somites are fused to the head, the appendages on those somites are modified as maxillipeds. The first pair of maxillipeds has a complicated structure. The endopod functions as a feeding device, and the epipod elaborates into a large and sometimes convoluted gill. The second pair of maxillipeds also is involved in feeding, but in females the coxae of these appendages have small posteriorly directed brood plates. The third pair of maxillipeds usually is leg-like but can have other functions, such as forming an opercular covering over the more anterior mouth appendages. The other five pairs of thoracic appendages function as walking legs. These appendages may have exopods, or the exopods may be reduced or absent. The first five pairs of abdominal appendages are known as pleopods. With a single exception (a species from the deep sea), pleopods are not found on females. Among males, pleopods may be absent, may occur in reduced numbers, or may be fully formed. A freely articulated telson is present in five families but is fused to the last abdominal somite in three families. The last pair of abdominal appendages is the uropods. In most cumaceans the uropods are composed of a peduncle that leads to two rami, the endopod and the exo-pod. The structure of the uropod often has bearing on classification at the genus level.
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