Copepoda Harpacticoida: General Structure (Part 4)

The first four pairs of swimming legs (pereiopods P1-P4) are basically biramous with an exopod and endopod. The two members of each leg pair are united into a single functional unit by an intercoxal sclerite. Each ramous is at most 3-segmented. The arrangement of setae and spines on the leg rami is the character most widely used for species identification and a setal formula denotes this arrangement. The fifth pais of legs (pereiopd P5) are each basically biramous and leaf-like, but the endopod and basis remain separated only in a few species. In all other species, the two elements have fused into a single baseoendopod. The sixth pais of legs (pereiopod P6) is the last in both sexes. In males each member is primitively represented by a single plate with three setae. In females, the sixth leg is vestigial and incorporated in the closing apparatus of the genital antrum. Substantial modifications may be found in the shape and structure of the caudal rami. However, a basic pattern of seven (three lateral, one dorsal and three terminal) ramal setae can be recognized.
Males are nearly always smaller than females but are also distinguished by the structure of the antennules, genital somite and the P5 (which is smaller and less ornate than the females)