There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first thoracic somite is completely fused with the head. All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite. The eyes are not stalked, but are sessile on the sides of the head. The antennules have only a single branch, and in the Woodlice are very small. The antennæ have no exopodite, although in a few other Isopods a minute vestige is present. The thoracic limbs never have any trace of exopodites. The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea. The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills. In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter. In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female. They are almost the only Crustacea, except the Cirripedes already mentioned, which are normally hermaphrodite. [Illustration: FIG. 21--AN AMPHIPOD (_Gammarus locusta_). ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.) _a´_, Antennule; _a´´_, antenna; _acc_, accessory (inner) flagellum of antennule; _br_, gill plate; _cx_, coxal plate (the expanded first segment of the leg); _gn_, the two pairs of gnathopods (prehensile legs); _plp´´´_, abdominal appendage of third pair; _prp´_, _prp´´_, first and second peræopods, or walking-legs; _t_, telson; _ur_, uropod; II, VIII, second and eighth thoracic somites; 1, 6, first and sixth abdominal somites]