They agree with the true crabs in not having appendages (uropods) to the sixth segment of the pleon, the atrophy being complete in the Homolidae and Homolodromiidae, whereas in the Dromiidae and Dynomenidae a pair of small plates appear to be vestiges of these organs.
The slender pleon has always six distinct segments, the sixth carrying two-branched uropods, the preceding five armed with no pleopods in the female, whereas in the male the number of pairs varies from five to none.
The genuine Isopoda are divided among the Flabellifera, in which the terminal segment and uropods form a flabellum or swimming fan; the Epicaridea, parasitic on Crustaceans; the Valvifera, in which the uropods fold valve-like over the branchial pleopods; the Asellota, in which the first pair of pleopods of the female are usually transformed into a single opercular plate; the Phreatoicidea, a fresh-water tribe, known as yet only from subterranean waters in New Zealand and an Australian swamp nearly 6000 ft.
T, telson, having the uropods or appendages of the last abdominal somite spread out on either side of it, forming the " tail-fan."
In nearly all Malacostraca the last pair of abdominal appendages (uropods) differ from the others, and in the more primitive groups they form, with the telson, a lamellar " tail-fan " (fig.