Isopods are possibly the most diverse in overall body form of all crustacean orders. Besides occurring in shallow marine water where crustaceans are typically found, isopods have succeeded both on land and in the deep sea where they have radiated into a variety of bizarre forms. Habitat can be useful as a guide to identification.
Such is the diversity of form of the isopods that no species can be regarded as typical. Some are flat, some are thin, some are free living and others are parasitic. Isopods are recognised by having most of the seven pairs of legs very similar to each other and in having only one pair of 'uropods' or tail-limbs. Virtually all of the 95 world families are represented by numerous species in Australia.
Read more at http://www.mov.vic.gov.au/crust/isopogal.html