An isopod is any member of the order Isopoda. Isopods are segmented crustaceans, the best known of which are woodlice, pill bugs, sow bugs, and other land-dwelling species. In caves, aquatic isopods are often studied because many are troglobitic and depend on caves for survival. Most isopods are small; however, there are about nine species of so-called giant isopods in the genus Bathynomus.