Isopoda is an order of crustaceans that live in the sea, fresh water, or on land. Most are small grayish or whitish animals with rigid, segmented exoskeletons. They have two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Aquatic species mostly live on the seabed or bottom of freshwater bodies of water, but some more derived taxa can swim for a short distance. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball to conserve moisture or as a defense mechanism . _.