Isopoda is an order (group) of crustaceans that includes woodlice, sea slaters and their relatives. Isopods live in the sea, in fresh water, or on land, and most are small greyish or whitish animals with rigid, segmented exoskeletons (external skeletons). 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. Females brood their young in a pouch under their thorax. Isopods have various feeding methods: some eat dead or decaying plant and animal matter, others are grazers or strain food particles from the water around them, a few are predators, and some are internal or external parasites, mostly of fishes. Aquatic species mostly live on the seabed or bottom of freshwater bodies of water, but some more derived taxa (advanced groups) 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 defence mechanism.