Any of various slow-moving, minute invertebrates of the phylum Tardigrada. Tardigrades have a head and four fused body segments, each of which has a pair of stubby legs ending in claws. They live in water, damp moss, flower petals, or sand, and are usually 1 mm (0.04 inches) or less in size. Tardigrades are able to resist extremely low temperature, pressure, and humidity, and go into dormant states for months or years. They are believed to be intermediate in evolutionary development between annelids and arthropods. Also called water bear.