any of a phylum (Tardigrada) of minute water animals with segmented bodies and four pairs of unsegmented legs, often regarded as primitive arthropods
See tardigrade in American Heritage Dictionary 4
(tärˈdĭ-grādˌ)
noun
Any of various slow-moving, microscopic invertebrates of the phylum Tardigrada, related to the arthropods and having four body segments and eight legs and living in water or damp moss. Also called water bear.