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a phylum of very small invertebrates, closely related to the Arthropoda, to which they were assigned until recently. The short body, which measures 0.1–1.0 mm in length, lacks distinct segmentation. It has four pairs of unjointed nodulelike legs, with two to nine long, setaceous tarsal digits. There are two ocelli. The mouth organs are a pair of sharp styles that serve to pierce the cell membranes of the algae and mosses that the Tardigrada feed on. The digestive tract is a straight tube extending the length of the body. Respiration is cutaneous. The organisms are dioecious. The young, which resemble adults, emerge from deposited eggs.
Representatives of the phylum, which comprises approximately 300 species, are ubiquitous, inhabiting seas, fresh waters, moss, and soil. Terrestrial species after drying are capable of surviving years in a state of anabiosis. Members of the phylum can tolerate several hours at temperatures as low as - 271°C and as highasl50°C.