The phylum Tardigrada belongs to the Panarthropoda group, together with the onychophorans (velvet worms) and arthropods, and comprises almost 1,000 described species. However, taxonomists expect that at least 10,000 species exist. The phylum is divided into three classes: Hetero-tardigrada, Eutardigrada, and Mesotardigrada. The latter was established on the basis of a single species, Thermozodium esakii, found in a hot sulfur spring in Nagasaki, Japan. However, the species has not been recorded since the end of World War II. The Heterotardigrada consists of two orders, Arthro-tardigrada and Echiniscoidea, and is characterized by the presence of cephalic appendages, so-called cirri and clavae, that function as mechano- and chemoreceptors, respectively. The arthrotardigrades are marine forms that usually have median cirrus and telescopic legs, with or without toes, while the echiniscids are terrestrial armored or marine unarmored forms. The echiniscids have no median cirrus and the legs lack toes. All heterotardigrades have a separate gonopore and anus.
Eutardigrada consists of two orders, Parachela and Apoc-hela, and is characterized by the absence or reduction of external sensory structures. The cuticle is unarmored and the legs have no toes. The so-called double claws of eutardigrades are differentiated into a primary and a secondary branch. Gametes, excretory products, and feces are released through a cloaca. True hetero- and eutardigrades are found in Cretaceous amber from Canada and the United States, and an aberrant tardigrade has recently been recorded in Siberian limestone from the Middle Cambrian.
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