Mesotardigrada is one of three classes of tardigrades,[3] consisting of a single species, Thermozodium esakii.

This species was described in 1937 by German zoologist Gilbert Rahm from a hot spring near Nagasaki, Japan.

The inability of taxonomists to replicate Rahm's finding has cast doubt on the accuracy of the description, making T. esakii, and by extension the entire class Mesotardigrada, a taxon inquirendum.

[11] The type specimen Rahm used as the basis of his description has either been lost[12] or it was never preserved in the first place, which Grothman et al. (2017) suggest is consistent with the lax taxonomic standards of the 1930s.

[11] Grothman et al. (2017) suggest that Rahm might have observed and misinterpreted a species in the class Heterotardigrada, possibly belonging to the genus Carphania or Oreella.