Ecdysozoa classes

Secernentea was a class of nematodes in the Classical Phylogeny System (Chitwood, 1958) and is no longer in use. This morphological-based classification system has been replaced by the Modern Phylogeny system, where taxonomy assignment is based on small subunit ribosomal DNA (SSU rDNA).

Eutardigrada are a class of tardigrades (Tardigrada) without lateral appendages. Primarily freshwater bound, some species have secondarily gained the ability to live in marine environments (Halobiotus). By cryptobiosis many species are able to live temporarily in very dry environments. More than 700 species have been described.

thumb|right|240px|Mononchidae eating a Mononchidae Adenophorea or Aphasmidia was a class of nematodes (roundworms). It has been by and large abandoned by modern taxonomy, because there is strong evidence for it being a motley paraphyletic group of unrelated lineages of roundworms.

Heterotardigrades (class Heterotardigrada) is a class of the tardigrades (water bears) that have cephalic appendages and legs with four separate but similar digits or claws on each. 444 species have been described.

The Chromadorea are a class of the roundworm phylum, Nematoda. They contain a single subclass (Chromadoria) and several orders. With such a redundant arrangement, the Chromadoria are liable to be divided if the orders are found to form several clades, or abandoned if they are found to constitute a single radiation.

Enoplea (enopleans) is a class within the phylum Nematoda.

Mesotardigrada is one of three classes of tardigrades, consisting of a single species, Thermozodium esakii. The animal reportedly has six claws of equal length at each foot. 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.

Cyclorhagida is a class of kinorhynchs, small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos, commonly known as mud dragons.