aschelminth, a name referring to an obsolete phylum of wormlike invertebrates, mostly of microscopic size. Previously, phylum Aschelminthes included seven diverse classes of animals: Nematoda (or Nemata), Rotifera, Acanthocephala, Gastrotricha, Kinorhyncha (or Echinodera), Nematomorpha, and Gnathostomulida. (According to some authorities, Gnathostomulida was replaced by Priapula in this list.) At present, each of these classes, including Priapula, has been reclassified as a separate phylum.

These animals were originally grouped together because all seemed to possess a peculiar type of body cavity called a pseudocoel (that is, a body cavity that does not contain a lining of mesoderm), which develops differently from the body cavities of other animals. It has become clear, however, that these animals do not have close evolutionary linkages with one another, and each group has been placed in its own phylum. On the other hand, rotifers appear to be strongly allied with acanthocephalans, and eventually these two groups may be classified together in the same phylum. On the whole, the other former aschelminths may be closely related to arthropods because all exhibit molting at some point during development.