Because of their conspicuous ciliation and microscopic size the rotifers were originally classified as infusoria and not distinguished from monocellular organisms. Dutrochet (1812) was the first to use the term Rotifera and to regard the group as a biological unit set apart from the protozoa. Ehrenberg (1838) classed the Rotatoria among the worms, and Metschnikoff (1864) established the distinction between them and the Gastrotricha; Zelinka (1889) included both groups with the Aschelminthes. Although the phylogenetic origin of the Aschelminthes and the relationships within this group continue to present a number of unsolved problems there can be no doubt that Rotifera, Gastrotricha and Nematoda have many structural features in common.