The gastrotrichs are a phylum of microscopic animals, found in fresh water and marine environments. They are bilaterally symmetric, with a complete gut. The body is covered with cilia, especially about the mouth, and has two terminal projections that serve as adhesive tubes. Like many microscopic animals, their locomotion is primarily powered by hydrostatics, and they reproduce entirely by parthenogenesis. Originally they were thought to have a pseudocoelom, but this was an artifact created by preservation methods, and they are now known to be acoelomate. Genetic studies place them as close relatives of the flatworms. About 450 species are known.
Another set of characters is presented that supports the monophyly of Ecdysozoa: molting under influence of ecdysteroid hormones, loss of locomotory cilia, trilayered cuticle and the formation of the epicuticle from the tips of epidermal microvilli.
Comparative morphology suggests Gastrotricha as the sister group of Ecdysozoa with the synapomorphies: triradiate muscular sucking pharynx and terminal mouth opening.
Proposed relationships under the Ecdysozoa hypothesis: Panarthropoda and Cycloneuralia are sister taxa, the closest sister group of Ecdysozoa is Gastrotricha.
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