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.