GastrotrichaGastrotricha is a phylum of tiny, free-living, freshwater and marine-dwelling animals. They are bilaterally symmetrical and acoelomate. They have a mouth and an anus, as well as circulatory and nervous systems with some poorly understood sensors. The skeleton is hydrostatic. The body surface has cilia, scales, and adhesive tubes. About 450 species are known, with no fossils.