Gastrotricha 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.