Gastrothrichs are small (less than 4 mm), dorsoventrally flattened, transparent worms. At the anterior end they have sensory bristles, cilia, sensory pits, and some species have eye spots. The body ends in a tail or fork. Over 400 species are known. They inhabit interstitial spaces in sand, and move using epidermal cilia, or loop in a leech-like fashion using adhesive tubes, see Chaetonotus sp. on the right which has two adhesive tubes. Males tend to be rare and poorly developed.