The elongate body of a gnathostomulid, which is typically less than 2 mm long (range 0.3 to 3.5 mm), usually has distinct head, trunk, and tail regions. Outer epithelial cells each have single cilia which collectively help the animal glide between sand grains. The ventral mouth leads to a blind gut (i.e., the gut has just a single opening, rather than a separate mouth and anus), but undigested solid waste may leave not only through the mouth but also via a temporary anus or temporary dorsal connection to the epidermis.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)