Gnathostomulida
. Gnathostomulid, group of microscopic marine worms that live between grains of sand in shallow ocean water. These invertebrates are able to live for long periods without oxygen (see Anaerobe ) and are common in stagnant bottom muck and black sand. More than 80 species of gnathostomulids have been described. Gnathostomulids resemble flatworms and were once classified in the same phylum. Like flatworms, gnathostomulids are bilaterally symmetrical (both sides of the body are identical along the midline) and have waving body cilia (tiny hairlike projections); they lack an anus and a coelom (internal body cavity) and are hermaphroditic (individuals have both male and female sex organs). Unlike flatworms, however, each epithelial (skin) cell in gnathostomulids has only one hairlike cilium. In addition there are significant differences in sperm structure between gnathostomulids and flatworms. Gnathostomulids are generally 0.5 to 1.0 mm (0.02 to 0.04 in) in length with a slight constriction