Gnathostomulida

Gnathostomullida...

Since their description and recognition as a separate phylum, Gnathostomulida have been encountered worldwide as na often abundant constituent od marine meiofauna. Most members of this phylum are of meiofaunal size and live in the interstitial spaces between sediment particles. At present, there are 18 genera with over 80 known species, but many more can be expected. The phylum is unique in having a body epithelium in which each cell carries only one cilium, and a bilaterally symmetrical muscular pharynx, equipped with cuticular mouth parts. Gnathostomulida are worm-shaped, cylindrical or with a slight dorsal-ventral flattening, and range from 300 to 3000um in length and 50 to 100 um in diameter; however, the most frequently observed genera have a body width of 50 to 70 um. The body is divided into a head which is commonly expanded and well delimited, a long trunk which constitutes 90% of the body, and a relatively short tail.
All gnathostomulids are hermaphrodites. The phylum is divided into two orders, the elongated Filospermoidea (with filiform sperm: without paired sensory organs, and without a bursa, vagina, or penis stylet), and the usually more compact Bursovaginoidea (with sperm not filiform; and usually with paired siensory organs and a bursa). Representatives of the phylum have been encountered in all the world’s oceans, from the supratidal brackish groundwater to the intertidal, and the subtidal to 800m. As a rule they occur in fine to coarse sand with a high admixture of organic detritus as typically found in sheltered bays, intracoastal waterways, between coral reefs, or wherever slightly reducing conditions may exist.