Problognathia minima.   Problognathia minima, a jaw worm   This transparent, microscopic jaw worm is a Gnathostomulid which lives in the spaces between sand grains in shallow seas. All of the worms in this phylum are characterized by unique hard jaws. The dark structures at the front end are parts of a hard feeding apparatus Gnathostomulids use to graze on fungi, bacteria, and protoctists dwelling among underwater algae and sand grains. More than 6,000 Gnathostomulids per liter can be extracted from fine black sand; because of hydrogen sulfide produced by bacteria, this sand smells like rotten eggs. Such environments, rather rare now, probably dominated the Earth in its earliest history.   Gnathostomulids also live on leafy underwater plants and algae. Bilaterally symmetrical beings, the Gnathostomulids resemble Platyhelminthes, Nemertines, Mesozoans and other Bilateria. Jaw worms share some characteristics with flatworms: dead-end digestive tract and ciliated body surface. However, some zoologists think jaw worms are degenerate forms, derived from pseudocoelomates (as snakes and whales have evolutionarily "degenerated" from animals with four limbs).

Phylum Gnathostomulida   Greek gnathos, jaw; stoma, mouth   Gnathostomulids are microscopic acoelomate marine worms that have a characteristic feeding apparatus made of a hard noncellular material. Gnathostomulids are found mainly in shallow salty waters down to a depth of several hundred meters and in the spaces between sand grains. The worms are tiny &emdash;they can flow through pores 900 µm2 in cross-sectional area. Species are known from Maine, the Florida Keys, Bimini, the Caribbean, Bodega Bay (California), the Indo-Pacific Ocean and the White Sea. They probably live worldwide. About 80 species in 20 genera of gnathostomulids have been described so far, and there are probably more than a thousand species alive today. They live in the sulfuretum&emdash;deep, black, fine sand smelling of hydrogen sulfide produced by bacteria &emdash;and also on the thallus (leafy parts) of algae or on leaves of marine plants, such as the eel grass Zostera and the marsh grass Spartina. In certain sediments, gnathostomulids may outnumber even nematodes: there can be more than 6000 individuals per liter of sediment. Despite their enormous numbers, gnathostomulids remained unrecognized and undescribed for so long because they stick to sand and other particles and because they degenerate so quickly after death. It requires sophisticated extraction techniques to pull them off these surfaces alive. Gnathostomulids are hermaphrodites. The single ovary produces large eggs, which mature one at a time. Posterior to the ovary is at least one testis. They do cross-fertilize&emdash;some, like the Problognathia shown here, have a penis stiffened by a stylet. One gnathostomulid injects sperm packets between the skin and gut of its mate. The undulipodiated sperm swim to a storage sac called a bursa. Fertilization takes place internally, after which the fertilized egg ruptures the body wall and is released. Development is direct, from egg to adult hermaphrodite worm; there are no larval stages. However, at least in some species, a nonsexual feeding stage may alternate with a distinct nonfeeding sexual stage&emdash;taking a year to complete the cycle. Gnathostomulids have transparent bodies 0.3 to 1.0 mm long; a slight constriction separates the head from the trunk, and there is no external cuticle. They have no circulatory or respiratory organs and no skeleton. There is a modest nervous system in the outer epithelial layer. In some species, the head bears well-developed sense organs&emdash;stiff bundles of cilia and pits lined with cilia, collectively called the sensorium. On the belly side of the head are the feeding structures: a hard comblike basal plate (used for grazing on films of bacteria, protoctists, and fungi), two lips, a mouth, and a pair of toothed lateral jaws within a muscular pharynx. The jaws work rapidly, snapping open and closed in about a quarter of a second. Particles of food are passed into the gut, a tubular sac having no anus&emdash;undigested food leaves by the mouth. A pair of excretory organs, each made of two or three cyrtocytes (curved cells), is located on the sides of the bursa. Gnathostomulids move by using their body cilia and by rapid muscle contractions. They nod their heads from side to side, swim, glide, constrict, and twist by using three or four paired longitudinal muscles. The cells of the external epithelium bear only one cilium each, and the ciliary propulsion can reverse direction&emdash;these traits distinguish gnathostomulids from flatworms, which they otherwise resemble. Both phyla have body cilia, lack a coelom and anus, and are hermaphrodites. In fact, gnathostomulids have been recognized as an independent phylum only since 1969. Evidence against a close relationship of the two phyla is that sperm tails of gnathostomulids have a typical 9 + 2 cross section, whereas those of flatworms have a 9 + 1 cross section. Their jaw structure may relate gnathostomulids to rotifers; on the other hand, monociliated epithelium has been found only in gastrotrichs and gnathostomulids. Some famous fossils called conodonts were once thought to be remains of the hard parts of ancient gnathostomulids. They are toothlike fossils found in black, silty shale deposited under anaerobic conditions from the late Cambrian to late Triassic, from about 580 to 200 million years ago. It is surmised that conodonts were used as "teeth" to tear apart fungal hyphae and algal mats. However, the basal plates of modern gnathostomulids differ from conodonts in that they are made of tough organic matter rather than of calcium phosphate. In the early 1980s conodont animals were assigned a (fossil) phylum of their own, Conodonta.   Genera: Austrognatharia Gnathostomaria Gnathostomula Haplognathia Mesognatharia Nanognathia Onychognathia Problognathia Semaeognathia   Literature: Durden, C., J. Rodgers, E. Yochelson, and R. Riedl, "Gnathostomulida: Is there a fossil record?" Science 164:855&endash;856; 1969. Fenchel, R. M., and R. J. Riedl, "'The sulfide system: A new biotic community underneath the oxidized layer of marine sand bottoms." Marine Biology: International Journal on Life in Oceans 7:255&endash;268; 1969. Gould, S. J., "Nature's great era of experiments." Natural History 92:12&endash;21; 1983. Riedl, R. J., "Gnathostomulida from America." Science 163:445&endash;452; 1969. Sterrer, W., "Systematics and evolution within the Gnathostomulida." Systematic Zoology 21:151&endash;173; 1972.