The phylum gnathostomulida is one of the more obscure classifications in biology, just as the organisms involved are obscure (although apparently quite numerous). (Common name is "jaw worm" or sometimes, "sand worm," thanks here to Tem42!)
These are tiny worms (mostly microscopic, measuring in at 0.2 to 3 mm in length) who live on marine shores in fine-grained sand, all over the world, but best studied in Western Europe, the Mediterranean Sea, the US coastline, the Caribbean and a few localities in the Pacific Ocean. They are hermaphroditic but do not, apparently, self-fertilize; however, that each worm is both a boy and a girl means that just any other member of the species he/she encounters will be a good mating partner. Contrary to the practice of most marine invertebrates, gnathostomulids produce only a few gametes, yolky eggs produced one at a time (not millions of gametes tossed into the water like sea urchins, for example). The young develop immediately into worms, without an intermediate swimming stage, and so, goes the theory, are more likely to remain in the suitable but rather restricted habitat of their parents. ("Stay near home, baby, we know what's good for you!")
The collection technique will illustrate the habitat. One gathers a bucket of foul-smelling sand reeking of hydrogen sulfide from some intermediate area (dig deep) of sandy beach, including the black layer. This is combined with just enough seawater to cover the sand, and left "to go from bad to worse," as one authority (see Pearce, cited below) has it: that is until the vile odor is even stronger and black spots have appeared on the surface of the sand. At this point the gnathostomulids emerge. They may also live on plant leaves, roots and algae in the same general locations, again, thanks to Tem42 for the information.
They have no anus, and no body cavity by most definitions. They do have a little pair of toothed jaws with which they scrape algae, diatoms, bacteria, and other organic matter from the surface of the sand grains. Because there is little light in their intermediate area of sand, most species have no eyes. Their respiration may be anaerobic; no one knows for sure, because they are difficult to impossible to keep alive in the laboratory. (Too much oxygen?) What we know of them has been developed by examining the hard parts of dead specimens.
So, they're not very important, right?
Most scientists doubt this last conclusion, if only because what we do know suggests that while they are tiny they are also very numerous, perhaps an important part of most shore environments, almost certainly an important part of the ecology of the seashore.
(KINGDOM, Animalia; PHYLUM, Gnathostomulida; number of species unknown.
Sources:
Pearse/Buchsbaum, Living Invertebrates, Blackwell Scientific Publications, Boston 1987
Smithsonian Tropical Research Institute, http://biogeodb.stri.si.edu/bocas_database/search/phylum/25/