Nematoda
There are about 61,400 spesis of Nematodes, 11,400 of those being marine species. They live in mud, sediment, soil, and in other living things. They feed on bacteria, cryptomonads, and the bodies/food of larger orgamisms. Nematodes have a notoriety for being parasites. Although many species of nematode are parasitic, most are free living.