Rotifer

The rotifers a phylum of microscopic pseudocoelomate[?] animals. Most rotifers are around 0.1-0.5 mm long, and are common in freshwater throughout the world with a few saltwater species.

Rotifers get their name (derived from Latin and meaning "wheel-bearer") from the corona, which is composed of several ciliated tufts around the mouth that in motion resemble a wheel. These create a current that sweeps food into the mouth, where it is chewed up by a characteristic pharynx (mastax) containing tiny jaws. When unattached, it also pulls the animal through the water. Most free-living forms have pairs of posterior toes to anchor themselves while feeding.

There are a variety of different shapes of rotifer. There is a well-developed cuticle which may be rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape. A few of these move by inchworming along the substrate. Other rotifers are sessile, living inside tubes or gelatinous holdfasts, and may even be colonial. In most rotifers the males are reduced, and may even be absent, in which case reproduction is by parthenogenesis. In some species, parthenogenesis produces two distinct types of eggs; one type of egg goes on to develop into a normal female, while the other develops into a degenerate male form that cannot even feed itself and exists only to produce sperm. In these species fertilized eggs form resistant zygotes that are able to survive when the pond they live in dries up, only resuming development into a new female generation when conditions improve again.

An interesting feature is that adult rotifers frequently have a precise number of cells, usually on the order of a thousand. There are about 2000 species, classified into three classes. Sometimes the Acanthocephala are included as a class as well, though they are more usually treated as a separate phylum, which then becomes the closest relative to the rotifers. The Gnathostomulida may be close relatives as well. These phyla make up a group which may be related to the Trochozoa, as suggested by the presence of trocophore-like larvae in some groups, or possibly the Ecdysozoa.