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The rotifers are 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 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. An interesting thing 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:

Seisonoidea - marine forms commensal on crustacean Nebalia, bisexual
Monogonata- free-swimming and sessile forms, small and sometimes seasonal males, single female gonad
Bdelloidea - free-swimming and crawling forms, retractable head with bipartite corona, no males, paired female gonads.

The closest relatives to the rotifers are the Acanthocephala (spiny-headed worms). Further relationships to other phyla are not so clear, but they are probably related to the Trochozoa, as suggested by the presence of trocophore-like larvae in some groups.
Bdelloid, feeding - Photo by JG