Rotifer
| Rotifers | ||||||
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| Scientific classification | ||||||
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| Classeses | ||||||
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Seisonoidea Bdelloidea Monogononta |
Rotifers get their name (derived from Latin and meaning "wheel-bearer"; they have also been called wheel animalcules) 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. Like many other microscopic animals, adult rotifers frequently have a fixed number of cells within a species, usually on the order of one thousand.
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.
There are about 2000 species, divided into three classeses. The parasitic Acanthocephala may belong among the rotifers as well. These phyla belong in a group called the Gnathifera, which may be related to the Platyhelminthes.