Rotifer
The
rotifers make up 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"; 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.
