Rotifer

The rotifers a phylum of microscopic
pseudocoelomate animals. Most rotifers are aroun
0.1-0.5 mm long, and are common in freshwater
throughout the world with a few saltwater          Scientific classification

species.                                           Kingdom: Animalia
Rotifers get their name (derived from Latin and    Phylum:  Rotifera

meaning "wheel-bearer") from the corona, which is           Classes
composed of several ciliated tufts around the      Seisonoidea
mouth that in motion resemble a wheel. These       Bdelloidea
create a current that sweeps food into the mouth,  Monogononta
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.

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