Because of their very small size and mostly soft bodies, rotifers are not commonly favored for fossilization. The rotifers are microscopic animals, and under high magnification will look something like the picture at upper left, for most perople using a light microscope. Those with more sophisticated microscopes and lighting techniques can give rotifers such as Philodina , grazing at lower left, a beautiful glow. At right, Collotheca is another monogonont rotifer, shown here bearing an egg on its stalk end. Notice the extemely long coronal cilia this rotifer uses to catch food.
Rotifers are multicellular animals with body cavities that are partially lined by mesoderm .
These organisms have specialized organ systems and a complete digestive tract that includes both a mouth and anus. Since these characteristics are all uniquely animal characteristics, rotifers are recognized as animals, even though they are microscopic. Most species of rotifers are about 200 to 500 micrometers long. Rotifers are thus multicellular creatures who make make their living at the scale of unicellular protists. The rapid movement of the cilia in some species makes them appear to whirl like a wheel.