Rotifers are mainly freshwater, but one Class (see below) is marine and some species can be found world wide, but most commonly in in humid terrestrial habitats. They tend to inhabit shallow waters, living on the bottom or on submerged vegetation or other surfaces. About 1800 species are known, all ranging in length from 0.1 - 3.0 mm, and their normal life span is just a few days. Many tissues are syncitial and the number of nuclei is fixed. The anterior corona of cilia is used for movement and feeding. The body is often covered in sculptured cuticle into which the corona can be drawn. The foot can also be drawn into the cuticular covering. The toes or spurs, when present, serve as an anchor. They are almost transparent (see left), with only the digestive tract showing colour. They move by creeping along surfaces, aided by the foot, or by swimming through the water with the beating of the coronal cilia providing propulsion. When they creep the corona is usually retracted into the body, and the adhesive secretions produced by the pedal glands stick to the substrate. When swimming the foot is retracted..

There are three Classes. Class Bdelloidea (see Philodina roseola below left) does not have a cuticular covering, males have never been observed, and the females appear to be obligately parthenogenic. They can survive extremes of temperature and desiccation for years.

Class Monogonata (see Notommata copeus right)contains the largest number of species. Sexual reproduction has been observed, although males are few, absent for most of the year, and much smaller than females.

Class Seisonidea is the marine Class; they are relatively large and live in the gills of crustaceans.