As the name Rotifera suggests, one of their most diagnostic features is the corona or “Wheel”-like complex of cilia in the head region which functions is locomotion and feeding. They are commonly vermiform metazoans, sometimes with elaborate cuticular spines or ornamentation, but rarely exceeding 2mm in length and more commonly less than 0.5 mm long. An estimated 2,000 species of rotifers have been described and divided into three classes, five orders up to 22 families and 123 genera and subgenera. Ten of the families have meiobenthic representatives. Although the majority are known from freshwater, rotifers are also found in brackish water and in the sea.
The Rotifera are composed of three classes. The class Pararotatoria is represented by a single genus, Seison, an exclusively marine taxon epizoic on the gills of the crustacean Nebalia. The remaining two classes include the Digononta and the Eurotatoria: the latter class contains most of the known species of the phylum.
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