Rotifers occur at all except polar latitudes. Almost all are freshwater and only two genera are exclusively marine. Rotifers are pseudocoelomate animals with a body cavity containing fluid and a loose syncytium of amoeboid cells. There is no peritoneum. These animals are eutelic and individuals of each species have a characteristic and constant number of cells.
Tissues are often syncytial.
The characteristic number of cells originates in the embryo and remains for life with growth occurring through increase in cell size, rather than through addition of new cells. Rotifers possess a characteristic ciliary ring, or corona, at the anterior end to generate the feeding and locomotory current. The body wall is a syncytial epidermis, with an intracellular, epidermal lorica, and a loose arrangement of somatic muscles. The lorica may be thin and flexile or thick and rigid. The gut is usually complete and the pharynx is an elaborate, muscular mastax with up to 7 hard jaws, or trophi.