Rotifera, or Wheel-Animals, a class of small aquatic animals, which belong to the Worms or subphylum of Vermes. They are of interest from their beauty when examined under the microscope, and from numerous points in their anatomy and life-history. The body never consists of more than one segment. At the anterior end is a disc-shaped mass provided with cilia, the slashing of which causes the disc to appear as. if it were revolving: whence the name of the class. The body is generally attached by a process or foot, but in some cases the animal is free-swimming, but has a bifid foot by which it can anchor itself at will (e.g. Hydatina). The animal is usually protected by a hard cuticle or skin, and this may be surrounded by a tube made of pellets of mud as in Melicexta. The body cavity is probably an archicoele (q.v.). In most cases the male is small and degraded, and appears only to live in order to fertilise the female; it has not yet been discovered in one family (the Philodinadcv). In some cases the animals can survive for a long period after being completely dried. The systematic position is obscure; the resemblance of the ciliated disc to the similar organ in the larva known as the Trochosphere leads to their inclusion in the Worms. They are world-wide in distribution, live in either fresh or salt water or in damp moss and grass.