ROTATOR´IA (_róta_, a wheel) or ROTIF´ERA (_rota_ and _fero_, to bear).--The animals contained in this class are minute, being just distinguishable to the naked eye as white specks. They are common in long-kept infusions and among _Confervæ_ in the water of pools and ditches. Their body is usually longer than broad, often presenting indications of rings; and at or near the posterior end is frequently found a prolongation resembling a tail, but terminated by two short moveable thumb-like processes, rarely a sucker, which enable the animals to cling to objects. The most characteristic organ, however, is a kind of rounded or oval disk, placed at the anterior end of the body, and furnished with cilia. When these are in active motion, the organ appears as a revolving wheel, whence the name of Wheel-animalcules, by which they are sometimes designated. The wheel-organ enables the animals to swim through the water, and also brings their food to the mouth by the currents which it produces. It is usually cleft into two or more lobes, and can be retracted, as is commonly the case when the animals are disturbed. In many of these animals the body is more or less covered by a horny shell or carapace; and in some it is fixed at the bottom of a tube, within which it can be withdrawn. On the anterior part of the body are frequently seen two or more red spots, which represent eyes. The alimentary canal is mostly distinct, being indicated by the colour of its contents, and it is lined with cilia. Towards its front portion is a gizzard (Pl. XI. fig. 2 _a_) containing teeth, which are sometimes attached to a jointed jaw-like framework; these are usually in active motion. No heart or blood-vessels have been observed in the Rotatoria; but on each side of the body in many of them is a long wavy tube, containing at intervals minute ciliated bodies, the cilia propelling the water through the tubes, and so exerting an aërating or respiratory function. The reproduction of the Rotatoria takes place by the formation of ova, which may often be distinguished within the body of the parent. _Rot´ifer vulgáris_ (Pl. XI. fig. 2) is a common species. It has a spindle-shaped body, which is capable of contraction almost into a ball. The front or head end is sometimes protruded (fig. 2), at others retracted and obscured by the exserted disk (fig. 2*); and beneath it is a tentacle-like organ, supposed to represent an antenna (fig. 2 _b_). The position of the jaws is indicated at _a_. The alimentary canal is seen running down the body; and two ova exist, one on each side of it, these being often recognizable by the existence of the eyes and jaws. At the end of the body are two lateral processes, and a tail-like piece, which can be withdrawn or protruded and is furnished with two moveable portions or toes. PLATE XI. [PAGE 150.] ROTATORIA, INFUSORIA, &c. Fig. 1. _Anguillula_ (_Dorylaimus_), species of.