If size alone were a criterion of classification, the Rotifera would have to be ranked among the microscopic Protozoa, as they are scarcely visible to the naked eye; but a more complicated organisation separates them widely from these lowest members of the animal kingdom, and entitles them to be placed next to the worms. [Illustration: Ptygura mehcerta.--(A rotifer highly magnified.) 1. Partially expanded. 2. Completely expanded, the cilia in action causing currents indicated by the arrows. 3. Contracted. _a._ Contractile vesicle. _b._ Situation of the anal orifice. ] They are chiefly characterised by a remarkable rotatory or ciliary apparatus, whose vibrating motions, whirling the water about in swift circles or eddies, engulf in a fatal vortex their microscopic food, or enable them to swim from place to place. Such is the crystal transparency of these curious little creatures that their internal structure can be easily recognised. The mouth is placed immediately below the rotatory apparatus, and when once an unfortunate animalcule has been driven into its gaping portals, it is presently crushed between a pair of formidable sharp-toothed jaws, which are perpetually in motion, whether the animal is taking food or not. After having undergone the action of this lively apparatus, the aliment passes into a tubular stomach surrounded by a cushion-like mass of cells commonly coloured with the hue of the food, and, therefore, concluded to be connected with the digestive system. [Illustration: Conochilus volvox.--(Highly magnified.) _a._ Jaws and teeth. _b._ Papillæ. _c._ Glands. _d._ Ovarium. ]