[Illustration: Fig. 105. Kerona silurus.—_a_, contractile vesicle; _b_, mouth; _c_ _c_, animalcules which have been swallowed by the Kerona.] None of the Infusoria have regular jointed limbs, but certain families of the higher genera have peculiar and powerful organs of locomotion partly consisting of strong ciliary bristles placed on the anterior in rows, used for crawling or climbing, and partly consisting of groups of strong processes which serve as traction feet, generally trailing behind the animal while swimming, or used to push it forward. When the bristles or cilia of this high group of Infusoria are used for crawling their motions may be traced to the contraction of the skin; but in the Infusoria that are never fatigued though their cilia vibrate incessantly night and day, it may be presumed that these motions are altogether independent of the will of the animal, in as much as there are innumerable cilia in the human frame that are never at rest during the whole course of our existence, nor do their vibrations cease till a considerable time after death—a striking instance of unconscious and involuntary motion. The cell which constitutes the body of the Infusoria is filled with sarcode, which is the receptacle of the food, and in that substance all the internal organs of the animalcule are imbedded. In the higher genera it is full of granular particles of different sizes and forms, and it contains a nucleus in its centre, characteristic of cellular protozoa generally. The nucleus is of a dull yellow colour, and is enclosed in a transparent capsule, which in the smaller Infusoria reflects light brilliantly. It is generally of an ovoid form and single, but in several species the nucleus is double, and in others there are several nuclei. The Infusoria have a distinct mouth and gullet, and for the most part another aperture for ejecting the indigestible part of their food, though some discharge it by the mouth, others through any part of their surface. A few of the larger Infusoria devour the smaller; others feed on minute vegetable particles, chiefly diatoms. Solid substances that are swallowed are collected into little masses mixed with water, and enter into clear spherical spaces called vacuoles in various parts of the sarcode, where they are partially digested. When the animal has not had food for some time, clear spaces only filled with a very transparent fluid are seen, variable both in size and number. It was on account of the digestive vacuoles that the Infusoria were called Polygastria by Ehrenberg.