The most striking character of the Infusoria is the presence of vibratile cilia, which are variously arranged; in some entirely covering the body, irregularly or in regular rows, in others being situated at definite parts only. By the action of the cilia they are enabled to swim freely in the water, also to obtain their food, which consists of minute Algæ or fragments of animal matter. In many of them there is a special row or set of cilia, which, by the currents it produces, urges the particles of food suspended in the water towards the mouth. The cilia also act as respiratory organs, by changing the water with which their bodies are in contact. In some of the species there are stout bristles or setæ, by which they are enabled to crawl upon water-plants. On carefully examining the bodies of the Infusoria, rounded granular spots will be seen, frequently containing minute Algæ, &c. (fig. 28 _b_). These spots are the digestive cavities, and have been called _gastric sac´culi_; but whether they are definite sacs or mere excavations, formed by the particles of food having been forced into the softer internal substance of the body, has not been positively determined. The sacculi may be filled artificially by mixing very fine indigo, or carmine, on a slide with the water in which the Infusoria are contained. A definite food-tube or alimentary canal has been detected in a few of the Infusoria; but it cannot be shown to exist in the majority of them. A mouth exists in most of them, and is sometimes indicated by a row or set of cilia somewhat larger than those existing upon other parts of the body, and leading to or placed near it. The particles of food which have entered the body are often seen to pass round it, as if circulating, descending on one side and ascending on the other. In addition to the gastric sacculi, certain clear transparent spots may also be seen within the body, appearing light or dark according to the adjustment of the focus. If these are attentively watched, they will be seen to contract and finally disappear, becoming again distended and vanishing at tolerably regular intervals. These are the _contractile vesicles_ (figs. 27 _a_, 28 _a_, 37 _a_), and they contain a clear liquid, the nature of which is uncertain.