entirely the metabolism of the Infusorian, whereas the micronucleus is essentially a generative nucleus from which macro- and micro-nuclei are again and again produced. Encystment amongst the Infusoria is very general, and is essentially a means of protection when the surrounding medium dries up. Doubtless these cysts are frequently carried long distances by the wind, which explains the wide geographical distribution of most species. Also, multiplication often takes place in the encysted condition. Some Infusoria live a free life, others are sedentary; the latter form colonies in fresh as well as in salt water. Numerous species are parasites of various lower and higher animals,[245] and a few also are parasitic in man. [245] It may be stated that numerous peculiarly shaped species live in the stomach of ruminants, others in the colon of horses. Several species are found in the rectum of frogs and toads; others, again, on the surface of the bodies of fishes; and various other species exist in and on the bodies of invertebrate animals. The Prague zoologist, v. Stein, introduced a classification of the Infusoria that has been almost universally adopted. It is founded on the different position of the cilia on the body. Though, no doubt, artificial, it is a convenient system. Bütschli has compiled a better one.[246] But for our purpose Stein’s system is sufficient:-- [246] Bronn’s _Cl. u. Ordn. d. Thierr._, i, Protozoa, Part 3, Infusoria. Order 1. _Holotricha_, Infusoria with cilia that are evenly distributed over the entire body. Order 2. _Heterotricha_, ciliated all over like the _Holotricha_, but having stouter cilia about the peristome. Order 3. _Hypotricha_, only ciliated on the ventral surface. Order 4. _Peritricha_, with only a ring of spiral cilia, mostly sedentary. The Infusoria observed in man belong to the order _Heterotricha_, with few exceptions. Genus. *Balantidium*, Claparède et Lachmann. Heterotrichous Infusoria of oval or bag-like form and almost circular on transverse section; the anterior extremity narrowed, the posterior end broad and rounded off, or also narrowed; the peristome starting at the anterior end is broadest there and becomes narrower as it gradually obliquely approaches towards the posterior extremity. There are coarse cilia along the entire left border and the anterior part of the right border. Longitudinal striation is distinct and regular. There are two contractile vacuoles on the right, and occasionally also two or more to the left. The anus (cytopyge) is terminal. The macronucleus is usually horse-shoe or kidney-shaped, sometimes oval; the micronucleus contiguous. Reproduction is by binary fission and conjugation, and encystment occurs. The cysts are spherical or oval. These ciliates are parasitic in the large intestine of human beings and pigs, in Amphibia, and in the body cavity of polychæte Annelida. *Balantidium coli*, Malmsten, 1857.