IV. _Peritricha._ Typically of a sedentary habit and the cilia are reduced to a zone round the adoral region of the body. A typical soil form is _Vorticella microstomum_. The above classification is far from complete, but should be sufficient to give an idea of the general grouping of the organisms. For a more detailed account reference must be made to the numerous text books on protozoa. LIFE HISTORIES. The life history of each species has its own characteristic features as regards nuclear division, etc., and in many forms, notably the amœbæ, it is impossible to identify them with certainty unless the chief stages of the life history are known. In general, however, the soil protozoa pass through very similar phases and develop in a perfectly straightforward way. Broadly speaking, there are two main phases of the life history--a period of activity often mistermed vegetative, and a period of rest. In the former the animal moves, feeds and reproduces, while in the latter there is secreted round the body a thick wall, capable of resisting adverse external influences. This condition is termed the cystic stage, and by means of it the animals are distributed from place to place by air, water, etc. Indeed, so resistant are the cysts that many of them are capable of withstanding the action of the digestive juices of the intestines of animals, through which they pass to be deposited by the fæces on fresh ground. This cystic stage of the life history is found in practically all free-living protozoa, though it is not formed in exactly the same manner in every case. In the majority of instances the cyst is the product of a single organism, round which is formed a delicate gelatinous substance which soon hardens and gradually acquires the peculiar characters of the wall. Concerning the chemical nature of this wall there is little known, but Goodey,[11] working on the cysts of _Colpoda cucullus_, found it to be formed of a carbohydrate, different from all carbohydrates previously described, to which the name “Cytose” was given. When in this state the animals are able to remain dormant for considerable periods until favourable conditions once more obtain when the wall is ruptured and the animal again resumes the active phase of its life history. This simple process is characteristic of such species as _Heteromita globosus_, _Cercomonas spp._, and many others. It will be noted that no increase of numbers, i.e. reproduction, occurs. A more complex condition is, however, sometimes found, as, for example, in the ciliate _Colpoda steinii_, where actual reproduction into small animals takes place within the cyst.