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.
Finally there is the less common type of cyst formation, such as is found in the flagellate Oicomonas termo described by Martin.[19] This flagellate, in common with all other forms, reproduces by dividing into two; the division of the nucleus initiating the process. At certain undetermined periods of the life history, however, conjugation occurs between two similar animals forming a large biflagellate body known as the zygote. After swimming about for varying periods of time, during which the size increases and a large vacuole appears, the zygote secretes a thick wall, loses its flagella, and becomes a cyst. While in this condition the two gamete nuclei fuse to form one, and eventually a single Oicomonas emerges from its cyst.
Similarly in A. diploidea the cysts are formed after two individuals have come together. In the young cysts two amœbæ are found in close association, and according to Hartmann and Nägler[12] a sexual process occurs inside the cyst involving a “reductive” division of the nuclei. This requires confirmation, but it is certain that only one individual comes out of the cysts, which originally contained two amœbæ.
Such cysts have been termed by some writers “reproductive,” evidently a misleading term, since no increase in numbers, but rather a decrease, results from the process. A better term is, perhaps, conjugation cyst.
In soil protozoa, then, three different modes of cyst formation obtain, and failure to make the distinction inevitably leads to confusion.
Before leaving the question of life histories, reference must be made to a peculiar and characteristic feature of Nægleria gruberi. This amœba under certain circumstances assumes a free-swimming biflagellate stage. After variable periods of time the flagella are lost and the ordinary amœboid condition resumed. What are the factors concerned in the production of flagellates is unknown, but flooding the coverslips with distilled water is an effective method for causing their appearance.