The Protozoa, or one-celled animals, belonged to an unknown world before the invention of the microscope. The first of these instruments enabled the early observers to see some of the larger and more conspicuous members of the group and each improvement of the microscope has enabled us to see more and more of them and to study in detail not only the structure but to follow the life-history of many of them. _The Amoeba._ With some, as the common amoeba (Fig. 8), a minute little form that is to be found in the slime at the bottom of almost any body of water, the life-history is extremely simple. The organism itself consists of a minute particle of protoplasm, a single cell with no definite shape or body-wall and no specialized organs or apparatus for carrying on the life-functions. It lives in the slime or ooze in fresh or salt water, takes its food by simply flowing over the particle that is to be ingested, grows to a certain limit of size, then divides into two more or less equal parts, each part becoming a new animal that goes on with its development as did the parent form. This process of growth and division may go on for many generations, but cannot continue indefinitely unless there is a conjugation of two separate individuals. This process of conjugation is just the opposite to that of division. Two amoeba flow together and become one. It seems to rejuvenate the organism so that it is able to go on with its division and thus fulfil its life-mission which is the same for these lowly animals as with the higher, that of perpetuating the species. _Classes of Protozoa._ The group or Phylum Protozoa is divided into four smaller groups or classes. The amoeba belongs to the lowest of these, the Rhizopoda. Rhizopoda means "root-footed," and the name is applied to these animals because most of them move about by means of root-like processes known as pseudopodia or "false feet." This is by far the largest class and contains thousands of forms, mostly living in salt water but there are many fresh-water species. They are non-parasitic, but some of them by their presence in the body may cause such diseases as dysentery, etc. [Illustration: FIG. 7--Typhoid Fever bacilli. (After Muir and Ritchie.)] [Illustration: FIG. 8--_Amoeba_, showing the forms assumed by a single individual in four successive changes. (From Kellogg's Elementary Zoöl.)] [Illustration: FIG. 9--_Euglina virdis._ (After Saville Kent.)] [Illustration: FIG. 10--_Spirocheta duttoni_, × 4500. (After Breinl and Carter.)]