The experimental difficulties of dealing with soil protozoa are considerable, and without a thoroughly sound technique investigators may easily go astray. CLASSIFICATION. The animal kingdom is divided into two main groups or sub-kingdoms--the Protozoa and the Metozoa. In the latter the characteristic feature is that the body is composed of several units, called cells, and consequently such animals are often spoken of as multicellular. The Protozoa, on the other hand, are usually designated as uni-cellular, since their bodies are regarded as being homologous to a single unit or cell of the metozoan body. For various reasons exception has been taken by Dobell[9] and others to the use of the term uni-cellular, for, as Dobell says, “If we regard the whole organism as an individual unit, then the whole protozoan is strictly comparable with a whole metozoon, and not with a part of it. But the body of a protozoan, though it shows great complexity of structure, is not differentiated internally into cells, like the body of a metozoon. Consequently it differs from the latter not in the number of its cellular constituents, but in lacking these altogether. We therefore define the sub-kingdom of the protozoa as the group which contains _all non-cellular animals_.” It should be pointed out that this view does not find favour with many zoologists, but it is useful in bringing into prominence the fact that each protozoan is comparable as regards its functions with the multi-cellular animals. The protozoa are again further divided into four main classes:-- I. Rhizopoda. II. Mastigophora. III. Ciliophora. IV. Sporozoa. Of the above classes, representatives of each of the first three are found living in the soil, but up to the present there is no evidence that any sporozoon is capable of living an active life in the soil, though the cysts of such organisms may be present. The class _RHIZOPODA_ consists of those protozoa whose organs of locomotion and food capture are _pseudopodia_, that is, temporary extensions of the living protoplasm. The body is typically naked, that is to say, without any cuticular membrane, though in some forms, ex. _Amœbæ terricola_, the external layer of protoplasm is thickened to form a pellicle. A skeleton or shell may be present. The class is further sub-divided into various sub-classes, only two of which concern the soil protozoologist, viz., the _Amœbæ_ and the _Mycetozoa_, of which the most important representative is _Plasmodiophora brassicæ_, which attacks the roots of many cruciferous plants, causing the disease familiarly known as “Fingers and Toes.” The _Amœbæ_ are again divided into two orders:-- (_a_) _Nuda_, without shell or skeleton; (_b_) _Testacea_, with shells often termed _Thecamœbæ_.