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.
Representatives of the “naked” amœbæ commonly found in soils are Nægleria (Dimastigamœba) gruberi, Amœba diploidea (possessing two nuclei) and A. terricola, the last two forms possessing a comparatively thick skin or pellicles. Trinema enchelys, Difflugia constricta and Chlamydophrys stercorea are examples of soil Thecamœbæ.
The class MASTIGOPHORA consists of those protozoa whose typical modes of progression are by means of flagella, whip-like filaments which, by their continual lashing motion, cause movement of the animal.