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. The body may be naked or corticate. The only organisms which concern the soil biologist belong to the _Flagellata_ order. The Flagellates differ considerably among themselves, both as regards their mode of feeding, and the number of flagella, thus making their classification difficult and outside the scope of this book. Suffice it to say that in the soil such organisms occur possessing one, two, three or four flagella, ex. _Oicomonas termo_, _Heteromita globosus_, _Dallengeria_ and _Tetramitus spiralis_. Further, their mode of feeding may be _saprophytic_ in which nourishment is absorbed by diffusion through the body surface in the form of soluble organic substances, _holozoic_ where solid food particles are taken in, or _holophytic_ in which food is synthesised by the energy of sunlight. This last group is commonly spoken of as the _Phyto flagellates_, which are to all intents and purposes unicellular algæ, and as such will be dealt with in Chapter VI. The class _CILIOPHORA_ consists of those protozoa whose typical organs of locomotion are threads or cilia. These organisms can in one sense be regarded as the highest of the protozoa, since in no other division does the body attain so great a complexity of structure. Moreover, they are typically characterised by a complicated nuclear apparatus with the vegetative and generative portions separated into distinct bodies, the macro-nucleus and the micro-nucleus. Their mode of nutrition is _holozoic_, though recently Peters has brought forward evidence that certain species can obtain their nourishment saprophytically. The sub-class Ciliata comprises four orders, all of which are represented in the soil. I. _Holotricha._ The cilia are equal in length and uniformly distributed over the whole body in the primitive forms, though restricted to special regions in the specialised forms. Typical soil forms are _Colpoda cucullus_, _Colpidium colpoda_. II. _Heterotricha._ There is a uniform covering of cilia, and a conspicuous spiral zone of larger cilia forming a vibratile membrane and leading to the mouth. III. _Hypotricha._ The body is flattened dorso-ventrally and the cilia are often fused to form larger appendages or cirri confined to the ventral surface. Movement is typically a creeping one. Typical soil forms are _Pleurotricha_, _Gastrostylis_, _Oxytricha_.