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.
IV. Peritricha. Typically of a sedentary habit and the cilia are reduced to a zone round the adoral region of the body. A typical soil form is Vorticella microstomum.
The above classification is far from complete, but should be sufficient to give an idea of the general grouping of the organisms. For a more detailed account reference must be made to the numerous text books on protozoa.