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.
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.
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.