Protozoa

“ In pure drinking water there may be very few, but in stagnant water they are very numerous, living not on the organic material in solution in this, but on the bacteria which find in such fluid favorable conditions for existence. The food of protozoa consists chiefly of other organisms, particularly bacteria, and they are classed with the animals. The protozoa are the most widely distributed and the most universal of the parasites. The infectious diseases which they produce in man, although among the most serious are less in number than those produced by bacteria. ”

“ AT the present time the importance of protozoa-study is recognized in all branches of biological science where, as single-celled organisms, they illustrate the manifold principles of living things. Thus it is in physiology, in cellular biology, in psychology and in general biology. There is one field, however, a field that is daily growing more extensive, in which the importance of protozoa has only recently been recognized, and this is the field of pathology. ”

“ Myxosporidia, or Parasitic Protozoa.—Internal parasites are very numerous and varied. Some of them are bacteria, giving rise to infectious diseases, especially in ponds and lakes. Others are myxosporidia, or parasitic protozoans, which form warty appendages, which burst, discharging the germs and leaving ulcers in their place. In the report of the U. S. Fish Commissioner for 1892, Dr. R. R. Gurley has brought together our knowledge of the protozoans of the subclass Myxosporidia, to which these epidemics are chiefly due. ”

“ The recent discoveries of the protozoic origin of malaria, sleeping-sickness, and other human and many other animal diseases, has directed attention both to the protozoa, with their complicated life-histories, and to the insects which convey them from one creature to another. Both protozoa and insects are highly specialized groups of animals. The establishment, by the aid of the Quick bequest, of a chair of protozoology will do something to meet the necessities of the case, so far as the protozoa are concerned ”

“ With the knowledge of these other pathogenic protozoa and of their modes of life investigation will bring out the means of combating them and thus of reducing human suffering, and this prospect if for no other reason, is a full justification of the many commissions that have been appointed, and of the vast sums of money that have been spent, to further protozoan research. ”

“ The protozoa, like the bacteria, are unicellular organisms and contain a nucleus as do all cells. They vary in size from forms seen with difficulty under the highest power of the microscope to forms readily seen with the unaided eye. Their structure in general is more complex than is the structure of bacteria, and many show extreme differentiation of parts of the single cells, as a firm exterior surface or cuticle, an internal skeleton, organs of locomotion, mouth and digestive organs and organs of excretion. ”

“ So marked is the tendency to parasitism that they are often parasitic for each other, smaller forms entering into and living upon the larger. Variation does not seem to be so marked in the protozoa as in the bacteria, though this is possibly due to our greater ignorance of them as a class. We are not able, except in rare instances, to grow them in pure culture, and study innumerable generations under changes in the environment, as the bacteria have been studied. ”

“ Following out this sub-division of the entire series of Protozoa, the several groups range themselves into four readily distinguishable sections. In the first, the most lowly organised and most abundant have no oral orifice in the literal meaning of the word, food being intercepted at any point of the surface of the body. This most simple elementary type of structure of the Protozoa is represented in the Amœba and Actinophrys, the various representatives of the Foraminifera, and certain Flagellata, as Spumella and Anthrophysa. ”

“ Protozoa are unicellular organisms possessing a nucleus distinct from the cytoplasm. They belong to the Protistenreich of Haeckel, a kingdom which unites the simplest and most primitive forms of life, and which may be considered equivalent in systemic value to the animal and vegetable kingdoms.
The protozoal cell constitutes an entire individual; it may exist singly, or combined in the form of cell colonies.
The body is a mass of protoplasm sometimes enclosed in a limiting envelope. ”

“ The protozoa are widely distributed, being most abundant in the richer types of soil, especially during the spring and autumn. A great amount of research has been undertaken at Rothamsted, England, and elsewhere, on the part played by protozoa in soil fertility; the evidence thus secured points to the probability that some of these organisms may be detrimental in that they devour certain kinds of bacteria responsible for the production of nitrates and other substances of nutritive value to plants. ”

“ Many as yet unknown species of blood protozoa will doubtless be added to the already considerable list of those attacking man and the lower animals. As regards man, it is doubtful if we have as yet completed the list even of the much-studied malaria parasites, much less determined their respective life histories and zoological relationships. Moreover, there are certain genera of blood protozoa which, although occurring in other mammals, have, so far, not been found, or, at all events, recognized in man, Babesia and Hæmogregarina for example. ”

“ Therefore the reproductive elements of the metazoa and metaphyta are in this respect precisely analogous to the protozoa: potentially, or in their own nature, they are immortal; and, like the protozoa, if they die, their death is an accident due to unfavourable circumstances. But the case is quite different with all the other parts of a multicellular organism. Here, no matter how favourable the circumstances may be, every cell contains within itself, or in its very nature, the eventual doom of death. ”

“ To the medical world for the most part, the group of protozoa consists of the few types of parasitic forms that cause human disease, and in this world any one who has a knowledge of Trypanosoma, or Amæba, or Plasmodium, is a student of the protozoa, while a deeper knowledge makes him a biologist. At the present time there are many students of the group in this sense, and the relations of protozoa to human welfare bid fair to be the most popular aspect of protozoan study, while in the public mind already the term protozoa is apparently the synonym of some new and fearsome thing. ”

“ The protozoa are the lowest group of the animal kingdom. Like bacteria they are composed of single cells so small as to be visible only under the microscope. They play an important part in causing certain diseases of man, especially in the tropics. Among the well-known human diseases of protozoan origin are malaria, amoebic dysentery, and sleeping-sickness. Protozoa also cause several wide-spread and serious plagues of domestic animals.
VISIBLE PARASITES
A few diseases are caused by parasites large enough to be seen with the naked eye. One of the most important is hookworm disease. ”

“ Stuber, 1886) .
8 However, in the Protozoa there are examples of hereditary transmission of parasites, e.g., in the case of Babesia (Piroplasma) bovis and Babesia canis in their invertebrate hosts (ticks) ; in Crithidia melophagia and Crithidia hyalommæ; and in the case of Spirochæta duttoni in its invertebrate host (a tick) .
9 Independently of propagation, many protozoa protect themselves from death by encystment when the water in which they are living dries up; in this condition the wind may carry them over wide tracts of land. ”

“ The foregoing definition does not distinguish the Protozoa sharply from the primitive forms of plant-life, with which, as stated above, they are connected by many transitions; but the differentiation of the body-substance into nucleus and cytoplasm separates them at once from the Bacteria, in which the chromatin is distributed evenly through the body-protoplasm. ”

“ If it be true that there is among protozoa, among unicellular animals, anything comparable to the gradual decline in the growth power which occurs in us, we shall expect it to be revealed in the condition of the cells—to see in those cells which are old an increase in the proportion of protoplasm, and consequently a diminution in the relative amount of nucleus. ”

“ It has recently been discovered that under some conditions these bacteria are preyed upon by tiny one-celled animals (protozoa) living in the soil and are so reduced in numbers that they cannot do their work effectively. If, then, the soil is heated artificially or treated with antiseptics so as to kill the protozoa, the bacteria which escape multiply so rapidly as to make the land much richer than before. ”

“ On the other hand, attachment may be transient (conjugation) when, after the exchange of portions of the nucleus, the couple separate, to multiply independently of each other. Sometimes there is an ALTERNATION OF GENERATIONS, as there may be several methods of propagation combined in the same species, either direct multiplication, conjugation, or copulation being practised; the different generations may thus, in certain cases, be unlike morphologically.
Protozoa inhabit salt water as well as fresh water; they are also found on land in very damp places, and invade animals as parasites. ”

“ It is quite possible, therefore, that Protozoa exist in which syngamy does not occur. In view, however, of the widespread occurrence of sexual processes amongst unicellular organisms, both of animal and vegetable nature, and the fact that extended observation continually brings to light new instances of this kind, it is safer, in cases amongst the Protozoa in which syngamy is not known to occur, to explain its apparent absence by the imperfections of the present state of our knowledge, than to suppose that in such forms sexual phenomena are entirely lacking in the life-cycle. ”

“ The protoplasm of the Protozoa (see p. 5 f.) varies greatly in consistency and in differentiation. Its outer layer may be granular and scarcely altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria, Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the Acrasieae; it is continuous with a firm but delicate superficial pellicle of membranous character in most Flagellates and Infusoria ”

“ The life history of each species has its own characteristic features as regards nuclear division, etc., and in many forms, notably the amœbæ, it is impossible to identify them with certainty unless the chief stages of the life history are known. In general, however, the soil protozoa pass through very similar phases and develop in a perfectly straightforward way. ”

“ Finally, as Nasir has shown, the protozoa play a part in the complicated nitrogen cycle, and work of this type needs extending.
Such, then, are a few of the outstanding problems that confront the soil protozoologist; but he must always remember that the organisms he studies are but a small fraction of the total, and that any influence affecting one part of the complex will be reflected in another. As Prof. Arthur Thomson said in his Gifford Lectures, “No creature lives or dies to itself, there is no insulation. ”

“ That quinine does not kill all blood protozoa is certain, for it has no effect on the hæmoprotozoa of birds and reptiles, or on the trypanosomes. Certain it is that in man, with the exception of the crescent body, it usually quickly causes the parasite to disappear from the general circulation. ”

“ These are classics in the study of the protozoa, and they showed the method by which results are to be reached in the search of the parasites of human disorders. In each of these life cycles there follow upon each other a long line of generations, with great dissimilarity of the successive generations; Schaudinn drove home the conclusion that the unit of study should be the whole life cycle, and his results rendered it probable that many forms of protozoa that had hitherto been regarded as different species might be merely stages of one and the same life cycle. ”

“ The protozoa parasitic in blood of the vertebrate host undergo a vegetative or non-sexual multiplication, the process being then known as schizogony; but in order to ensure their further existence, and their passage from one vertebrate host (intermediary host) * [1] to another, a further cycle in a cold-blooded invertebrate host (definitive host) is necessary, during which the sexual cycle or sporogony takes place.
In the vertebrate blood the young parasite is known as a schizont. The products of the asexual fission are known as merozoites ”

“ The Protozoa are single units of living matter (protoplasm) , and may be referred to as the one-celled animals; they are mostly microscopic, and lead an independent life, or are associated in colonies, but are capable, as a rule, of carrying on independently all the functions of life, though there are no organs such as those of digestion, respiration, and circulation, as we know them in the higher animals. It is amongst such simple forms that the distinction between the lowest animals and plants ceases to be clear. ”

“ Besides the Amœba, Paramœcium, and Vorticella there are thousands of other Protozoa. Most of them live in water, but a few live in damp sand or moss, and some live inside the bodies of other animals as parasites. Of those which live in water some are marine, while others are found only in fresh-water streams and lakes. ”