Protozoan

“ The presence of protozoa is not peculiar to British soil since they have been found by various workers in Germany, France, the United States, and elsewhere. In view of their probable importance in the soil economy there has been instituted a survey of the protozoan species of soil from all parts of the world. ”

“ 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. ”

“ It is difficult to trace the origin of this view, since early observers, viz., Ehrenberg and Dujardin, in 1841, were of the opinion that the protozoa [69] were living in the trophic active condition in the soil, and it was not until 1878 that Stein showed that free living protozoa can encyst. To Martin and Lewin, however, must be ascribed the distinction of first proving that the soil possesses an active protozoan population, for by a series of ingenious experiments these observers isolated several flagellates and amœbæ in a trophic condition from certain of the Rothamsted soils. ”

“ 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 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. ”

“ The amœba belongs in the class of protozoans called Rhizopoda “root footed.”
Other classes of Protozoans are the Infusorians (in the broad sense of the term) , which have many waving cilia (Fig. 17) or one whiplike flagellum (Fig. 18) , and the Foraminifers, which possess a calcareous shell pierced with holes (Fig. 19) . Much chalky limestone has been formed of their shells. To which class does the paramecium belong?
Protozoans furnish a large amount of food to the higher animals. ”

“ It is not always easy to tell whether certain organisms are plants or animals, and because of this some scientists have suggested that these “in-betweens” be placed in a separate kingdom—the Protista. The protistans are primarily unicellular organisms and are represented by such forms as bacteria, algae, diatoms, and the protozoans (see below) . ”

“ The Protozoans are unicellular or simple combinations of cells. Above the Protozoa, animals are organized on four different plans of structure, but they are all compounded of cells, though many of the cells are greatly modified. In these modifications the cellate structure reappears as a fundamental homologue in every organ of all of the higher animals, and it is still found in the animals themselves. The phytons of plants are the homologues of organs in animals. There may be many phytons serving the same functions in plants, as there may be many organs serving the same function in animals ”

“ This explanation also makes inevitable the assumption that the ancestors of our present unsymmetrical protozoans were symmetrical and swam in straight courses without revolving, a condition of affairs which contrasts strongly with present conditions, for none of the most nearly symmetrical unicellulars and swarm spores now swims without revolving on the long axis. It is therefore exceedingly improbable that spiral swimming is the result of an acquired habit. ”

“ Protozoology.—It is of interest that about the year 1890, when bacteriologists ceased to announce discoveries with their accustomed regularity, owing to the fact that all readily recognized pathologenic bacteria had been discovered, the systematic study of protozoa began and some of the single-cell forms of life in the animal kingdom soon took a place as disease-producers alongside the corresponding form of the vegetable kingdom. Until this time, protozoa had been found in only two diseases of man, dysentery and malaria. ”

“ Under no conditions is multiplication so rapid as with the bacteria, and in general the simpler the form of organism the more rapid is the multiplication. It is common to all of the protozoa to develop forms which have great powers of resistance, this being due in some cases to encystment, in which condition a resistant membrane is formed on the outside, in others to the production of spores. A fluid environment is essential to the life of the protozoa, but the resistant forms can endure long periods of dryness or other unfavorable environmental conditions. ”

“ Protozoa occurring as blood or intracellular parasites of other organisms. They possess no definite organs for locomotion or for ingestion of food. The typical reproduction takes place by the formation of spores and minute germs called sporozoites.
EXAMPLES: Gregarina, Coccidium, Plasmodium.
Class IV. INFUSORIA
Protozoa in which small filaments called cilia serve as organs of locomotion; these are distinguished from flagella by their much smaller size and in being present in great numbers, forming a fine covering over the whole of the body. The body protoplasm is always corticate. ”

“ 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. ”

“ 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. ”

“ From the first discovery of the Protozoa, their life-history has been the subject of the highest interest: yet it is only within our own times that we can say that the questions of their origin and development have been thoroughly worked out. If animal or vegetable matter of any kind be macerated in water, filtered, or even distilled, various forms of Protista make their appearance; and frequently, as putrefaction advances, form after form makes its appearance, becomes abundant, and then disappears to be replaced by other species. ”

“ 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. ”

“ 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. ”

“ 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 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 ”

“ 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. ”

“ 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 ”

“ Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. ”

“ 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. ”

“ 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. ”

“ It is favoured by conditions favourable to trophic life in the soil, and finally becomes so active that the bacteria become unduly depressed.
“It is difficult to see what agent other than a living organism can fulfil these conditions. Search was therefore made for a larger organism capable of destroying bacteria, and considerable numbers of protozoa were found. The ciliates and amœbæ are killed by partial sterilisation. Whenever they are killed the detrimental factor is found to be put out of action; the bacterial numbers rise and maintain a high level. ”

“ Perhaps the most remarkable class of stimulations, however, is that due to the presence of the entire protoplasmic body of one organism in the cell of another, each living its own life for the time being, but the protoplasm of the host cell showing clearly, by its abnormal behaviour, that the presence of the foreign protoplasm is affecting its physiology. A simple example is afforded by Zopfs' Pleotrachelus, the amoeboid protoplasmic body of which lives in the hypha of Pilobolus, causing it to swell up like an inflated bladder, in which the parasite then forms its sporangia. ”

“ In certain cases the adaptation of the organism is for a narrow environment; for example, the parasitism may extend to a simple species only, in others the adaptation may extend to a number of genera. In certain cases the adaptation is mutual, extending to both parasite and host and resulting in symbiosis, and this condition may be advantageous for both. Certain of the protozoa harbor within them cells of algæ utilizing to their own advantage the green chlorophil of the algæ in obtaining energy from sunlight and in turn giving sustenance to the algæ. ”