The Coccidia are another important group of Sporozoa which are parasitic within the cells of higher animals; they are not found in body cavities, nor do they enter blood cells ; they are oval in shape and motionless. Their method of reproduction is completely known, and is the same in principle as that of the Sporozoon which causes malaria in man ; it will there fore be described here, although it cannot be studied practically.
The Development of Coccidium schubergi. The method of reproduction differs from that of Monocystis in two respects. In Monocystis reproduction occurs only by the formation and union of gametes; repro duction of this kind is called sexual. In Coccidium, however, there are two kinds of reproduction, sexual and asexual. Gametes play no part in asexual reproduction.
The second respect in which Monocystis and Coccidium differ refers to the outward appearance of the gametes.
In Monocystis they appear alike, but in Coccidium they are outwardly of two kinds, as in the vast majority of animals. Coccidium schubergi lives in the intestinal cells of the centipede Lit hobiits forjicatus.
The Trophozoite. The trophozoite is a motionless oval body which lies wholly within the cell of the intes tine of the host. It absorbs nourishment and grows ; having reached maturity it undergoes changes for the sake of reproduction. As we have already mentioned, these changes are of two kinds.
Asexual Reproduction (Schizogomj). By this method the parasites become multiplied within the body of the centipede, so that more and more of the cells of the intestine become infected until five days after the entrance of the first parasite, when most of the intestine is destroyed and the host begins to die. When the tro phozoite is mature its nucleus divides many times ; subsequently the protoplasm also divides, so that each division contains a portion of nucleus. The dividing trophozoite is called a schizont, and becomes resolved into a large number of elongated bodies called mero zoites. The merozoites part company, fall into the gut of the host, and, by means of their pointed ends, pene trate the other healthy cells which line the gut; after penetrating a cell, they grow rapidly, and become mature trophozoites, which again become schizonts. The events therefore succeed one another in a cycle which is repeated again and again until few healthy cells remain in the centipede's intestine ; other events then occur.
Sexual Reproduction (tiporoyoiiy). In this process resistent spores are fonned which pass out of the centi pede and into the earth, where they may be eaten by other centipedes. Spores are formed, as in Monocystis, by the union of gametes, which are of two kinds, large macroganietes and small microgametes. The gametes are produced in the following way. "When the centipede is becoming exhausted by the repeated asexual cycles, the trophozoites become altered in appearance: some of them become macroganietes, others produce a number of microgametes. A macrogamete is a large bean-shaped cell ; it is merely an altered trophozoite. A microgamete is a minute filamentous body which moves actively, being impelled by two lashing threads or flagella, which are attached at either end of it. The micro gametes are formed much in the same way as mero zoites, by repeated divisions of a trophozoite, which is in this case called a gametocyte. An active microgamete moves towards the nearest macrogamete, which is usually lying passively in the gut of the host, and becomes fused with it. Thus the body called a zygote is formed, from which the sporozoites are derived. The zygote becomes covered with a tough, chitinous membrane, in which condition it can survive outside the body of the centipede. The zygote is a single cell with a single nucleus formed by the union of two nuclei, which may be called male and female: the nucleus of the macro gamete is called the female nucleus, the other which was contributed by the microgamete is the male nucleus.
The nucleus of the zygote divides into four pieces, each of which becomes surrounded by a cyst wall, and further divides into two sporozoites. The zygoto therefore resembles that of Monocystis in that it is covered with a tough, resistant wall, and breaks up into eight sporo zoites, but in Coccidium the sporozoites are formed in couples. When a centipede swallows a zygote along with its food, the sporozoites issue from the cyst, enter the cells of the intestine and become mature tropho zoites, which undergo schizogony. The foregoing de scription is of the main facts of the reproduction, but many other important details are known, which have not been mentioned.
A union of two unlike gametes, the one small and motile, the other large and passive, occurs in the repro duction of all the higher animals; this sexual method of reproduction has long been known in them, but that a similar process occurs among the Protozoa is a more recent discovery.
HAEMOSPORIDIA
In this group the trophozoite is a parasite within the red blood cells of warm-blooded animals. The most important of them are Plasmodium and Laverania, which cause malarial fever in man. Tho life-history of these parasites is essentially like that of Coccidium. In it there are two separate cycles. The asexual cycle (schizogony) occurs within the blood of man. The malarial parasites use the red corpuscles of man in exactly the same way as Coccidium uses the intestinal cells of the centipede; in other words, as a sporozoite they enter a red blood cell and feed upon it as a t>-ophozoite ; when full grown they divide into a number of merozoites, which disperse and infect other healthy blood cells, just as the merozoites of Coccidimn infect other healthy intestinal cells. This cycle of schizo gony is repeated several times; at each dispersal of merozoitos the infected man gets an attack of ague.
The asexual cycle of Plasmodium malariae occupies seventy-two hours, and causes quartan ague ; that of P. vivax occupies forty-eight hours, and therefore causes tertian ague. The trophozoites of these parasites are amoeboid; in this respect they differ from Coccidium.
The method and meaning of the sexual cycle is essentially the same as in Coccidium. By schizogony, Coccidium cannot pass from centipede to centipede, neither can the malarial parasite pass from man to man. Only by the sexual cycle (sporogony) can a fresh host be infected.
In both cases the sexual process results in the formation of sporozoites ; in Coccidium these lie in the earth for a time, and are therefore confined within a resistent cyst. In the malarial parasite the whole sexual cycle is passed in the body of a mosquito. Some of the trophozoites in a man's blood become gametocytes.
When the mosquito sucks the blood of an infected man, the gametocytes pass with the blood into the stomach or mid-gut of the mosquito, where unequal gametes are formed and unite to produce zygotes, which are soon after found projecting from the outer surface of the organ. Within each zygote a large number of sporozoites are formed, which leave the zygote cyst and find their way into the salivary glands of the mosqu;to and lie ready to infect a healthy man. The details of this process are usually treated as part of Pathology.
