In some cases the oogametes are at first oblong, like ordinary merozoites, and round off in the gut. The microgametocyte, or spermatogone, has the same character, but is smaller; it applies itself like a cap to one pole of the oogamete, which has rounded off; it then divides into four sperms, whose cytoplasm is not sharply separated; one of these then separates from the common mass, enters the oogamete, and so conjugation is effected, with an oosperm as its result. This latter mode of conjugation is that of Adelea ovata and Coccidium lacazei: the former is probably the more primitive and the commoner. The sperms of Coccidiidae, when free, usually possess two long flagella, either both anterior, or a very long one in front and a short one behind, both turned backwards.
The genus Coccidium affects many animals, and one species in particular, C. cuniculi Rivolta, attacks the liver of young rabbits,[[108]] giving rise to the disease "coccidiosis." Coccidium may also produce a sort of dysentery in cattle on the Alpine pastures of Switzerland; and cases of human coccidiosis are by no means unknown. Coccidium-like bodies have been demonstrated in the human disease, "molluscum contagiosum," and the "oriental sore" of Asia; similar bodies have also been recorded in smallpox and vaccinia, malignant tumours and even syphilis, but their nature is not certainly known; some of these are now referred to Flagellata (see p. [121]).
Closely allied to the Coccidiidae are the Haemosporidae, dwellers in the blood of various cold-blooded Vertebrates,[[109]] and entering the corpuscles as sporozoites or merozoites to attain the full size, when they divide by schizogony; they are freed like those of the next family by the breaking up of the corpuscle. The merozoites were described by Gaule (1879) as "vermicles" ("Würmchen"), and regarded by him as peculiar segregation-products of the blood; though Lankester had described the same species in the Frog's blood as early as 1871, with a full recognition of its true character. His name, Drepanidium, has had to give way, having been appropriated to another animal, and has been aptly replaced by that of Lankesterella. The sexual process of Karyolysus has been found to take place in a Tick, that of Haemogregarina in a Leech, thus presenting a close analogy to the next group, which only differs in its less definite form in the active state, and in the lack of a cell-wall during brood-formation.
Laveran was the first to describe a member of the Acystosporidae, in 1880, as an organism always to be found in the blood of patients suffering from malarial fever; this received the rather inappropriate name of Plasmodium, which, by a pedantic adherence to the laws of priority, has been used by systematists as a generic name. Golgi demonstrated the coincidence of the stages of the intermittent fever with those of the life-cycle of the parasite in the patient, the maturation of the schizont and liberation of the sporozoites coinciding with the fits of fever. Manson, who had already shown that the Nematodes of the blood that give rise to Filarial haematuria (see Vol. II. p. 149) have an alternating life in the gnats or mosquitos of the common genus Culex,[[110]] in 1896 suggested to Ronald Ross that the same might apply to this parasite, and thus inspired a most successful work. The hypothesis had old prejudices in its favour, for in many parts there was a current belief that sleeping under mosquito-netting at least helped other precautions against malaria. Ross found early in his investigations that Culex was a good host for the allied genus Haemoproteus or Proteosoma, parasitic in birds, but could neither inoculate man with fever nor be inoculated from man. He found, however, that the malaria germs from man underwent further changes in the stomach of a "dappled-wing mosquito," that is, as we have since learned, a member of the genus Anopheles. Thenceforward the study advanced rapidly, and a number of inquirers, including Grassi, Koch, MacCallum (who discovered the true method of sexual union in Halteridium[[111]]), and Ross himself, completed his discovery by supplying a complete picture of the life-cycles of the malaria-germs. Unfortunately, there has been a most unhappy rivalry as to the priority of the share in each fragment of the discovery, whose history is summarised by Nuttall, we believe, with perfect fairness.[[112]]
The merozoite is always amoeboid, and in this state enters the blood corpuscle; herein it attains its full size, as a schizont, becoming filled with granules of "melanin" or black pigment, probably a decomposition product of the red colouring matter (haemoglobin).
Fig. 35.—Life-history of Malarial Parasites. A-G, Amoebula of quartan parasite to sporulation; H, its gametocyte; I-M, amoebula of tertian parasite to sporulation; N, its gametocyte; O, T, "crescents" or gametocytes of Laverania; P-S, sperm-formation; U-W, maturation of oosphere; X, fertilisation; Y, zygote. a, Zygote enlarging in gut of Mosquito; b-e, passing into the coelom; f, the contents segmented into naked spores; g, the spores forming sickle-germs or sporozoites; h, sporozoites passing into the salivary glands. (From Calkins's Protozoa, after Ross and Fielding Ould.)
The nucleus of the schizont now divides repeatedly, and then the schizont segments into a flat brood of germs (merozoites), relatively few in the parasite of quartan fever (Haemamoeba malariae, Fig. 35, E-G), many in that of tertian (H. vivax, Fig. 35, M). These brood-cells escape and behave for the most part as before. But after the disease has persisted for some time we find that in the genus Haemamoeba, which induces the common malarial fevers of temperate regions, certain of the full-grown germs, instead of behaving as schizonts, pass, as it were, to rest as round cells; while in the allied genus Laverania, (Haemomenas, Ross) these resting-cells are crescentic, with blunt horns, and are usually termed half-moons (Fig. 35, O, T), characteristic of the bilious or pernicious remittent fevers of the tropics and of the warmer temperate regions in summer. These round or crescent-shaped cells are the gametocytes, which only develop further in the drawn blood, whether under the microscope, protected against evaporation, or in the stomach of the Anopheles: the crescents become round, and then they, like the already round ones of Haemamoeba, differentiate in exactly the same way as the corresponding cells of Coccidium schubergi. The female cell only exhibits certain changes in its nucleus to convert it into an oosphere: the male emits a small number of sperms, long flagellum-like bodies, each with a nucleus; and these, by their wriggling, detach themselves from the central core, no longer nucleated. The male gametogonium with its protruded sperms was termed the "Polymitus form," and was by some regarded as a degeneration-form, until MacCallum discovered that a "flagellum" regularly undergoes sexual fusion with an oosphere in Halteridium, as has since been found in the other genera. The oosperm (Y) so formed is at first motile ("ookinete"), as it is in Haemosporidae, and passes into the epithelium of the stomach of the gnat and then through the wall, acquiring a cyst-wall and finally projecting into the coelom (a-e). Here it segments into a number of spheres ("zygotomeres" of Ross) corresponding to the Coccidian spores, but which never acquire a proper wall (f). These by segmentation produce at their surface an immense quantity of elongated sporozoites (the "zygotoblasts" or "blasts" of Ross, Fig. 35, g), these are ultimately freed by the disappearance of the cyst-wall of the oosperm, pass through the coelom into the salivary gland (h), and are discharged with its secretion into the wound that the gnat inflicts in biting. In the blood the blasts follow the ordinary development of merozoites in the blood corpuscle, and the patient shows the corresponding signs of fever. This has been completely proved by rearing the insect from the egg, feeding it on the blood of a patient in whose blood there were ascertained to be the germs of a definite species of Haemamoeba, sending it to England, where it was made to bite Dr. Manson's son, who had never had fever and whose blood on repeated examination had proved free from any germs. In the usual time he had a well-defined attack of the fever corresponding to that germ, and his blood on examination revealed the Haemamoeba of the proper type. A few doses of quinine relieved him of the consequences of his mild martyrdom to science. Experiments of similar character but of less rigorous nature had been previously made in Italy with analogous results. Again, it has been shown that by mere precautions against the bites of Anopheles, and these only, all residents who adopted them during the malarious season in the most unhealthy districts of Italy escaped fever during a whole season; while those who did not adopt the precautions were badly attacked.[[113]]
Anopheles flourishes in shallow puddles, or small vessels such as tins, etc., the pools left by dried-up brooks and torrents, as well as larger masses of stagnant water, canals, and slow-flowing streams. Sticklebacks and minnows feed freely on the larvae and keep down the numbers of the species; where the fish are not found, the larvae may be destroyed by pouring paraffin oil on the surface of the water and by drainage. A combination of protective measures in Freetown (Sierra Leone) and other ports on the west coast of Africa, Ismailia, and elsewhere, has met with remarkable success during the short time for which it has been tried; and it seems not improbable, that as the relatively benign intermittent fevers have within the last century been banished from our own fen and marsh districts, so the Guinea coast may within the next decade lose its sad title of "The White Man's Grave."
So closely allied to this group in form, habit, and life-cycle are some species of the Flagellate genus Trypanosoma, that in their less active states they have been unhesitatingly placed here (see p. [119]). Schaudinn has seen Trypanosomic characters in the "blasts" of this group, which apparently is the most primitive of the Sporozoa and a direct offshoot of the Flagellates.