For one thing the resistance of many free-living Protozoa to the digesting fluids released by insectivorous plants, e.g., pitcher plant, shows a remarkable degree of adaptability. Another thing is the extraordinary adaptability of some parasitic protozoans; the gut-parasites of the frog tadpole live for instance in the gut of the horse-leech, which eats the tadpole, and there grow and mul tiply; and some gut-parasites can live equally well in the blood of their host, if by chance they should get into it.
Most striking are the reproductive adaptations of the parasites, for the reproductive mechanism is often governed absolutely by its requirements in relation to its host or hosts as the case may be. First the remarkable fertility of parasites must be mentioned as an adaptation to their form of life, for in spite of their easy existence the possibility of their dying in or with their hosts is a serious menace to the maintenance of parasitic species, for only a minute proportion of the germs can have a chance of finding another host. We have already referred to an adaptation of an entirely different order, namely the division of the life-cycle into two periods spent in different hosts; the combination of an alter nation of hosts with an alternation of generations. The classical example of this is that of the malaria parasite. The bite of the mosquito injects the sporozoites into the blood; there they bore their way into the red blood corpuscles and develop into the agamonts ; these break up by multiple division into the merozoites and in doing this burst the corpuscles whose substance is already depleted by the parasite. The merozoites bore into fresh cor puscles and repeat the story, or they can transform themselves into macrogametocytes and microgametocytes, which wait for some time until chance takes them into the stomach of a sucking mosquito. If they have to wait too long they die, but if they succeed in getting into the mosquito's stomach without undue delay they are able to complete their life-cycle; the microgame tocyte breaks up into microgametes, these fertilize the macro gametes and the motile zygote bores into the stomach-wall of the mosquito. Here it grows, its nucleus divides many times and finally the sporont, as we now call the adult zygote, breaks up into a number of sporozoites. These migrate to and collect in the salivary glands of the mosquito and, if this should bite a human being, make their way with the injected saliva into the blood stream of the victim. The cycle is now complete.
When we speak of parasitism it is difficult to avoid thinking that the host is necessarily injured by the parasite. Most para sitic Protozoa however are actually harmless ; they live on surplus food and body-substance and do their host no harm at all. Every man, every frog, cockroach and earthworm, shelters innumerable parasitic protozoans without being injured by them in the slight est degree. Other protozoans on the contrary injure their host in a greater or less degree, not by taking away food, but by demol ishing the body-cells of their host. Some types secrete poisons (toxins). As we have already mentioned, the malaria parasite destroys the red blood corpuscles of man. (See MALARIA.) The malaria parasite is not the only pathogenic (i.e., disease causing) protozoan. Trypanosomes (fig. 7) have been shown to be the cause of African sleeping-sickness and of a human disease not rare in South America, schizotrypanosis. Leishmania donovani (fig. 20) causes the kala-azar disease in India; Entamoeba histolytica (fig. 21) the tropical amoebic dysentery of man; many trypano somes cause serious cattle plagues. A coccidian is responsible for a dangerous rabbit disease, some microsporidians are a great dan ger to fish and others attack the silk-worm caterpillar. These are only a few examples of the menace that parasitic protozoans con stitute to other organisms. The pathogenic forms, however, are few in number compared with the harmless ones.