Polymorphism Variability

Free-living Protozoans.

We can divide the Protozoa into ecological groups according to their modes of existence, remem bering however that these groups are joined by every intergrade. The one all-important distinction is of course that between free living and parasitic Protozoa.

The free-living Protozoa are composed of plankton forms, swim ming forms, bottom-dwelling forms and soil forms. Plankton forms, though also present in fresh water, are characteristically found in the sea. They are Protozoa that, for the most part, al though unprovided with any special motile organs, are able to float in the water and are carried by the currents.

Swimming forms (Nectonta) are protozoans that swim about with cilia and flagella, and in so far as they are heterotrophic do not as a rule waft in their prey but actually chase it. We find them in all kinds of water, from the sea down to mere puddles.

Bottom dwellers (creeping forms) live only on the bottoms and sides of ponds, lakes and so on, and creep only upon firm ground; some can swim a little, others are rooted firmly in their place. Their food is varied ; at very great depths the autotrophic forms cannot of course exist because they need light, but they occur nevertheless in every possible kind of watery place. Most for aminiferans and amoebae are typical bottom dwellers.

Parasitic Forms.

Amongst Protozoa endoparasites are in the majority. Most parasitic Protozoa flourish in or on animals including other Protozoa, and there is scarcely any animal group that is free from them. A few forms parasitize plants; some, e.g., flagellates, are found in the latex (or "milk") of the spurges (Euphorbia). Ectoparasites are naturally commonest on water animals and on moist or mucilaginous parts of the skin of land animals ; these can either creep about or swim or remain rooted to one place. Most ectoparasites however are not true parasites, and merely use their foothold on their host to extract food for their environment, moving away as occasion requires.

Endoparasites inhabit body-cavities like the gut, veins, gall bladders and so forth, as well as every conceivable tissue, where they lie between the cells, as tissue parasites, or bore their way inside the cells, as cell-parasites. Some move about fairly freely, others are almost immobile.

The great majority of the endoparasites feed by osmotic ab sorption; that is they live on dissolved nutriment that they either find ready for them, as in the blood or the digestive tract, or that they have to prepare by secreting substances to dissolve the cells of the host.

Most parasitic protozoa are transferred from one host to an other by f orming cysts, which escape into the open and are then picked up by the new host quite by chance—usually by being eaten. Direct transmission from one animal to another of the same species is rare ; it is found in Trypanosoma equiperdum, which causes the so-called stallion plague, and is transmitted from mare to stallion and vice versa at coitus. The germs of other protozoans penetrate into the eggs while in the bodies of the fe males and are thus carried on to the next generation. Very fre quently a second host serves as a carrier; this is the case in those trypanosomes that pass between human or mammalian blood and the intestines of insects such as mosquitoes; the insect when biting absorbs blood and at the same time injects its saliva into the wound. The insect is called in these cases the intermediate host. In other instances the host plays a more passive part, as in the transmission of the coccidian Aggregata: the asexual reproduction of this protozoan takes place in the gut of a crab; the germs (or merozoites as they are called) pass into the gut of a second—the chief—host, the cuttlefish Sepia that feeds on the crab. In the cuttlefish the merozoites develop into male or female gametes; fertilization follows and the spores produced by the zygote pass out with the excrement. The spores are eaten again by the crab.

Adaptation of

Parasites.—Parasites, as we have mentioned before, are for the most part so well adapted to their habit of life that they are incapable of feeding or of multiplying outside their host ; trypanosomes from the blood put into water die imme diately. Yet we cannot escape the question as to how parasitism started, for we cannot assume that all parasites have been para sites since the beginning of time ; on the contrary, they must have arisen from free-living forms. We have now a few facts that make the transition from a free-living to a parasitic form if not entirely clear at least to some extent intelligible.