PROTOZOA. The forms of life included under the name Protozoa are enormously varied. The separation of the group from the Metazoa (higher animals), plants and bacteria, is based to some extent on convenience in classification, but it does rest on certain definite characters which distinguish the Protozoa from the three main groups.
Single protozoans are as a rule extremely small, so small that they can only be seen with the help of the microscope, and the un aided and inexperienced eye will only pick them out where by chance a great mass of one kind or another have collected to gether; heaped together in this way they will scarcely look like liv ing creatures. When we see the surface of a small stagnant pond covered with a sheet of green, we really have in front of us a great host of the flagellate organism Euglena (fig. 16). When, on board ship, we see phosphorescence at night so strong that every drop of water seems to sparkle we are really watching the antics of less millions of the luminous protozoan Noctiluca (fig. 18). Chalk is in reality the mud of an ancient sea purified by various chemical processes but consisting ultimately of the cal‘ careous shells of Foraminifera and little else.
But much oftener people come into contact with Protozoa with out knowing it ; in water of every kind there are Protozoa without number; nearly every human being harbours protozoans of vari ous innocuous species, living as parasites; not rarely other para sitic species will make their presence felt most disagreeably, although they do not play as important a part in this connection as Bacteria do. The exciting agent concerned in blackwater fever, malaria, sleeping sickness, amoebic disentery, Nagana cattle plague in East Africa, coastal fever of cattle in South Africa, to mention a few examples, is in each case a protozoan. Apart from these practical considerations, interest centres on the Protozoa on account of the picture they show us of the infinite diversity of living organisms.
In common usage the Protozoa are said to be "unicellular ani mals" in contrast to the higher animals and plants which are built up of many cells. The protozoan corresponds to a single such ele ment ; it is an isolated cell (q.v.). What do we mean by this? A
comparison between a protozoan and a multicellular animal of comparable size may make this clear. When we look at a small nematode (an organism related to the thread worm) under the microscope we find that its body is made up of separate organs— gut, nervous system, sexual organs and so forth ; these organs appear to consist of more or less clearly defined and separated parts which we call cells. These cells are not all alike ; the cells of the gut look quite different from those of the nervous system or of other organs; these small, fundamental elements of the animal body only resemble one another in the ground plan of their struc ture : In each of them we can distinguish a cell-body and a cell nucleus. Now let us consider the protozoan chosen as a type (fig I) ; it has organs : its body has an opening through which it takes in its food; a skeletal structure provides the body with a distinc tive form and ensures its rigidity; fine hair-like processes serve for locomotion, but the body of the animal as a whole, instead of being made up of individual elements, consists of one single coher ing mass of protoplasm. And if we look for cell nuclei, we find only one. The protozoan thus agrees with the individual cell of a higher animal or plant in this essential character of the organiza tion of the cell—its division into cell-body (cytoplasm) and cell nucleus; but only in this, for if we look carefully at the individual cells of the worm we have just taken as an example we find none that equal in complexity the single cell of the protozoan.
It is not hard to see why this should be so. The protozoan is a cell by itself ; it swims about freely, finding its own food ; it leads a life of its own. The muscle-cells of the worm on the other hand are incapable of independent existence ; they rely for nourishment on the cells of the intestine, while these cells in their turn, absorb ing nutriment, have no power of contraction. In other words, the protozoan is constitutionally versatile, the cells of the worm spe cialize; they stand in relation to one another on the principle of division of labour; they are viable only when brought together into a many-celled individual.