There are further the symbiotic Protozoa, which not only do no injury to, but actually benefit, their host ; some to such an ex tent that they have become indispensable. We will begin with this latter group. In the gut of all wood-eating termites live peculiar flagellate protozoans, the so-called Trychonymphidae. These proto zoans eat small particles of wood, the masticated food of the termite, and digest them; and it has now been determined that this activity of the trychonymphid is essential to the life of the termite.
This is certainly an extreme case of symbiosis between a proto zoan and a higher animal. In the stomachs of the ruminants (cat tle and sheep) vast numbers of infusorians are found, which con sume the cellulose of the host's food and are probably as useful as the flagellates are to the termites ; but they are not indis pensable. Most of this work is done by bacteria, which also inhabit the stomach. Similarly, some radiolarians can easily do without the autotrophous peridineans that live with them.
Many autotrophic planktonic Protozoa serve as food for other plankton organisms, such as the larvae of worms, sea-urchins and fishes—not exclusively, indeed, for the diatoms which far surpass them in number are more important as a source of food. In so far as, with diatoms, they supply food for fishes and smaller animals eaten by fishes, these protozoans are not without impor tance to man, fish being one of his staple sources of food supply.
Other marine Protozoa (Radiolaria and Foraminifera) that form shells of mineral matter, play quite a different part. In cer tain circumstances they can go to form solid rock. The shells of dead individuals collect, often in enormous numbers, on the sea floor and form, generally with other mineral particles but some times without any other constituent, the fine silt or ooze that covers the bottom of the deep sea. In the Pacific (Rose atoll) a cubic metre of Globigerina ooze is laid down yearly over a surface of 150 sq.m.; in other words a layer o.66cm. thick every year or over two feet in a hundred years. 29.2% of the whole floor of the ocean, i.e., over 4o million square miles, is covered with Globi gerina ooze ; the area covered with radiolarian ooze is estimated at nearly 12%. Chalk and sandstone are nothing but the ooze and
silt of antediluvian seas; and certain rocks consist, entirely or almost entirely, of the shells of fossil Protozoa, Foraminifera, Radiolaria and Coccolithophorida.
As compared with the marine forms the fresh water Protozoa play a modest part ; they certainly provide food for some animals, but they do not figure very largely in the diet as a whole. In waters with much decomposing animal and vegetable debris the protozoans take some part in what is called biological water cleansing. In a ditch or pond containing much decaying matter the water is polluted with all kinds of bodies, but gradually cleans itself. First the decaying matter is broken down by bacteria and partly consumed. In this materials are found—mostly evil-smell ing—that are needed by certain Protozoa for food and the water is therefore freed of them. The bacteria themselves are eaten by other protozoans. A similar part is played apparently by the soil Protozoa. In one gramme of earth from 100-50,000 amoebae, 1.000-100,000 flagellates and as many as 1,000 infusorians have been found. On the one hand they keep down the nitrifying bac teria (see BACTERIOLOGY) so necessary for higher plants; on the other hand they may effect nitrogen fixation themselves and so have the same effect as manure.
The species of fresh water and soil Protozoa are cosmopolitan; the protozoan fauna of pools and rivers is the same in Africa as in North America. This is due to the fact that the conditions of life in pools are much the same all over the world, and that the fresh-water protozoans can be carried very easily, especially in the encysted form, by wind and animals. In the sea the case is rather different ; the water of the Pacific is distinctly different in salt content, temperature and other respects from that of the Mediter ranean and corresponding to these differences we find different species limited to different seas. As an example of a cosmopolitan species on the other hand we have Noctiluca miliaris, the luminous flagellate (fig. 18). The geographical distribution of parasites naturally follows that of their hosts.