often through the so-called cytopyge or cell anus.
Like all other forms of life the Protozoa do not need to use at once, for building up their bodies, all the food they absorb ; they put a proportion of it by as reserve material to be used in case of necessity. Various kinds of materials—such as starch and the so-called paramylon, a starch-like body—are stored in this way both by the autotrophic species and by those living on decom posing organic matter; the other heterotrophous Protozoa store fat, glycogen and volutin.
Intestinal parasites and some living free in very putrid water not only do not require free oxygen but are incapable of living in a medium containing any appreciable amount of it ; they are "anaerobic." Excretion.—We know very little of the expulsion of the prod ucts of the decomposition of the body substance and reserve ma terials. In the bodies of many protozoans are to be found crystals partly consisting of salts of uric acid ; these are probably to be considered as excretions. In all probability most excreta escape through the surface of the body or are expelled from the con tractile vacuole.
Active locomotion on the other hand is achieved in some form by nearly all Protozoa; there are certainly a few almost incapable of movement, either because they have settled and grown on a fixed body (fig. 12) or because they do not possess true motile organs. The movement of protozoans can be effected either by proper organs for the purpose or in various other ways.
The simplest organs of motion are the so-called pseudopodia, which are outgrowths from the body, of the most varied form; these can be extended or with drawn at will. They serve for a
kind of creeping movement ; the pseudopodium is formed and sticks to the material underfoot; then a new pseudopodium is stretched out on the side of the old one in the direction in which movement is to take place, and protoplasm now flows on to this. In this simple way the whole protoplasmic mass is rearranged bit by bit and the protozoan moves forward. Protozoans whose pseudopodia are relatively small in proportion to the whole body form their pseudopodia just like Amoeba, but use them much more like real feet, putting them out in the direction of move ment and then drawing them in again. These pseudopodia may be quite thin and delicate, branched or netted, and are then called rhizopodia. The mechanism of amoeboid movement is not yet f ally explained.
Quite another kind of move ment is that by means of cilia and flagella. The whip-like flagella (fig. 16) either rotate, as in a circle described by an index-finger with the hand at rest, or strike like the lash of a whip; the rotation has a sucking effect like that of an aeroplane propeller and this drives the protozoan forward. The whipping action of the flagellum is not so well understood.
The cilia are usually arranged in thick complex bands (fig. 6) and beat, i.e., they move like an index-finger that bends to strike a keyboard and then straightens again. Usually the cilia work together, every cilium in the row beating a little later than the next before it, so that a wave appears to pass completely over the row. This phenomenon has often been compared to the effect of a gust of wind passing over a cornfield. The blows of the cilia have a rowing action very like that of a duck's foot. The cilia of sessile protozoans produce currents in the water which carry the particles of food into the neighbourhood of the cyto stome. The lightning quickness with which the forward movement may be modified is due to an alteration in direction of the beats of the particular cilia or through co ordinated work of individual rows of cilia. "Cirri" strike just like cilia but being so much stiffer function more like legs.