Protozoa

Nutrition.

We can divide the Protozoa according to their method of nutrition into four groups. The first consists of organ isms provided with chromatophores (figures 1, 2, 16, 17, 19).

These organisms create their own food just like plants, by produc ing starch out of carbonic acid gas their chlorophyll enab ling them to use the energy of sunlight for this purpose; then, absorbing various salts from the water, they build up with the starch those albuminous substances of which their bodies are com posed. These organisms are therefore self-supporting in the fullest sense of the word, for they can exist without the help of any other living creatures : they are "autotrophic." There are, we ought to say, some amongst them that are also capable of feeding on other organisms. All other Protozoa are "heterotrophic," i.e., they use for their nutrition either other organisms directly or, more rarely, substances produced by other organisms. Protozoans forming this second group of nutrition types (the saprophytes) live on the products of putrefaction; that is, on substances formed when bacteria decompose organic materials.

A third group, including most heterotrophic Protozoa, live like so many animals by eating more or less solid food. They will attack almost anything; living organisms from the most minute bacteria to small multicellular animals water-fleas, small worms and so forth. As parasites they prey upon the tissue-cells of the animals in which they live. Still others live on dead bodies, or parts of bodies of other protozoans or multicellular animals, or particles of nutritive material yielded by other organisms—starch, wood, and so on.

The last group of nutrition types com prises the rvajority of the parasitic Proto zoa. They live on the dissolved food they imbibe through their body-surface from the blood, stomach-content and lymph of their host.

A word is necessary with regard to the ingestion of the food. The autotrophous species, those living on putrefying material, and also most of those living as parasites, feed osmotically; the food they need is dissolved in water ; it permeates and is absorbed through the protoplasm sur face. The solid food of the other hetero trophic protozoans must be swallowed whole before it can be absorbed. This can be done in various ways; those protozoans that have no cytostome are often capable of taking in food at any point of their surface and eat merely by flowing round and engulfing their prey, a process which is called "amoeboid inges tion." A variation of this habit is the engulfing of the food mate

rial not by the body of the protozoan itself but only by special outgrowths of the body called "pseudopodia" and its absorption into the body proper only later or not at all. The delicate, hair like or netted pseudopodia of some protozoans, e.g., heliozoans, are sticky and are used as traps ; the victim comes up against the pseudopodium, sticks to it and is engulfed.

Protozoa having a cytostome naturally absorb their food materials or exist as superfluous products of the organism's econ omy. We must just mention the fact that some protozoans have pigments (blue, red, etc.) that are distributed in the form of granules in various parts of the body ; colourless forms are however in the majority.

Colony Formation.

All protozoans, as we have already shown, are not unicellular and uninucleate ; actually there are all through it ; whether they are of the kind that wafts its food with cilia into the cytostome, or whether the prey is seized by the cyto stome, the process of absorption is completed by the protozoan pushing itself over the prey in such a way that the latter is grad ually covered up by the body (fig. 5). In the first case a sort of fore-mouth is often developed; this is called a peristome, and is lined with cilia which drive the prey towards the cytostome, as in fig. 4.

The so-called Suctoria actually do not devour their prey at all but suck it with the aid of stiff suckers or tentacles (fig. 12) . Although most Protozoa swallow their prey alive, there are certain kinds that first kill their victims with poisons.

The food particles are absorbed into the inside of the cell-body in both eating and sucking proto zoans; the space in which they lie engulfed is spoken of as the "food vacuole." Its walls are of living protoplasm, but we can regard it as a primitive gut. Substances are secreted into it to kill and dissolve the engulfed substances ; the food-materials extracted from these latter are absorbed by the protoplasm. We do not yet know very much of the details of digestion, although certain digestive ferments have been identified in some proto zoans. In many species the food vacuole follows a definite course round the body, while digestion is in progress. When an indiges tible remnant is left at the end, the vacuole is brought to the sur face and its contents expelled, .