Feeding habits of protozoa

Feeding habits of protozoa vary widely. Some contain chlorophyll and behave essentially like plants. Thus the various species of Euglena are able to manufacture starch from simple inorganic materials. These forms and also ciliates without mouths, such as the Opalinidae, are able to absorb food materials through their surfaces. In all of the sporozoa, moreover, food enters the organism through the body wall. These forms are parasites and live on the organic materials provided by their hosts.

Most ciliates and flagellates have definite mouths, but it does not follow that all of the food taken up by these organisms passes through the mouth opening. In flagellates, especially, it seems probable that much of the food enters the cell by diffusion through the body wall. Ciliates like paramecium sweep food particles into their gullet by the beat of their cilia. Although various useless materials may enter the gullet and pass into the cell, there is apparently some power of discrimination.

The ingestion of food by ameba is essentially the same process as that which occurs when leukocytes or other ameboid cells take up foreign particles. Such phagocytosis occurs in practically all metazoa, and is of great importance both in the normal life of animals and in the resistance of animals to disease. Phagocytosis plays an essential role in metamorphosis and, both in insects and in amphibia, larval organs are destroyed bit by bit by the ameboid cells which ingest them. Because of its importance as a protection against disease, phagocytosis has been very widely studied by bacteriologists, pathologists, and students of medical sciences generally. Just as amebae select their food, so do leukocytes exercise a selection of the particles they ingest. The rate of ingestion of bacteria by leukocytes has often been studied under a variety of conditions. Bacteriologists have tried to obtain quantitative data by determining either the number of bacilli taken up per leukocyte or the percentage of leukocytes participating. Important factors (not always properly considered) are the absolute and relative concentrations of bacteria and leukocytes. Phagocytosis increases with increase in the number of bacteria, but the rate of ingestion does not keep pace with the increasing possibilities for collision between bacteria and leukocytes. Various authors have studied the effect of temperature on phagocytosis; curves are obtained like those found for ingestion of food by ameba. Any explanation of the mechanism of phagocytosis could at the same time serve as an explanation of the ingestion of food by ameba. At the present time, however, none of the theories of phagocytosis seems very satisfactory.

In higher animals, phagocytosis is not restricted to leukocytes. Scattered through various tissues and organs of the animal are numerous cells capable of ingesting solid objects. Of these, the macrophages move somewhat more freely than the histiocytes, and there are also morphological differences between the two types of cells. Together, they constitute the reticuloendothelial system.