THE FOOD-VACUOLE IN PARAMECIUM

¹ The Biological Laboratory, Johns Hopkins University and The Marine Biological Laboratory, Woods Hole, Mass.

1. The feeding apparatus in Paramecium consists of a shallow ciliated groove, a ciliated tube which leads into the body, and a bundle Of fibers (esophageal fibers) which extend from the tube nearly to the posterior end of the body. The tube is composed of an outer part (the vestibulum) and an inner part (the pharynx).

2. Paramecia ingest all sorts of small particles, but more digestible than indigestible ones. Selection takes place in the vestibulum and the proximal end of the pharynx.

3. In forming a food-vacuole, the cilia in the pharynx force fluid with particles in suspension against the membrane over the distal opening of the pharynx, producing a sac, the esophageal sac.

4. As the esophageal sac enlarges, the particles in suspension in it become greatly concentrated, owing largely, if not entirely, to the passage of water out through the membrane into the cytoplasm.

5. A portion of this sac is constricted , as a food-vacuole, probably by the action of the esophageal fibers.

6. The initiation of the constriction of the sac is probably due to periodicity in the constrictive action of the fibers, the size of the sac, and the composition of its contents.

7. There is much variability in the size of the food-vacuoles. This is correlated with the quantity and the quality of the particles in the surrounding fluid, the chemical composition of this fluid, the rate of ingestion, the rate of loss of water from the esophageal sac, and the length of the intervals between consecutive constrictions of the esophageal fibers. The frequency of formation of food-vacuoles is correlated with the quantity and the quality of the particles in the surrounding fluid and the acidity, and the temperature of this fluid. The shape of the food-vacuoles depends largely, if not entirely, upon the viscosity of their content.

8. After the food-vacuole has left the pharynx it passes rapidly on a fixed course toward the posterior end of the body, and slowly on a varied course to the anus. The former is probably due to the action of the esophageal fibers; the latter is due to cyclosis.

9. On its course through the body, the food-vacuole usually decreases greatly in size, and the acidity of its content increases greatly; then it enlarges very rapidly and the acidity of its content decreases greatly. The extent of these changes varies enormously. Under some conditions there are no perceptible changes; under others the acidity in some vacuoles increases to a maximum at least as high as pH 1.4 and then decreases approximately to pH 7.8.

10. There is no "preliminary alkaline phase" of the food-vacuole.

11. The change in acidity is definitely correlated with change in size. The changes in size are due to difference between internal and external osmotic concentration and the action of the stretched vacuolar membrane. The increase in acidity is probably due to secretion of acid by the cytoplasm adjoining the vestibulum and the pharynx and to impermeability of the vacuolar membrane to hydrogen-ions, and loss of water. The decrease in acidity is due to entrance of alkaline fluid from the cytoplasm.

12. The increase in acidity probably causes hydrolysis and thereby increase in osmotic concentration resulting in inflow of fluid containing digestive enzymes.

13. Death of ingested living organisms is probably largely due to toxic substance produced by the pharynx and concentrated in the food-vacuole, owing to impermeability, of the vacuolar membrane to it, and loss of water.

14. Paramecia digest protein, fat, and starch. Digestion takes place during the alkaline phase of the food-vacuole. The enzymes involved originate in the cytoplasm and are carried into the food-vacuole by the cytoplasmic fluid which enters during its rapid enlargement.

15. The neutral-red granules and the mitochondria are probably not involved in digestion.

16. All these phenomena are essentially the same in the four species studied, namely P. caudatum, P. nucleatum, P. aurelia, and P. trichium.