THE FOOD-VACUOLE INPARAMECIUM

Abstract

The feeding apparatus in Paramecium consists of a shallow ciliated groove, a ciliated tube which leads into the body, and a bundle of esophageal fibers which extend from the tube nearly to the posterior end of the body. The tube is composed of an outer (the vestibulum) and an inner part (the pharynx). 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. 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 the esophageal sac. As the sac enlarges, the particles in suspension in it become greatly concentrated, owing largely to the passage of water out through a membrane into the cytoplasm. A portion of this sac is constricted off, as a food-vacuole, probably by the action of the esophageal fibers. 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. The great variability in the size of the food-vacuoles 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 is correlated with the quantity and quality of the particles in the surrounding fluid and the acidity and temp. of this fluid. The shape depends largely, if not entirely, upon the viscosity of their content. After the food-vacuole has left the pharynx is passes rapidly on a fixed course toward the posterior end of the body, and slowly on a varied course to the anus. The former movement is probably due to the action of the esophageal fibers; the latter is due to cyclosis. On its course 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 may increase to pH 1.4 and then decrease to pH 7.8. There is no "preliminary alkaline phase" of the food-vacuole. The changes in acidity are definitely correlated with changes in size, which are due to differences between internal and external osmotic conc. and the action of the stretched vacuolar membrane. 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. Decrease in acidity is due to entrance of alkaline fluid from the cytoplasm. Increase in acidity probably causes hydrolysis and thereby an increase in osmotic conc. resulting in inflow of fluid containing digestive enzymes. Death of ingested living organisms is probably due largely 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. 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 cyto-plasmic fluid which enters during its rapid enlargement. The neutral-red granules and the mitochondria are probably not involved in digestion. All these phenomena are essentially the same in the 4 spp. studied, namely P. caudatum, P. multimicronucleatum, P. aurelia, and P. trichium.