the wind may have scattered some of these little organisms in the dried mud or dust. Some may have existed in a dormant state on the hay and the water awakened them to active life. In the water, too, there may have been some living cells, plants and animals. At first the multiplication of the tiny animals within the hay infusion is extremely rapid; there is food in abundance and near at hand. After a few days more, however, several kinds of one-celled animals may appear, some of which prey upon others. Consequently a struggle for life takes place, which becomes more and more intense as the food from the hay is used up. Eventually the end comes for all the animals unless some green plants obtain a foothold within the jar. If such a thing happens, food will be manufactured within their bodies, a new food supply arises for the animals within the jar, and a balance of life may result. REFERENCE BOOKS ELEMENTARY Hunter, _Laboratory Problems in Civic Biology_. American Book Company. Sharpe, _A Laboratory Manual for the Solution of Problems in Biology_, pp. 133-138. American Book Company. ADVANCED Eggerlin and Ehrenberg, _The Fresh Water Aquarium and its Inhabitants_. Henry Holt and Company. Furneaux, _Life in Ponds and Streams_. Longmans, Green, and Company. Parker, _Biology_. The Macmillan Company. Sedgwick and Wilson, _Biology_. Henry Holt and Company. XIII. SINGLE-CELLED ANIMALS CONSIDERED AS ORGANISMS _Problems.--To determine:_ _(a) How a one-celled animal is influenced by its environment._ _(b) How a single cell performs its functions._ _(c) The structure of a single-celled animal._ LABORATORY SUGGESTIONS _Laboratory study._--Study of paramoecium under compound microscope in its relation to food, oxygen, etc. Determination of method of movement, turning, avoiding obstructions, sensitiveness to stimuli. Drawings to illustrate above points. _Laboratory demonstration._--Living paramoecium to show structure of cell. Demonstration with carmine to show food vacuoles, and action of cilia. Use of charts and stained specimens to show other points of cell structure. Laboratory demonstration of fission. [Illustration: Pleurococcus. A very simple plant cell.]