negative reaction is useful in avoiding obstacles; the positive in securing contact with food. The food-taking reaction is the enveloping of food by throwing pseudopodia about it. These three reactions together with the ordinary crawling locomotion and the throwing out of pseudopodia in search of a solid on which to crawl, constitute the entire variety of Amœba's experiences as displayed in behavior.
The structure of the Ciliata is much more complicated and in certain respects marks a distinct advance in equipment for the struggle for existence. Paramecium may represent the type. In this animal not only are the cilia modified locomotory structures, but there is a definite region for food taking. A groove extends obliquely down one side of the body, terminating at its lower end in a mouth. It is to the cilia, chiefly, however, that Paramecium owes its superiority over Amœba. These are usually inclined backward and their stroke then drives the animal forward. The most interesting characteristic of the stroke is its obliqueness so that Paramecium always rotates on its long axis, whether it moves forwards or backwards. In consequence of this fact, according to Jennings, Paramecium solves the problem of how an unsymmetrical organism, without eyes or other sense organs, may, nevertheless, maintain a definite course through trackless water. Not even man succeeds in maintaining a straight course under similar but simpler conditions. On the trackless snow-covered prairie the traveler without compass, landmarks or other guide wanders in circles, though it is possible to err only to right or left, not up and down as in the water.
Paramecium, by rotation, compensates for any wandering by equal wandering in the opposite direction. Nature anticipated the modern rifle by several asons. With this equipment whenever a Paramecium reacts negatively to some stimulus it is able to continue swimming in a direction away from the stimulus until the stimulation ceases. Through the additional power of slightly accentuating the rotation-swerve the animal is enabled to swim in various directions and to remove itself successively from many different environmental conditions, until it has found what Jennings terms the "optimum." This behavior Jennings characterizes briefly as a "selection from the environmental conditions resulting from varied movements." It is in fact a "trial and error" method with selection of the optimum—a method not unknown to man himself.
This characterization of it, however, may seem to imply too much. Paramecium "tries" over and over again, but what is "tried" is always the same thing—there seems to be no profiting by experience. Whether the response of Paramecium to stimulation is conditioned in any degree by subjective phenomena, or is even accompanied by them, is a disputed point. Even if present there remains the further question as to the extent such subjective phenomena may be considered similar
