stimulus sets free a certain amount of energy previously latent in the protoplasm, and the energy set free takes on the form of movement. Any living matter which, when acted on by a stimulus, thus suffers an explosion of energy, is said to be "irritable." The irritability may, as in the amœba, lead to movement; but in some cases no movement follows the application of the stimulus to irritable matter, the energy set free by the explosion taking on some other form (heat, etc.) than movement. Thus a substance may be irritable and yet not contractile, though contractility is the most common manifestation of irritability.
The amœba (except in its prolonged quiescent stage) is rarely at rest. It is almost continually in motion. The movements cannot always be referred to changes in surrounding circumstances acting as stimuli; in many cases the energy is set free in consequence of internal changes, and the movements which result are called spontaneous or automatic movements. We may, therefore, speak of the protoplasm of the amœba as being irritable and automatic.
3. It is receptive and assimilative.—Certain substances serving as food are received into the body of the amœba, and, being there in large measure dissolved, become part and parcel of the body of the amœba become, in fact, fresh protoplasm.
4. It is metabolic and secretory.—Pari passu with the reception of new material, there is going on an ejection of old material, for the increase of the amœba by the addition of food is not indefinite. In other words, the protoplasm is continually undergoing chemical change (metabolism), room being made for the new protoplasm by the breaking up of the old protoplasm into products which are cast out of the body and got rid of. These products of metabolic action have, in all probability, subsidiary uses. Some of them, for instance, we have reason to think, are of value in the solution and preliminary changes of the raw food mechanically introduced into the body of the amœba; and hence are retained within the protoplasm for some little time. Such products are generally spoken of as "secretions." Others, which pass more rapidly away, are generally called "excretions." The distinction between the two is an unimportant and frequently accidental one. The energy expended in the movements of the amœba is supplied by the chemical changes going on in the protoplasm by the breaking up of bodies possessing much latent energy into bodies possessing less. Thus the metabolic changes which the food undergoes in passing through the protoplasm of the amœba (as distinguished
- This word has recently acquired a meaning almost exactly opposite to that which it originally bore, and an automatic action is now by many understood to mean nothing more than an action produced by some machinery or other. In this work I use it in the older sense, as denoting an action of a body, the causes of which appear to lie in the body itself. It seems preferable to "spontaneous," inasmuch as it does not necessarily carry with it the idea of irregularity, and bears no reference to a "will."