THE IMPORT OF PROTOPLASM. 423 



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 amoeba, lead to movement ; but in some cases no move- 

 ment 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 amoeba (except in its prolonged quiescent stage) is rarely at 

 rest. It is almost continually in motion. The movements cannot al- 

 ways be referred to changes in surrounding circumstances acting as 

 stimuli ; in many cases the energy is set free in consequence of in- 

 ternal changes, and the movements which result are called sponta- 

 neous or automatic ' movements. We may, therefore, speak of the 

 protoplasm of the amoeba as being irritable and automatic. 



3. It is eeceptive and assimilative. Certain substances serv- 

 ing as food are received into the body of the amoeba, and, being there 

 in large measure dissolved, become part and parcel of the body of the 

 amoeba become, in fact, fresh protoplasm. 



4. It is metabolic and secretoet. Pari passu with the recep- 

 tion of new material, there is going on an ejection of old material, for 

 the. increase of the amoeba 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 

 amoeba ; and hence are retained within the protoplasm for some little 

 time. Such products are generally sj^oken of as "secretions." Others, 

 which pass more rapidly away, are generally called " excretions." The 

 distinction between the two is an unimportant and frequently acci- 

 dental one. The energy expended in the movements of the amoeba is 

 supplied by the chemical changes going on in the protoplasm by the 

 breaking up of bodies possessing much latent energy into bodies pos- 

 sessing less. Thus the metabolic changes which the food undergoes 

 in passing through the protoplasm of the amoeba (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." 



