UNICELLULAR FORMS 57 



Nutrition. With this seemingly meagre equipment the amoeba 

 carries on all the functions of life. We may now examine the 

 means by which these functions are performed. Since the energy- 

 consuming functions depend on the potential energy in food, the 

 method of securing and utilizing foods, the nutrition of the ani- 

 mal, is first to be considered. The amoeba feeds on bits of plant 

 material that it may encounter, and on other Protozoa. Like many 

 animals that live in water, the amoeba cannot swim, but creeps 

 about over such surfaces as may be present, a stem, bits of detritus, 

 or the bottom. On contact with a food bit it sends out pseudopodia 

 on either side of the object that soon engulf it, forming a food 

 vacuole. Thus the wall of the food vacuole is formed from a por- 

 tion of the body wall of the animal. Then from the cytoplasm there 

 diffuses into the vacuole certain substances that act upon the food 

 in several ways, dissolving the soluble portions and changing it 

 chemically and physically into a condition that will permit the 

 useful ingredients to pass through the wall of the vacuole and into 

 such chemical forms that the metabolic processes of the animal 

 may make use of them. This process of altering food so that it 

 will pass through living semi-permeable membranes and so that 

 it is chemically available to protoplasm is called digestion. The 

 process in Man, while much more complicated than in amoeba, 

 includes these general and necessary processes. In the amoeba the 

 solid food particles are taken into the cell and there digested. This 

 is INTRACELLULAR digestion. The processes of utilizing the food after 

 it has been digested are collectively termed assimilation. 



The amoeba apparently has little power of discrimination between 

 particles that contain food and other substances, for it will engulf 

 bits of insoluble material, such as finely divided carbon. The indi- 

 gestible substances which it picks up, either as a part of a food 

 body or inert substances, for example carbon particles, traverse the 

 endoplasm of the amoeba in the food vacuoles and are eventually 

 left behind as the vacuole comes to the surface and opens. The fact 



