UNICELLULAR ORGANISMS 55 



extraneous material, and is always present in the living animal. 

 The other body commonly found is the contractile vacuole (Lat. 

 vacuus = empty) (Fig. 19 cv). This is a clear, pulsating drop, 

 at one moment appearing as a good-sized sphere, and the next 

 contracting and disappearing, to reappear again. It is thought 

 that when it contracts, its contents, which are liquid, are forced 

 out of the Amoeba's body through minute openings that appear 

 in its sides. These pulsations, which are fairly regular, plainly 

 indicate the performance of some important function. 



Assimilation and Growth. When the Amceba comes in con- 

 tact with a small plant or other bit of food, the pseudopodia 

 flow around and over it so that the food is taken bodily inside 

 the animal. The food may be taken in at any point on the 

 surface of the Amoeba's body, though more frequently it is 

 engulfed by the anterior pseudopodia. As shown in Fig. 19 B, 

 particles of food longer than the whole animal may be ingested. 

 After a time the bit of food thus ingested begins to show signs 

 of disintegration. It loses its sharp outline and becomes slowly 

 softened and dissolved. This change is produced by the action 

 of certain fluids which the animal secretes, and is a process of 

 digestion. The nutritious portions become in time absorbed 

 by the protoplasm and converted into new Amoeba substance; 

 the last process being assimilation. The refuse finds its way 

 eventually to the surface of the animal, a temporary opening- 

 appears and the Amoeba crawls away, leaving the refuse behind 

 it; Fig. 19 ex. Any part of the body may thus serve for the 

 ingestion of food or the ejection of refuse, although the food is 

 commonly taken in at the anterior end, and the refuse ejected 

 from the posterior end. 



Respiration. Amoeba is not only carrying on a process of 

 assimilation, by which new substances are built up, but is also 

 at the same time carrying on a process of disintegration, by 

 which the complex substances are broken down. This latter 

 is based upon oxidation or union with oxygen. As the result of 

 oxidation there is always formed carbon dioxid gas (C02) as a 



