148 



BIOLOGY FOR BEGINNERS 



trio* fnut*i 



similation go on directly in contact with the food, and undigested 

 particles are merely left behind when it flows away from them or 

 they pass out through any part of the cell. Oxygen is taken by 

 contact from the water in which it is dissolved and combines 

 directly with the food and protoplasm producing energy, just as 

 in all living things. The contractile vacuole acts as an excretory 

 organ, getting rid of waste. Locomotion is secured by the flowing 



of the protoplasm, projec- 

 tions being pushed out on 

 one side and withdrawn 

 on the other. Some form of 

 sensation must be present 

 because it responds to light, 

 food, moisture, or sudden 

 jars. 



Reproduction occurs as 

 soon as growth reaches a 

 certain size. The nucleus 

 first divides in two similar 

 portions, then the rest of 

 the protoplasm gradually 

 separates in two masses, 



each with a nucleus and capable of independent life and growth. 

 This simple reproduction by mere division is called fission. Repro- 

 duction by union of anything like the sperm and egg cells of plants 

 and other animals has never been observed in the amoeba, though 

 it seems almost necessary that there should be some such process. 

 There are nearly a thousand close relatives of the amoeba, some 

 of which attach a protective covering of tiny sand grains to their 

 body; others secrete. a layer of flint or lime. These shelled proto- 

 zoans are so abundant in the tropical seas that they tinge the water 

 white and their shells, falling to the bottom, make deposits of 

 limestone, such as the chalk cliffs of England. 



Paramoecium. Another common protozoan is the paramcecium 

 which is also abundant in stagnant water. We cultivate it in the 

 laboratory by putting some dry hay or leaves in water and leaving 



FIG. 45. Progressive stages of fission 

 of amoeba. After Schultz. 



