16 NATURE OF CELLS 



The contractile vacuole is an organ of excretion and aids 



in getting rid of the waste matter of the body. Carbonic 

 acid gas, water, etc., pass out threctly through any part of 

 the surface of the body into the surrouncUng water. In 

 other words, the excretion of waste products may go on 

 anywhere over the surface of the body. 



How the amoeba breathes. — This animal is surrounded 

 by water containing oxygen, and this gas is taken into the 

 body of the amoeba through any part of its surface. Vice 

 versa, carbonic acid gas is given off from any part of the 

 body. It is probable that the pseudopodia are formed 

 partly to increase the surface of the body in order to 

 facilitate respiration. 



Reproduction of the amoeba. — \Vlien a new amceba is 

 to be produced, all the pseudopodia are withdrawn and the 

 animal lies c[uiet for a time. Finally, the nucleus begins 

 to divide in half, and at the same time a constriction ap- 

 pears around the middle of the bodj^. The nucleus finaUy 

 chvides in two; but while it is doing so, the constriction 

 around the body becomes deeper and deeper, until finally 

 the body is cut in two parts, each part 'ndth a half of the 

 original nucleus (Fig. 6). Each part with its nucleus is 

 really a new amcelDa. This is the simplest method of 

 reproduction known and is called fission. 



Sensation in the amoeba. — "\ATien the amceba comes in 

 contact with a bit of food, it reacts toward the food, and 

 when it touches a grain of sand it moves out of the wa}-; 

 hence this primitive animal evidently possesses sensation. 



Animal cells in general. — As a rule, animal cells have 

 no well-defined walls about them, and for this reason the 

 term cell is a misleading one. The cell has lieen called 

 the unit of structure in an animal because animal tissues 



