PROTOZOA 13 



surface. Its movements require energy ; this, as in all ani- 

 mals, is furnished by the uniting of oxygen with the food. 

 Carbon dioxid and other waste products are formed by the 

 union ; these pass off at the surface of the ameba and taint 

 the water with impurities. 



Questions. — Why will the ameba die in a very small quantity of 

 water, even though the water contains enough food? Why will it die 

 still quicker if air is excluded from contact with the drop of water? 



The ameba never dies of old age. Can it be said to be immortal? 



According to the definition of a cell {Chapter I), is the ameba a 

 unicellular or multicellular animal? 



Cysts. — If the water inhabited by a protozoan dries up, 

 it encysts, that is, it forms a tough skin called a cyst. 

 Upon return of better conditions it breaks the cyst and 

 comes out. Encysted protozoans may be blown through 

 the air : this explains their appearance in vessels of water 

 containing suitable food but previously free from proto- 

 zoans. 



The Slipper Animalcule or Paramecium 



Suggestions. — Stagnant water often contains the paramecium as 

 well as the ameba ; or they may be found in a dish of water con- 

 taining hay or finely cut clover, after the dish has been allowed to 

 stand in the sun for several days. A white film forming on the 

 surface is a sign of their presence. They may even 'be seen with 

 the unaided eye as tiny white particles by looking through the side 

 of the dish or jar. Use at first a|or] in. objective. Restrict 

 their movements by placing cotton fibers beneath the cover glass ; 

 then examine with 4- or \ objective. Otherwise, study figures. 



Shape and Structure. — The Paramecium's whole body, 

 like the ameba's, is only one cell. It resembles a slipper 

 in shape, but the pointed end is the hind end, the front end 

 being rounded (Fig. 14): The paramecium is propelled 

 by the rapid beating of numerous fine, threadlike append- 



