PAEAMECIUM AND ITS RELATIVES 



169 



and this passes into the protoplasm through the thin mem- 

 brane surrounding the animal. When the oxygen combines 

 with the chemical elements found in foods and protoplasm, 

 oxidation is carried on, energy is released, and waste sub- 

 stances are formed which are given off in the process of 

 excretion. 



123. Excretion of wastes. At either end of the animal 

 is a clear space which is sometimes circular and at other times 

 star-shaped. These are the contractile vacuoles. The wastes 

 formed by oxidation (e.g. carbon dioxid and water) collect 

 to form the vacuoles. The protoplasm presses upon the 

 waste materials and periodically squeezes them out of the 

 animal. When this occurs, the contractile vacuole disappears. 



124. Reproduction and life history. In the interior of a 

 paramecium are two nuclei known as the large nucleus and 

 the small nucleus, both of which 



show readily when the animal is 

 stained with iodine or with other 

 chemicals. When the animal re- 

 produces, both the large and small 

 nuclei divide in halves (Fig. 119), 

 a new mouth and gullet are formed, 

 and two new contractile vacuoles 

 appear. The cell body then di- 

 vides transversely, the cells sepa- 

 rate from each other, and thus 

 from a single individual, two new 

 paramecia are formed. If condi- 

 tions are favorable, both animals 

 grow and may in turn reproduce at the end of twenty- 

 four hours. "It has been estimated that one paramecium 



large 

 nucleus 



FIG. 119. A paramecium di- 

 viding. 



