THE PROTOZOA 221 



highly colored body can be distinguished near the center of the 

 animal. This is called the nucleus. Every cell possesses a nu- 

 cleus. Reactions take place between the nucleus and the sur- 

 rounding protoplasm, and that these reactions are important is 

 proved by the fact that a cell deprived of its nucleus will not live 

 very long. 



Reproduction. — Paramecium reproduces only by simple 

 binary division. This process is interrupted occasionally by a 

 temporary union (conjugation) of two individuals. In binary 

 fission the nucleus first divides and then the animal is divided 

 into two by a constriction. The entire process occupies from 

 about half an hour to two hours. The daughter Paramecia 

 grow rapidly and divide again at the end of twenty-four hours or 

 even sooner, depending on the temperature, food, and other ex- 

 ternal conditions. It has been estimated that one Paramecium 

 may be responsible for the production of 268,000,000 offspring 

 in one month. 



Sometimes when two Paramecia come together in conjugation, 

 they remain attached to each other with their ventral surfaces 

 opposed, and a protoplasmic bridge is constructed between them. 

 As soon as this union is effected, the nuclei pass through a series 

 of complicated stages, during which part of the nucleus of each 

 animal passes over into the other. This has been likened to the 

 process of fertilization in higher animals. Then the two Para- 

 mecia separate and continue to grow and multiply by binary fission. 

 The causes and results of conjugation are not well understood. 



Reactions to Stimuli. — The reactions of Paramecium to 

 changes in the water are quite interesting. It will swim away 

 from salt if this is added to the water, but will swim into a drop 

 of -X per cent acetic acid and stay there. It avoids a high tem- 

 perature and swims against running water. These reactions 

 prove that the animal is capable of being stimulated and of re- 

 sponding to these stimuli. 



Life Activities of One-celled Animals and Many-celled Animals 

 Compared. — If we now compare the life activities of Parame- 



