MONOCELLULAR ANIMALS-THE PROTOZOA 



119 



varies with temperature, activity of the ani- 

 mal, and the concentration of salts in the 

 surrounding medium. As in the amoeba, the 

 contractile vacuole functions normally as a 

 bailer to rid the animal of excess water 

 constantly entering the cell. 



Behavior 



Paramecium is very sensitive to the rela- 

 tive acidity and alkalinity of its environ- 

 ment (Fig. 7-10). It responds positively by 

 going toward an acid environment, even 

 though the acid may be of sufficient 

 strength to destroy it. If a drop of weak 

 acid, for example, acetic acid, is placed on 

 a slide containing paramecia, they will 

 move toward the acid region and remain 

 there. If they get away from this region 

 and approach a less acid surrounding, the 

 paramecia give the avoiding reaction, and 

 thus return to the more acid medium. This 

 sensitivity is probably owing to the fact 

 that, in large numbers, paramecia give off 

 carbon dioxide in such quantity that the 

 water in the immediate vicinity becomes 

 weakly acid. 



Paramecium selects a temperature opti- 

 mal for its activities, usually around 25° C 

 (Fig. 7-10). If given a choice, it seeks this 

 temperature. If placed in an electrical field 

 of direct current, it responds in a very defi- 

 nite manner, always orienting itself witli 

 respect to the flow of the current (Fig. 

 7-10). It moves toward the negative pole, 

 indicating that externally it is positively 

 charged. 



Reproduction 



Paramecium, as in the case of amoeba, 

 maintains its numbers by dividing trans- 

 versely across its long axis (Fig. 7-12). The 

 first sign of division is a change in shape of 

 the nuclei. Paramecium has two nuclei, a 

 large macronucleus and a small micronu- 

 cleus; it is not quite clear how these differ 

 in regard to function. When the nuclei have 

 divided, a second gullet and two more con- 

 tractile vacuoles form. Other structures are 



F!g. 7-12. Paramecium divides by binary fission as 

 shown in this photograph of stained specimens. 



also duplicated before the two daughter 

 cells separate. After a growth period they 

 are ready to divide again. Under optimum 

 conditions division occurs about every six 

 to twelve hours. If division occurred three 

 times a day and all individuals survived, 

 their bodies would fill all the oceans of the 

 world within a month. Under natural con- 

 ditions they very soon cease dividing, be- 

 cause of accumulation of waste products, 

 lack of food, low temperatures, desiccation, 

 or falling prey to other aquatic animals. 



When placed in a suitable culture, para- 

 mecia periodically undergo a sexual process 

 called conjugation (Figs. 7-11 and 7-13). 

 Just why they do this is not clearly under- 

 stood, but it can be induced by reduc- 

 ing the bacterial food supply. During the 

 process two individuals interchange micro- 

 nuclear material, which means that genetic 

 factors are involved. This has the effect of 

 sexual reproduction, although there is no 

 increase in numbers, as the term repro- 

 duction would imply. Actually conjugation 

 seems to be unnecessary for division. Pro- 

 fessor L. L. Woodruff, some years ago, sep- 

 arated paramecia after each fission so that 

 conjugation was impossible. He followed 

 through 15,000 generations over a period of 



