AN INTRODUCTION TO ZOOLOGY 



size is reached, the walls contract and the contents are discharged 

 to the exterior, probably through a pore. The two vacuoles do 

 not contract at the same time, but alternately, the interval be- 

 tween successive contractions being ten to twenty seconds. The 

 expulsion of the fluid contents of the contractile vacuoles may be 



seen in the fol- 

 lo wing way. 

 Paramecia should 

 be mounted in 

 water into which 

 has been rubbed 

 up a stick of India 

 or Chinese ink. 

 They then appear 

 white against a 

 black back- 

 ground. Part of 

 the water should 

 be withdrawn 



FIG. 27. Paramecia swimming in a solution of India I rom beneath me 



ink, showing the discharge of the contractile cover glass, thus 



vacuoles to the outside. (From Dahlgren and slightly compress- 



Kepner after Jennings.) ing them jf now 



a specimen in profile is found and watched, the discharge pro- 

 duces a bright spot outside in the opaque liquid; this lasts from 

 one to two seconds, and is then driven off by the cilia (60, Fig. 

 27). What has been said of the function of the contractile 

 vacuole in Ameba (p. 39) applies as well to those of Parame- 

 cium, i.e. it acts as an organ of excretion and respiration, and is 

 probably hydrostatic (50). 



Locomotion. The only movements of Paramecium that in 

 any way resemble those of Ameba are seen when the animal passes 

 through a space smaller than its shorter diameter; it will then 

 exhibit an elasticity which allows it to squirm through. In a 

 free field Paramecium swims by means of its cilia. " These are 



