PARAMECIUM 73 



cause the death of the animal. Semiannual periods also occur, 

 but recovery from these does not take place if the animals are 

 kept under constant conditions or conjugation is prevented, but 

 the protoplasm degenerates and becomes vacuolated and the ani- 

 mals lose their energies and finally die. 



Experiments have been performed which seem to show that 

 in a varied environment neither conjugation nor death from old 

 age necessarily occurs. Thus Woodruff (1909, 1910) has carried 

 a culture of Paramecia through a period of thirty-seven months 

 by changing the character of the medium daily. During this 

 time there were seventeen hundred and ninety-five generations. 

 At the end of the thirty-seventh month, the animals were normal. 

 The cycle may thus be prolonged from six months to over thirty- 

 seven months by employing a varied culture medium. Since in 

 nature the stimuli derived from changes in the environment 

 probably are present, the length of the cycle may perhaps be 

 prolonged indefinitely. 



Behavior. Paramecium is a more active animal than Ameba, 

 swimming across the field of the microscope so rapidly that care- 

 ful observations are necessary to discover the details of its move- 

 ments. As in Ameba, its activities are either spontaneous, that is, 

 initiated because of some internal influence, or result from some 

 external stimulus. This stimulus is in all cases a change in the 

 environment. For example, if a drop of distilled water is added to 

 a drop of ordinary culture water containing a number of Para- 

 mecia, all of the animals will enter and remain in the distilled 

 water; they are stimulated to a certain kind of activity by the 

 change in the composition of the water. They will soon become 

 acclimated to their new surroundings, and will behave themselves 

 within the distilled water in a normal manner until another change 

 in their environment stimulates them to further reactions. 



Paramecium responds to stimuli either negatively or positively. 

 The negative response is known as the "avoiding reaction"; 

 it takes place in the following manner. When a Paramecium 

 receives an injurious stimulus at its anterior end, it reverses its 



