DESICCATION IN PHILODINA ROSEOLA. 355 



particles which were not before influenced by its trochal activity. 

 During all this time the mastax is active. Its grinding opera- 

 tions are easily observed in any feeding animal. 



C. The Contracted Animal.- Zelinka ('86) in describing the 

 movements of rotifers of the genus Callidina states that the 

 animals frequently retract the head and rotating organs without 

 any apparent reason. He observes further that their subsequent 

 extension may be rapid or slow and that no definite reason can be 

 assigned for the lack of uniformity in this respect among different 

 individuals. P. roseola contracts very rapidly but usually ex- 

 tends itself slowly. The factors which influence it to contract 

 seem to be (i) Sudden change in temperature of the medium; 

 (2) change in the chemical composition of the medium; (3) me- 

 chanical stimuli; and (4) desiccation. 



Gradual changes in temperature in the medium in which the 

 Pkilodina lives do not cause it to contract. Increase in tempera- 

 ture is conducive to more vigorous movements while a decrease 

 causes the animal to become sluggish. If the temperature is 

 suddenly changed by the addition of hot or cold water the animal 

 contracts and does not extend itself for many minutes. 



The addition of any active chemical to the culture causes the 

 rotifers to suddenly contract and if the foreign substance is not 

 removed they will remain in the contracted condition until death 

 ensues. Philodinas kept in infusions will frequently be found to 

 become inactive in this way. An examination usually shows 

 that the infusion is in too great concentration or that putre- 

 factive or acid forming bacteria have rendered the environment 

 unfavorable. 



A mechanical shock, of any kind will make the animals con- 

 tract completely but a contraction under such conditions usually 

 lasts but a few seconds unless the stimulus is continuous or 

 repeated. 



When the water around a Pkilodina begins to dry up the animal 

 creeps about rapidly apparently trying to escape from the 

 diminishing drop. This creeping continues until movement is 

 no longer possible. The animal then contracts into an almost 

 spherical mass and dries. 



Especially interesting to observe is the manner in which the 



