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THE AMERICAN NATURALIST [Vol. XLII 



This list is far from being complete but it represents 

 so many families of the free swimming rotifers upon 

 which the general statement in regard to desiccation is 

 supposedly based that the results obtained ought to 'indi- 

 cate whether the phenomenon of desiccation is wide- 

 spread among the common forms. 



Philodhia roseola and Philodina ritrina were the only 

 forms of the forty-five experimented upon which could 

 successfully withstand desiccation and resume normal 

 activities when again placed in water. Some of them re- 

 mained ten days in small masses of debris, 1-2 mm. in 

 diameter, which were as thoroughly dried as possible in 

 the laboratory atmosphere. Those that were dried in 

 the sun never revived when again placed in water. This 

 may have been due to a too complete desiccation or to the 

 high temperature, which was usually about 45° C. 



The cuticle in the Philodinadae is less specialized in the 

 structure than in any of the other families of the three 

 orders, and as this structural character is of great im- 

 portance in the present system of classification the family 

 may be considered the lowest or most primitive of all the 

 twenty-one families. It is interesting to note, however, 

 that some of the species of another genus, Rotifer, of the 

 same family, can not withstand complete desiccation. In 

 several experiments in which the four species of Philo- 

 dina and Rotifer were mixed together in the debris, sedi- 

 ment or water plants, all four species would revive if the 

 material in which they were contained was not completely 

 dried, but only the two species of Philodina revived when 

 the drying was complete. Systematists separate the two 

 genera by the position of the eyes but evidently there is 

 a more fundamental difference than this which means life 

 and death in times of drought. 



The common misconception regarding desiccation may 

 probably have arisen, in part, from the fact that when 

 mud or sediment from ponds in which rotifers live is 

 dried living rotifers appear after a few hours when water 

 is added to the sediment. These living rotifers prob- 



