246 Merkel Henry Jacobs 



certain experiments mentioned above show in a physical way, 

 namely, that the cuticle of dried rotifers is freely permeable to 

 water vapor, their death being caused by changes in the amount 

 of moisture. 



Since rotifers are so easily killed by rapid changes in moisture 

 content it must be supposed that in nature this is at least one of 

 the factors responsible for the higher mortahty during long periods 

 of desiccation. The causes of the injury as in the case of rapid 

 drying is probably mechanical, resulting in destruction of the 

 delicate internal organs and the cells composing them. Rotifers 

 living under natural conditions where much sand and organic matter 

 is present would not be subjected to such rapid changes as those 

 kept in small dishes and hence the mortality would naturally be 

 expected to be lower. The protective effect of sand, therefore, 

 is due not only to prevention of too rapid drying at the start but to 

 prevention of too rapid changes in the moisture content of the 

 animals after desiccation has occurred. 



e Effect of the Intensity of the Desiccation 



The mechanical injuries to the tissues caused by the swelHng 

 and shrinking under different atmospheric conditions are not the 

 only causes of death during a prolonged desiccation. Under 

 perfectly uniform conditions either in the dry air of a desiccator or 

 in air thoroughly saturated with water vapor the same relation 

 exists between mortality and duration of the desiccation. Other 

 things being equal, a long desiccation is more injurious than a short 

 one. Since the external conditions remain the same the injury 

 must be due to some progressive change occurring within the body 

 of the animal. Conceivably this change might be (l) a gradual 

 loss of water (2) an accumulation of injurious waste products or 

 (3) metabohc processes by which either the reserve supplies of 

 food or the body tissues themselves are slowly used up. 



There are many reasons to beheve that the first of these possi- 

 bilities is of Httle importance. The loss of water after the first 

 part of the drying has occurred would be extremely slight and 

 under ordinary conditions would come to an end as soon as an 



