DESICCATION IN PHILODINA ROSEOLA. 345 



Ehrenberg (1838) maintained that the presence of sand pro- 

 tected the rotifers from an actual loss of water. He believed 

 that during the apparent desiccation period all of the vital 

 processes continued, reproduction included. In regard to the 

 function of the sand in retarding desiccation he differed from 

 Spallanzani since the latter believed that the sand protected 

 the rotifer from the injurious effects of the air rather than from 

 loss of water. 



Bory St. Vincent, about this time, maintained that under no 

 conditions could the animals survive desiccation but that their 

 apparent revival was due to the hatching of eggs concealed in 

 the sand. 



Doyere (1842) after careful study confirmed in the main the 

 observations of Spallanzani. Doyere found, however, that a few 

 rotifers recovered in each lot dried on a clean slide. He dis- 

 proved the theory of Spallanzani as to the fatal effects of the 

 air, since he found that rotifers dried in air and then placed in a 

 vacuum showed a lower mortality than those dried directly in 

 the vacuum. He concluded that the rapidity of drying is an 

 important factor in the effects of desiccation. He decided from 

 his experiments with rotifers in vacua that the last traces of 

 water might be extracted without destroying the power of re- 

 vival, and that since life processes are impossible in the absence 

 of water, the dried animal possessed life poentially and not 

 actually. 



About 1860 a more general interest in the subject of desiccation 

 was revived. The advocates and opponents of the view that 

 desiccation occurs were so evenly divided and the evidence for 

 both views apparently so well founded that the arguments for 

 and against desiccation were considered in 1859 by a commission 

 appointed by the Societe de Biologic of France. It was the 

 opinion of this commission that in the dried state life existed 

 only in potentia. 



Up to this time no one had suggested that rotifers might 

 secrete a water-proof membrane during the dry season. In 1873 

 Davis said that in Philodina there was a secretion of a gelatinous 

 covering which prevented loss of the body fluids. He explained 

 that the presence of sand prevented rapid evaporation of the 

 water and thus gave the rotifers time to secrete the gelatinous 



