228 Merkel Henry Jacobs 



all cases in which recovery occurs the loss of the body fluids has 

 in some way been prevented. Among those who have held this 

 latter view, perhaps the most important name is that of H. Davis 

 ('73) ^^^ ^^^ ^^^ fi""^^ ^^ §^^^ ^ plausible explanation of the means 

 by which desiccation could be prevented. Since his views have 

 found a wide acceptance and are frequently quoted at the present 

 day, they will be considered at some length and an attempt made 

 to determine their truth or falsity. 



I The Views of Davis and Others 



Davis as well as many others before him, had noticed that roti- 

 fers dried on a clean glass slide are killed by the process while if 

 a little sand or moss is present recovery almost always occurs. He 

 supposed that in the latter case the rotifers had time to protect 

 themselves by secreting a gelatinous waterproof cyst which efi^ec- 

 tually prevented evaporation while in the former case, having no 

 protection, they were dried and consequently killed. According 

 to his view a true desiccation of the animal is always followed by 

 death and in all cases of recovery desiccation has been prevented. 

 He showed that grapes may be effectually protected against drying 

 by means of a thin coating of gelatine and assumed that in the 

 case of rotifers exposed to desiccation we are dealing with a phe- 

 nomenon somewhat similar. 



Others had claimed that the sand has a direct protective efi^ect. 

 Ehrenberg, for example, had compared it to the woolen mantle 

 of an inhabitant of the desert and others had considered that it 

 always holds in its interstices sufficient water to prevent complete 

 desiccation. But Davis and Hudson rightly objected that sand 

 at 100° C, at which temperature dried rotifers may survive for 

 a short time, would be but poor protection against loss of water 

 in the case of such a soft bodied animal, and Davis' explanation 

 that the rotifers are covered by an actual waterproof capsule has 

 the merit of being the first one to take this fact into consideration. 



Nevertheless, there are many reasons why this view cannot be 

 accepted. In the first place the evidence for the existence of such 

 a gelatinous secretion is extremely shght. Davis did not see it 



