2o8 Merkel Henry Jacobs 



the dust from the gutters of roofs and other places where water 

 is accustomed to stand at intervals, and even after prolonged 

 drying the addition of a little water is all that is necessary to start 

 them into activity. While in the dried condition they show no 

 visible signs of life. All movements cease and the body frequently 

 shrinks to a shapeless mass so that it is difficult to distinguish them 

 from the particles of sand among which they are found. When 

 water is again apphed the body gradually regains its original 

 form, movements appear, and after a longer or shorter time, 

 depending on the conditions of the desiccation, the animals re- 

 sume their normal activities apparently none the worse for the 

 experience. Not only may they be dried naturally in the air but 

 they may be subjected artificially to even more extreme conditions. 

 Various observers have kept them in desiccators and vacua for 

 long periods of time and have subjected them to temperatures at 

 which hfe ordinarily is impossible, without destroying their power 

 of again resuming their normal activities upon the application of 

 water. 



The length of time the animals may remain in this state of sus- 

 pended animation is often considerable; there are well authenti- 

 cated cases of rotifers, whose usual period of life is probably not 

 more than a few weeks or months at most, which have been 

 revived after a period of desiccation extending over three or four 

 years; one observer even claims to have revived them after fifteen 

 years' desiccation. In the case of the Anguillulidae even longer 

 periods have been recorded; Baker in 1771 succeeded in reviving 

 individuals of Tylenchus scandens which had remained in a dried 

 condition in grains of wheat for 27 years. 



As to the principal facts just given there seems to be httle doubt. 

 They have been confirmed by numerous observers and anyone 

 may repeat for himself with httle trouble the experiments on which 

 they are based. In the interpretation of these facts, however, 

 there has been, and still is, much diversity of opinion. What is 

 the actual effect of drying on the rotifer or tardigrade ^ Is the 

 water contained in its tissues really removed or does the animal 

 have some means of protecting itself against the loss of water, the 

 desiccation being only apparent ? If the former be the case, what 



