Effects of Desiccation on the Rotifer 223 



mained in the evaporating drop as long as possible thus being 

 brought together as the latter decreased in size. When drying 

 occurs under a cover glass or with sand, the contracting surface 

 film of the drop may frequently be seen to sweep a number of 

 rotifers together and by further contraction press them into a com- 

 pact mass; the part the rotifers themselves play in the process, 

 however, is purely a passive one. 



On the whole, the method of response to the first stages of des- 

 iccation shown by Philodina is an advantageous one. It is true 

 that the random movements sometimes lead single individuals 

 into places where they are caught by sudden evaporation of the 

 water and dried without any protection at all. On the other hand, 

 the majority of the rotifers, by continuing to move as long as pos- 

 sible, tend to find their way to the places where the water lingers 

 longest and where drying, when it does occur, is most gradual. 

 If they stopped at the first grain of sand they encountered they 

 would frequently be dried under far less advantageous circum- 

 stances. 



V VISIBLE CHANGES ATTENDING THE PROCESS OF DESICCATION 



When the water has so far evaporated that creeping is no longer 

 possible, the rotifer contracts in the manner already described, 

 drawing in both the head and the foot and then puckering the two 

 ends of the body as though they were drawn together by a purse 

 string. This part of the contraction is accomplished by muscular 

 action and occurs before the water surrounding the animal has 

 evaporated. Before the final contraction occurs the head may be 

 rapidly extended two or three times as if to test the external con- 

 ditions. When the process of drying is very slow contraction may 

 continue still further, the animal becoming noticeably smaller even 

 before all of the water has disappeared. Under these conditions 

 irregular wrinkles do not appear although the puckering at the 

 two ends is discernible. When dried more rapidly, even when in 

 contact with a large quantity of sand the animal assumes a more 

 or less irregular form, both longitudinal and transverse wrinkles 

 appearing in the cuticle and the internal organs shrinking away 



