ANIMALS— PHYSICAL ENVIRONMENT 121 



the antennae, etc., and that it is only developed in 

 ants from sandy and dusty places because the 

 strigil is peculiarly likely to become clogged in such 

 places. 



The protozoa of the sand in the neighbourhood 

 of Cairo have been investigated by Thomson and 

 Thomson. These microscopic animals exist in the 

 dry sand in a resistant envelope or " cyst," from 

 which they only emerge when the sand is moist, 

 and as it dries they again envelop themselves in 

 " cysts." In the climate of Cairo their period of 

 activity is reduced to a few days a year, and their 

 period of dormancy is extremely protracted. They 

 are exposed to light and heat and drought in the 

 sand, and yet the investigators were able to recover 

 about fifty species from the sand after eight months 

 of drought. Most of them belonged to forms known 

 elsewhere from fresh water ; that they were depend- 

 ent on occasional moistening is shown by their great 

 rarity in the sand of Luxor in Upper Egypt, in an 

 area which is without rain in the average year. 



