158 THE WONDER OF LIFE 



by the store of the estuaries and makes holes not only in the 

 softer ground, but in the hard clay of the grassy meadows. 

 In the latter the holes go down till they are below the 

 water-level. The animal seems able to live in poorly 

 aerated water, as Bate surmised long ago from his study of 

 preserved specimens. Its habits have been recently studied 

 by Mr. A. S. Pearse, who points out that the abihty to 

 breathe in poorly aerated water would be a distinct advan- 

 tage, and seems to have been secured by a simple contri- 

 vance. The gill-covers or side-flaps of the shield that covers 

 the front of the body are movable on the dorsal portion 

 of the carapace by a sort of flexible hinge joint. ' An 

 individual placed in a dish will often move the sides of the 

 carapace in such a manner that it resembles a Vertebrate 

 gasping for breath '. Such bellows-like movements must 

 serve to hasten the current of water that is drawn in over the 

 gills and thus facilitate respiration. 



As a good instance of the possession of a new home we 

 may refer to the freshwater sting-rays of the Ganges. No 

 fishes are more characteristically marine than rays and 

 skates, yet it is certain that there are several members of 

 this (Batoid) family in the Ganges. Two species, Trygon 

 fluviatilis and Hypoloihus sephen, have established them- 

 selves far up the great river. Even one thousand mUes 

 above tidal influence, they thrive and breed freely. 



While some creatures are sensitive specialists as 

 regards environment, others are tough cosmopohtans. 

 In illustration of the latter we may refer to Dr. Alcock's 

 account of the freshwater crabs (Potamonidse) of 

 India. They are typically freshwater animals, but some 

 can live both in brackish water and in damp jungle. ' They 

 are found in ponds, lakes, streams, rivers, and marshes ; 



