THE CONQUEST OF THE DRY LAND 223 



true vocal organs, the " scream," like the fainter 

 "cry" of our own bullhead, is probably the 

 sound made by the escape of air from its body. 

 For both Clarias and the Climbing Perch have 

 a special arrangement, a system of tubes branch- 

 ing from the gill-chambers, in which air is stored, 

 so that the fish is not altogether dependent on 

 its gills. 



Land-crabs illustrate terrestrial animals in 

 the making. In warm lands, such as Jamaica, 

 there are many kinds, often living in forests far 

 from the sea, sometimes doing great damage in 

 the sugar plantations. But once a year they 

 assemble in enormous numbers to make an 

 excursion to the seashore and deposit their 

 eggs below high-water mark, where they leave 

 them to be swept out to sea by the tide. Then 

 they return, weary and spent, to their inland 

 haunt for the rest of the year. 



Darwin, in his Naturalist's Voyage Round 

 the World, gives an account of the great Robber- 

 Crab which occurs in the Pacific Islands, 

 wherever the coco-nut palm grows. This 

 crab belongs to the same group as the hermit- 

 crab of the seashore, but it lives in a burrow 

 in the ground, and it lines it with the fibres from 

 the outside of the coco-nut shell. The Robber 



