THE CONQUEST OF THE DRY LAND 223 



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

 fainter "cry" of our own bullhead, is prob- 

 ably 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 branching from the gill-chambers, in 

 which air is stored, so that the fish is not alto- 

 gether 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 dam- 

 age 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. 



