THE CONQUEST OF THE DRY LAND 223 
has true vocal organs, the ‘ 
fainter ‘ 
‘scream,” like the 
‘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. 
