CHAP. IV 



SHIFTS FOR A LIVING 



49 



to suppose that mammals which passed from terrestrial 

 to more or less aquatic life, for instance beaver and 

 polar bear, otter, seals and whales, would enjoy a period 

 of relative immunity after the awkward time of transi- 







FIG. 10. THE LAND-CRAB OR ROBBER CRAB (Birgus latro). 



(From a specimen.) 



The animal really a kind of hermit-crab or Pagurid is often over 

 a foot. long. It breathes dry air by means of vascular ridges and folds 

 in the upper part of its branchial cavity. It has also rudimentary gills. 

 The structure of the female's abdominal limbs makes it practically cer- 

 tain that Birgus is descended from a shell-inhabiting ancestor, for while 

 the abdomen as a %yhole is symmetrical there are three appendages used 

 in egg-carrying which are present on the left side only. The male has 

 no abdominal limbs. The student should compare this figure with that 

 of the crayfish (fig. 72). Thus the last thoracic appendages are very 

 delicate in the land-crab. 



tion was over. So, too, many animals passed from the 

 battlefield of the sea-shore to relative peace on land or 

 in the deep-sea. In a change from open air to under- 



5 



