CHAPTER XIX 



THE FOOD OF ANIMALS— PLANT-EATING CRUSTACEA 

 AND LOWER INVERTEBRATES 



Having now passed in review examples of vegetarian Arthro- 

 pods belonging to the air-breathing series, we come to the two 

 classes of Crustacea and Xiphosura, which together constitute 

 the aquatic gill-bearing division of the same phylum. The latter 

 class includes only the carnivorous King-Crabs (p. 144), and the 

 former includes a host of forms, most of which are either car- 

 nivorous or else act as scavengers. There are, however, interest- 

 ing exceptions to this. 



HIGHER CRUSTACEA (Malacostraca) 



Among the Higher Crustacea (Malacostraca) some of the 

 Land-Crabs affect a vegetable diet, as, for example, the Country- 

 man-Crab (Gecarcinus ruricola) of Jamaica and other West Indian 

 islands. This has proved a great nuisance to planters, as the 

 shoots and juice of the sugar-cane constitute its favourite food. 



A still more interesting animal is the Robber- or Cocoa-nut- 

 Crab {Birgus latro) (fig. 448) of the Cocos- Keeling islands, which 

 appears to be a hermit-crab (see p. 137) that has taken to a ter- 

 restrial life, and given up the habit of sheltering its tail in a cast- 

 off snail-shell. Hermit-Crabs are believed to be descended from 

 hard-tailed forms, in which the posterior part of the body became 

 soft because it was protected in a new fashion. Should a hermit, 

 therefore, give up its habit of appropriating a snail-shell as a pro- 

 tection, we might expect the tail to become hard once more, and 

 this is actually what has happened to the Robber- Crab. This 

 species is mentioned here because it feeds upon fallen cocoa-nuts, 

 and the following is Darwin's graphic account of the way in 

 which it goes to work in order to extract the fleshy part from 

 the shell and fibrous husk by which it is invested (the extract is 



taken from A Natm^alist's Voyage): — "The front pair of legs 



220 



