THE ANIMAL IN THE WORLD 539 



need no further comment. We have seen instances of the 

 ways in which omnivorous, herbivorous, and carnivorous 

 animals are nourished. We have dealt with parasitism 

 and symbiosis. Under the head of commensalism a large 

 number of curious instances of co-operation between animals 

 is known. One of these must suffice here. The hermit 

 crabs are crustaceans related to the crayfish, with the 

 abdomen soft, owing to the thinness of its cuticle, and 

 twisted so that it will fit into the empty shell of molluscs 



Fig. 397. — Sea-anemones on the shell of a hermit crab. 

 — After Andres. 



like the whelk. They search for such shells, often fighting 

 one another for possession, so that they are sometimes 

 called soldier crabs, and they anchor themselves into their 

 shells by means of the limbs of the sixth abdominal seg- 

 ment. When they are attacked they withdraw into the 

 shell by the contraction of a muscle in the abdomen, but 

 often this does not save them from being eaten by fishes, of 

 which they are a very favourite food. They are very active, 

 and are constantly travelling in search of food, dragging 

 about their shells with them. Sea-anemones are, as we have 

 seen, polyps related to Hydra, but more complicated in their 



