414 ELEMENTARY ZOOLOGY 



and walking legs protruding. In some species of hermit- 

 crabs there is always to be found on the shell near the 

 opening a sea-anemone. "This sea-anemone is carried 

 from place to place by the crab, and in this way is much 

 aided in obtaining food. On the other hand, the crab is 

 protected from its enemies by the well-armed and dan- 

 gerous tentacles of its companion. On the tentacles there 

 are many thousand long slender stinging threads, and the 

 fish that would eat the hermit-crab must first deal with 

 the stinging anemone." If the sea-anemone be torn 

 away from the shell the crab will wander about seeking 

 another anemone. When he finds one, he struggles to 

 loosen it from the rock to which it is attached, and does 

 not rest until he has torn it loose and placed it on his 

 shell. 



In the case of the hermit-crab and the sea-anemone 

 there is no doubt of the mutual advantage derived from 

 their communal life. But this mutual advantage is not 

 so obvious in some cases of commensalism, where indeed 

 most or all of the advantage often seems to lie with one 

 of the animals, while the other derives little or none, but 

 on the other hand suffers no injury. For example, 

 "small fish of the genus Nomeus may often be found 

 accompanying the beautiful Portuguese man-of-war 

 (Physalid) as it sails slowly about on the ocean's surface. 

 These little fish lurk underneath the float among the 

 various hanging thread-like parts of the man-of-war which 

 are provided with stinging cells. They are protected 

 from their enemies by their proximity to these stinging 

 threads, but of what advantage to the man-of-war their 

 presence is is not understood." Similarly in the nests of 

 the various species of ants and termites many different 

 kinds of other insects have been found. " Some of these 

 are harmful to their hosts, in that they feed on the food- 

 stores gathered by the industrious and provident ant, but 



