PARASITISM 315 



situated, but is on the shell forming the crab's house, 

 it is less easy to understand the behavior of the animal 

 which is said at times to transfer its anemone to the 

 new shell when its home is changed. 

 Commensalism among the higher plants might be 

 exemplified by the epiphytes, plants, like orchids, that 



FIG. 109. Mutualism of diatom and bacteria. (Verworn.) 



cling to other plants, live upon them, yet not at their 

 expense or to their injury. In the tropics scarcely a 

 tree can be found upon which many varieties are not 

 present. 



Mutualism. This is a form of symbiosis in which one 

 or both of the symbionts derives advantage from the 

 association without injury to the other. The advantage 

 is usually physiological in character. Thus, certain 

 radiolaria not infrequently harbor a small green alga, 

 the zooxanthella. The alga gives off oxygen that is 

 advantageous to the little animal, which in turn gives 



