COMMENSALISM AND SYMBIOSIS 177 



mussels. The mussels and the crabs live together in per- 

 fect harmony and to their mutual benefit. 



There are a few extremely interesting cases of symbiosis 

 in which not different kinds of animals are concerned, but 

 animals and plants. It has long been known that some 

 sea-anemones pos- 

 sess certain body 

 cells which con- 

 tain chlorophyll, 

 that green sub- 

 stance character- 

 istic of the green 

 plants, and only 

 in few cases pos- 

 sessed by animals. 

 When these chlo- 

 rophyll -bearing 

 sea-anemones were 

 first found, it was 

 believed that the 

 chlorophyll cells 



,, * , , , FIG. 106. The crab Epizoanthus paguriphilus, with 



really belonged to the se a-anemone Parapagurus pilosiramus on its 



the animal's body, shell, 

 and that this con- 

 dition broke down one of the chiefest and most readily 

 apparent distinctions between animals and plants. But 

 it is now known that these chlorophyll-bearing cells are 

 microscopic, one-celled plants, green algae, which live ha- 

 bitually in the bodies of the sea-anemone. It is a case 

 of true symbiosis. The algae, or plants, use as food the 

 carbonic-acid gas which is given off in the respiratory 

 processes of the sea-anemone, and the sea-anemone breathes 

 in the oxygen given off by the algae in the process of ex- 

 tracting the carbon for food from the carbonic-acid gas. 

 These algae, or one-celled plants, lie regularly only in the 

 innermost of the three cell layers which compose the wall 

 13 



