THE MUTUAL RELATIONS OF ORGANISMS 



229 



dogs, and the human race. Among lower animals the associa- 

 tion of a hermit crab with a sea anemone is an illustration; 

 Fig. 105. Here the anemone gains an advantage from being 

 carried to and fro, while the 

 hermit crab is protected by 

 the nematocysts, which, as 

 in Hydra (page 144), are 

 abundant on the tentacles 

 of the anemone, and which 

 by their poison protect the 

 crab from the attack of 

 enemies. 



Mutualism is rather more 

 common among plants than 

 animals. An example is the 

 common gray mosses (Li- 

 chens) that grow on rocks FIG. 106. CLADO NIA\ A COMMON LICHEN, 

 or tree trunks; Fig. 106. GROWING ON ROCKS 



The microscope shows that , At B is show , n th . e y un ?. mycelium, beginning 



to grow around a single cell of the green alga. 



this plant is a combination 



of a fungus and a green plant; Fig. 107. In this association 



the green plant carries on 

 photosynthesis, furnish- 

 ing starch for both itself 

 and the fungus; on the 

 other hand, the fungus 

 furnishes, for the benefit 

 of the green plant, a 

 lodging place and a con- 



siderable quantity 01 

 carbon dioxid and water, 



FlG. 107. A MAGNIFIED SECTION OF 



A LICHEN 



Showing that it is made up of a fungus, m, and which it Collects from 

 cells of a spherical, green plant, a. 



tion, ^therefore, seems to be one of mutual advantage. An- 

 other example is a group of bacteria which grows within 



