228 



BIOLOGY 



Symbiosis 



Among both animals and plants, however, we not infre- 

 quently find different individuals associated and living in 

 mutual relations which may or may not be those of parasite 

 and host. The term symbiosis (Gr. sun = with + bios = life), 

 which may refer to either animals or plants (literally meaning 

 living together), is applied to a variety of relations where two 

 organisms live in close relation to each other, and is in con- 

 trast to free-living conditions where organisms live separately 

 from others. The purpose of symbiosis is not always the same. 

 Sometimes it is to the mutual advantage of both members; 

 sometimes it is to the advantage of one and the detriment of 

 the other, in which case it becomes parasitism. In accordance 

 with the relation of the two members of the group, symbiosis 

 may be divided into several types as follows: 



Helotism. In helotism (Gr. Helot = a slave) one organism 

 is enslaved by the other; neither is especially injured, but 



one does the work for the 

 other. Among animals the 

 best example of this is 

 among the slave-making 

 ants, where one species of 

 ants makes a slave of an- 

 other species, the slave do- 

 ing all the work for the 

 slave-maker. Both indi- 

 viduals carry on their life 

 in satisfactory fashion, and 

 neither is particularly in- 

 jured by the relation. 



Mutualism. When both 

 members of a group obtain 

 an advantage from associa- 

 tion, it is called mutualism. As examples of this, may be 

 mentioned the relation between domesticated animals, like 



sn 



FIG. 105. AN EXAMPLE OF 

 MUTUALISM 



A hermit crab cr, lives in the shell of the 

 snail, sn, and an anemone, an, fastens itself to 

 the outside of the shell. Both animals are bene- 

 fited. 



