138 ELEMENTARY ZOOLOGY. 



this instance there is believed to be a direct gain to both 

 species by the association. Similar associations of crabs 

 and sea anemones, crabs and mussels, are cited. This 

 relation is called symbiosis. 



FIG. 28. Argynnis cybele on thistle. Natural size. Photo by Folsom. 



Questions on the Figure. For what purpose does the butter-fly 

 visit the thistle? What special adaptations does the butter-fly 

 possess for this mode of life? What is the gain to the thistle from 

 the visits? 



It is perhaps more common between plants and animals. 

 In the case of the "ant-loving" plants it is believed that 

 the ant protects the plant, in some instances at least, 

 from other insects and that the plants secrete materials 

 or furnish homes for the ants. Certain ccelenterates 

 have unicellular algae imbedded in their own cells. These 



