n6 



ZOOLOGY. 



more successfully the demands made upon them by their en- 

 vironment. Similar partnerships may be formed by animals 

 of different species. The simplest of these associations are 

 known as commensalism or " mess-mat eism," in which the 

 degree of dependence and mutual advantage is perhaps not 

 very great. As instances may be cited the occupancy of the 

 same burrows by the prairie dog and a species of owl; the 

 attachment of barnacles to whales and sharks ; the hundreds of 

 species of other Insects which live in the nests of ants; the 

 lodging of fishes and other animals in the body-cavity of some 

 of the large tropical sea-anemones or among the tentacles of 

 some of the Hydrozoa. Each member of the association can 

 live without the other, but for some reason they often occur 

 together. The way in which species of rats and mice follow 

 man and occupy his habitations perhaps may be considered 

 under this head. 



156. Symbiosis. Under this term are included even closer 

 relationships between members of different species, where there 



FIG. 55- 



FIG. 55. Hermit crab in the shell of a Gasteropod. After Morse. 



Questions on the figure. What structural adaptations has the hermit- 

 crab to this mode of life? What conceivable gain has such a habit? What 

 animals are cited as symbiotic with the hermit-crab? 



