DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 



I2S 



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 to- 

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

 man and occupy his habitations perhaps may be considered 

 as illustrating a similar condition. 



1 60. Symbiosis. — Under this term are included even closer 

 relationships between members of different species, where there 



Fig. 57. 



Fig. s7. 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? 



seems to be a distinct advantage accruing to both members of 

 the partnership sufficient to account for it. The relation 

 of the ants to the aphids or plant-lice which they capture may 

 be so described. The aphids, although captives, are nourished, 

 often at great expense of labor to the ant, on the food which 

 they most prefer, and in return the ants use the sweet secretions 

 of their bodies as food. Symbiosis is probably more common 



