io8 The Insides and the Wor\ings of Living Things 



to these animals would suspect a " relationship " on the basis 

 of the similarity between the digging feet of the two forms. 

 2. Related species, coming to live under different con- 

 ditions, adapt themselves accordingly and so acquire diver- 



FiG. 33. Divergent Adaptation 



In the early stages of development all crabs look pretty much alike. The 

 crab Carcimis is shown lying on its back, with a parasite attached to the abdo- 

 men. This parasite is shown separately, above to the right, hardly more than a 

 sac with branching suckers through which it absorbs its nutrition from the 

 juices of the host. It is only by a study of the early stages of this parasite that 

 we recognize it also to be a crab, Sacciilina. 



gent forms and structures. A striking example is furnished 

 by the parasitic crustacean, Sacculina, and its host the crab 

 Carcinus (Fig. 33). The differences between the ordinary 



