ADAPTATION 



347 



the polyps are removed from the shell, the crab makes an effort 

 to replace them, showing that the association between the crab 

 and the polyp is by no means an accidental one. This tendency 

 for disguisement is shown by other species of crabs. The spider 

 crab, Libinia emarginata, is often provided with a covering of 

 sea weed, sponges, hydroids, etc. Kept in aquaria, spider crabs 

 have been observed to tear off bits of sea weed with their claws 

 and fasten them on their backs among the stiff hairs which 





Fig. 197. — Hermit crab and sea anemones. Usually the gasteropod shell 

 inhabited by the crab is completely covered by sea anemones, but only two of 

 the latter are represented for the sake of clearness. 



occur there. The transplanted plants and animals often grow 

 and provide an effective covering. 



An example of a closer symbiotic relationship is found in the 

 green hydra, Chlorohydra viridissima, which owes its green color 

 to the presence of microscopic green plants embedded among its 

 cells. The plants utilize the carbon dioxide given off by the 

 animal cells and in return yield oxygen which is readily absorbed 

 by the cells. The same sort of relations is found in some Protozoa 

 that contain numbers of unicellular plants. 



A rare symbiotic relationship between a mammal and algae is 

 illustrated by conditions found in the hair of the two-toed sloth, 

 Choloepus didactylus, a South American form. This animal is 

 largely arboreal in its habits, moving about in the trees, sus- 

 pended in an upside-down position from the branches by its 



