1 68 PHYLUM CCELENTERA. 



forms in deep water. Many polypes anchor upon the shells 

 of other animals, which they sometimes mask, and there 

 are most interesting constant partnerships between hermit- 

 crabs and sea-anemones, e.g. between Pagurus prideanxii 

 and Adamsia palliata. 



The hermit-crab is masked by the sea-anemone, and may 

 be protected by its stinging powers ; the sea-anemone is 

 carried about by the hermit-crab, and may get crumbs from 

 its abundantly supplied table. This illustrates a mutually 

 beneficial partnership or commensalism, which, however, in 

 some other animals may degenerate into parasitism (see 

 Fig. 81). 



Another kind of partnership is illustrated by many sea- 

 anemones and Alcyonarians. Minute unicellular Alg?e 

 (Zoochlorellae) live within the cells of the animals in close 

 physiological partnership with them (symbiosis). 



