i6o CCELENTERA. 



Bionomics. — The Ccelentera are almost all marine. In 

 fresh water we find the common Hydra, the minute Micro- 

 hydra without tentacles, the strange Polypodiuin, which in 

 early life is parasitic on sturgeons' eggs, the compound 

 Cordylophora, occurring in canals and in brackish water, and 

 the fresh water Medusoid {Limnocodium) found in a tank in 

 the Regent's Park Botanic Gardens, and another similar 

 form recently discovered in Africa. Most of the active 

 swimmers are pelagic, but there are also a few active 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., Bernhardiis prideatixii and Adanisia 

 palliaia. 



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. 



it 



