i8o 



Dwellers of the Sea and Shore 



Hydractinia. They form a soft mosslike covering, 

 usually of a whitish or pinkish color — the lighter-hued 

 colonies being male, and the darker-hued, female — and 

 are very seldom found growing elsewhere than on the 

 shells of hermit crabs. Here, then, we find that re- 



hydractinia; a colony of tubularian iiydroids. (photograph made of the 

 living animals growing on a shell occupied by a hermit crab; 

 greatly enlarged.) 



markable instance known as commensalism, a term 

 which, freely defined, means "dining at the same table." 

 But in this case, as indeed is the case with most true 

 commensal animals, they do more than merely dine 

 together; they really render each other a service. That 

 is to say, this sort of association is formed for mutual 

 benefit. The tubularians, beside the concealment they 



