THE SHELL-FISH OF THE COAST. 23 



coast are inhabited by hermit crabs, which in many- 

 cases, possibly, obtained possession through the right 

 of force of arms. Be this as it may, it is certain 

 that the hermit has been providentially provided 

 for, and that it has much to offer by way of grati- 

 tude to the dog-whelks. The Nassas are probably 

 all very tenacious of life; a specimen of Nassa obso- 

 leta submitted to me from Atlantic City survived 

 for a full year the dry atmosphere of a closet with 

 an artificially-heated wall. 



The N'assas are not the only inhabitants of the 

 tide-pools either, for with them we find associated 

 one or more forms of periwinkles, various small 

 fishes, and the ever- active hermit. Other organ- 

 isms, whose existence would scarcely have been 

 thought of, also lurk here. One of these is the 

 founder of a colony which has settled on the back 

 of old Nassa ohsoleta, and there spread out a crisp 

 brown covering, much resembling dry moss, which 

 might be readily mistaken for the horny covering 

 (epidermis) which belongs to most 

 shells. The colony is one of polyps, 

 next of kin to the Sertularia or sea-fir, 

 whose delicate bunchy masses lie scat- 

 tered over the beach, or gently oscil- ^ 

 late in the calmer waters of favored 

 localities. Indeed, our polyp is a near of hydbacti- 

 ally of the common fresh-water hydra, "'^ ^''hinata. 

 and, what may at first sight appear incredible, also 

 of the free-swimming Medusa or jelly-fish. Under 

 a magnifier the brown covering is seen to rise up 

 into simple and compound spines, from between 



Polyp colony 



