THE SHELL-FISH OF THE COAST. 23 
a 
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 Nassas 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 baclx 
of old Nassa obsoleta, and there spread out a crisp 
brown covering, much resembling dry nross, 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, °° 
OLYP COLONY 
localities. Indeed, our polyp is a near or Hypractr- 
ally of the common fresh-water hydra, “**S*"* 
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 
