THE 



POPULAR SCIEITCE 

 MONTHLY. 



SEFTEMBEE, 1888. 



THE GROWTH OF JELLY-FISHES. 



A CHAPTER IN THE NEW ZOOLOGY. 



By Pkofessob W. K. BEOOKS, 

 of johns hopkins ukivessity. 



I. 



ON any landlocked and sheltered sea-beach, where the waves 

 ripple up on to the sand without breaking, hundreds of small 

 spiral sea-shells may usually be found in the shallows dancing up 

 and down the sand at the water's edge, following the crest of each 

 little wave as it flows up and spreads out over the beach, and 

 turning to run back with it as it falls ; keeping always just within 

 the water, and exhibiting restless activity and agility, quite un- 

 like the sluggish habit of the snails which normally inhabit the 

 shells. 



If the loiterer by the waves should be inquisitive enough to be 

 attracted by them, and should search for the meaning of the un- 

 usual liveliness of the snails, he would find that each shell is in- 

 habited by a hermit-crab, that, after devouring the true owner of 

 the house, has thrust his own body into it, and carries it about, as 

 a defense against his many enemies, among whom his pugnacious 

 and cannibal brothers and sisters are perhaps the worst. 



So much the most superficial observer may discover for him- 

 self ; but if, with a naturalist's sharp sight and thirst for knowl- 

 edge, he examine more closely, he will find that about one in a 

 dozen of the shells is coated, upon the surface which is uppermost 

 as the crab carries it, by a white crust of a mossy substance which 

 is not found upon the empty shells which lie on the bottom, 

 nor upon the shells of living snails. If, impressed by this odd 

 fact, he detach a little of the moss and examine it under a micro- 

 scope, in a watch-glass filled with sea-water, he will find that it is 



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