Page:Popular Science Monthly Volume 33.djvu/595

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THE

POPULAR SCIENCE

MONTHLY.


SEPTEMBER, 1888.


THE GROWTH OF JELLY-FISHES.

A CHAPTER IN THE NEW ZOÖLOGY.

By Professor W. K. BROOKS,

OF JOHNS HOPKINS UNIVERSITY.

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 unlike 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 unusual liveliness of the snails, he would find that each shell is inhabited 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 knowledge, 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 microscope, in a watch-glass filled with sea-water, he will find that it is