in association with sand and other constituents, forms solid rock.

A feature of another of the coral rock disintegrating agents is its
extreme weakness. It is a rotund mollusc with frail white valves, closely
fitting the cavity in which it lives. As it cannot revolve, the
excavation of the cavity is, possibly, effected by persistent but
necessarily extremely slight "play" of the valves; but the animal
appears to be quite content in its cramped cell with a tiny circular
aperture (generally so obscured as to be invisible), through which it
accepts the doles of the teeming, incessant sea.




CHAPTER XV



BARRIER REEF CRABS


"Reasoning, oft admire
How Nature, wise and frugal, could commit
Such dispositions with superfluous hand."

MILTON.

So much of the time of the Beachcomber is spent sweeping with hopeful
eyes the breadths of the empty sea, policing the uproarious beaches,
overhauling the hordes of roguish reefs, and the medley concealed in cosy