SHELLS AXD MOLLUSCOUS ANIMALS. 199 



them. The apertures which lie exposed to the 

 surface are often empty, or contain shells in 

 which the fish are dead ; but break away a crag 

 from these pierced rocks, and we shall find numbers 

 of living shell-fish shut up within. These are the 

 different species of Pholas, well-named from the 

 Greek vrord. pholeo, "to hide," for they live and 

 die buried in their homes in the rock, save when 

 some naturalist brings them out for observation, 

 or the fisher goes and collects them for bait. It 

 is wonderful to look at their frail delicate shells, 

 and see how they can make their way into rocks, 

 hard clay, limestone and wood, enlarging their 

 cavity as they increase in size, and clustering in 

 multitudes in the wood-work of piers, sapping the 

 foundations of jetties, boring into the hulls of 

 ships, and, if their ravages are unperceived, 

 causing the shipwreck of the mariner, and the 

 destruction of the works of man's art. How 

 creatures apparently so helpless, and with such 

 fragile shells, should accomplish this, is yet a 

 mystery. Hidden as they are, within their rocks, 

 our means of observation on their habits are very 

 limited. Some naturalists think that they pierce the 

 hole by the rotatory movements of the shell within 

 the cavity, yet one would think that a slender 

 shell would rather break than force its way into a 

 hard rock. Others, familiar with the action of 

 minute cilia in causing currents in the water, have 

 thought that the Pholas, by producing these cur- 

 rents, wore away the stone. Yet this force, power- 

 ful as it is in entangling animalcules, is not great 

 when directed against a firm object, and one can 

 hardly imagine that the forest tree, lying on the 

 shore, and filled with these piercers a tree whose 



