200 SHELLS AND MOLLUSCOUS ANIMALS. 



wood was hard and firm, and whose constituent 

 fibres adhered so closely can have been per- 

 forated thus. Other naturalists believe that 

 some chemical solvent is possessed by the animal, 

 which exudes from it, and causes the cavity by 

 dissolving the rock. But this idea has its diffi- 

 culties too ; and we ask how the solvent which 

 destroys carbonate of lime, should leave the shell 

 uninjured, and how the same chemical substance 

 which dissolves limestone should act in a similar 

 manner on wood and clay. Each solution of the 

 mystery has its difficulties, and it remains yet an 

 unsettled question, by what means these dwellers 

 in the rock effect their purpose. 



The Pholades emit a most remarkable light. 

 This secretion not only shines during darkness, 

 but illuminates whatever it touches or happens to 

 fall on. " There is," says Dr. Priestley, "a remark- 

 able shell-fish, called Pholes, which forms for itself 

 holes in different kinds of stone. This fish illumi- 

 nates the mouth of the person who eats it, and it 

 is remarked, that contrary to the nature of other 

 fish, which give light when they tend to putres- 

 cence, this is more luminous the fresher it is, and 

 when dried, its light will revive on being moistened 

 either with salt-water or fresh ; brandy, however, 

 immediately extinguishes it." 



Whether this light is, or is not phosphorescent, 

 yet that this secretion does not effect the cavity 

 seems apparent, because, as it is always present, 

 it would be perpetually decomposing the rock ; 

 whereas the size of the cavity is increased at 

 the will of the animal, and is only made so large 

 as to enable it to move round in it. Our figure 

 represents a group of the common stone-piercer, or 



