PROLAS-STONE PIERCER. 201 



that the mollusks whose shells are perfectly 

 smooth, are capable of producing the same ex- 

 cavation as those which have a surface as rough 

 as a file. Many have supposed that these ani- 

 mals possess some liquid which acts as a solvent 

 upon the substance they enter. This appears 

 the more probable, as they are known to emit a 

 phosphorescent light. 



The entrance is the smallest part of the dwell- 

 ing of the Pholades, and hence it is evident that 

 they must have penetrated the rock when young 

 and small, and enlarged their perforations as 

 they themselves increased in dimensions. The 

 position of the hole is always oblique to the hori- 

 zon ; and its form is that of a truncated cone ter- 

 minated by a rounded cavity. This cavity re- 

 ceives the body, while the farthest end is oc- 

 cupied by the proboscis, which is continually 

 protruded to the orifice to procure the sea water, 

 upon which it subsists. The proboscis is long, 

 pliable, and fleshy, terminated by a corneous 

 substance, which is dentated like a saw at its ex- 

 tremity. There is an opening between the valves, 

 through which it is projected, and a case into 

 which it fits. At the approach of danger, these 

 mollusks use this instrument to eject water, often 

 to a considerable height, and its dentated margin 

 leads us to suppose, that they also employ it in 

 the work of perforation. 



