246 FUNCTIONS AND INSTINCTS. 



very fragile shells strengthened indeed by sup- 

 plementary pieces, and rough like a file, inhabited 

 by a very soft animal which appears to be fur- 

 nished with no organs adapted to boring so hard 

 a substance as a rock. When the young are 

 disclosed from the egg, being cast upon the rock 

 in which their mother resides, they bore a hole 

 in it which they enlarge daily, and which they 

 never leave, unless compelled by force. This 

 hole always communicates with the water, and is 

 the orifice through which the animal exerts its 

 double siphons ; one of these siphons is its mouth 

 and the other its anal orifice. Reaumur made 

 some observations upon their mode of boring, he 

 says, that it is by the rotation of the two valves 

 of their shell which form a rasp, and continually 

 wear away the rock which surrounds them. 

 The surface of the valves of the shell is ridged 

 longitudinally and transversely, and rough with 

 asperities at the intersections of the ridges which 

 seems to fit it for such an office, but still it is 

 usually so tender and friable, that one would not 

 expect it could act upon a rock, nor could it be 

 by this agency that they first make an entry 

 when young, or bore through shells, madrepores, 

 and wood as they are said to do. They are 

 stated principally to select calcareous rocks and 

 sometimes hardened clay, which seem better 

 adapted to the nature of their shells. Poli says 

 they use their foot as an auger in excavating 



