BIVALVE MOLLUSC A. 327 



thin, transparent, and white. The animal has a thick, white, 

 elongated, fleshy body ; its mouth opening anteriorly, throws out a 

 long tube traversed by two canals or siphons, through one of which 

 the water necessary for the respiration of the animal is absorbed, and 

 ejected through the other. Through another opening in the mantle 

 a very thick and short foot is protruded. 



In three ways also has this creature's method of boring been ac- 

 counted for the mechanical, the chemical, and the electrical; the first 

 being the one generally held. In this case it is supposed that the animal 

 uses its foot as a boring tool. The second presumes on the Pholas 

 secreting an acid which corrodes the rock ; the third that it possesses 



Fig. 131. Pholas crispata (Linnaeus). 



a galvanic battery with similar powers. It is not impossible but that 

 all these three theories may have a measure of truth. That the foot 

 of the borer is used is clear. The luminosity which is so characteristic 

 of the animal is in favour of an electric current, which is almost 

 always accompanied by chemical decomposition, which would set 

 free the hydrochloric acid of the sea water. The small size of the 

 entrance to the chambers of the Pholas is accounted for by the 

 increase of its size during its residence there. De Blainville thought 

 that a simple movement of the shell incessantly repeated would suffice 

 to pierce the stone, macerated by the sea water which passed through 

 the breathing apparatus. 



Mr. Robertson, of Brighton, exhibited the living Pholas in the act 

 of boring through masses of chalk, and thinks the process entirely 



