BIVALVE MOLLUSCA, ; 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 
a i / 
Fig. 131.—Pholas crispata (Linnzeus). 
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 
