THE PHOLAS AT WORK. 253 



How much more shrewd and philosophical are the 

 opinions of such a man as Professor Owen, who, 

 when speaking of the mechanical action of the valves 

 of P. crispata, says, ' To deny this use of the Pholas 

 shell, because the shell of some other rock-boring 

 bivalves is smooth, is another sign of a narrow mind.' 

 Again, this learned author forcibly remarks, in direct 

 opposition to the writer previously quoted, 'The 

 diversity of the organization of the boring molluscs 

 plainly speaks against any one single and uniform 

 boring agent at all!' 



The more I study this subject, the more does the 

 truth of the last-mentioned statement become appa- 

 rent to my mind. 



An examination of engravings of the shells, or 

 even of the Pholas itself, when lying loose in the 

 tank, or quietly seated in the rock, extending and 

 retracting its siphons, fails to give one the slightest 

 idea of its extraordinary appearance when enlarging 

 its dwelling. At such times it seems to be a totally 

 different animal, and to have suddenly acquired a 

 most marvellous degree of power, energy, and per- 

 severance, forming a striking contrast to its usual 

 quiet, passive habits. 



In the first place, as I have elsewhere written, it 

 retracts its tube to, and even under, the level of its 

 shell, just as a man, about to urge onwards some 

 heavy mass with his shoulders, would depress his 

 head to increase and concentrate his muscular power. 



