156 THE SAXICAV^E THEIR HABITS. 



this fact, and Mr. Osier has adduced several others which 

 fell under his own notice. 



To answer the objection that this solvent of limestone 

 must act destructively on the shell itself, which is -of the 

 same composition, and certainly not more insoluble than the 

 rock, Mr. Osier supposes that the animal has the power of 

 applying its solvent to a limited space external to the shell, 

 where it is quickly neutralized and rendered harmless. The 

 instrument of the application he believes to be the foot, an 

 organ which admits of being extended to a length fully equal 

 to that of the shell, and appears to be perforated by a tube, 

 which passes forward from the part where it joins the body 

 and terminates abruptly on the under surface near its extre- 

 mity. By this instrument the liquid can be applied remote 

 from the shell, which is thus removed from its destructive 

 influence. Where the Saxicavae are numerous, their holes 

 communicate very freely ; and it is common to meet with one 

 which has attached its byssus to another. In this case, it is 

 always found that the shell of the second has been acted on 

 in a direction, and to an extent, which corresponds with the 

 range of the foot of the assailant. The neighbouring shells 

 are very often thus corroded. " On examining a considerable 

 number," says Mr. Osier, " taken indiscriminately from the 

 same rock, 1 have found that the shells of more than half had 

 been thus injured. As long as the injury is superficial, no 

 attempt is made to repair it ; but, when the shell is nearly or 

 quite penetrated, the breach becomes filled, not with new 

 shell, but with a firm yellow substance, which is insoluble 

 even in a strong mineral acid. It would be difficult to con- 

 ceive a fact, short of absolute demonstration, which could 

 give a more decisive support to the theory of a solvent. A 

 peculiar provision is given to the animal to preserve it from 

 destruction by an injury to which it is particularly exposed. 

 The supposition of mechanical penetration would require us 

 to believe that a newly formed substance, much softer than 

 that which has been destroyed, can stop the progress of the 

 mischief, and even repair it, under the continued application 

 of the original destructive force." 



These arguments seem good, yet a solvent has never been 

 detected ; and every experiment which Mr. Osier made for 

 this purpose was quite unsuccessful. Had the question been 

 previously balanced, the inability to detect a solvent would 

 justify strong doubts of its existence : but, while all the 

 facts connected with the natural history of the Lithophaga 

 afford consistent support to the theory of a solvent, and are 

 opposed to the supposition of penetration by a mechanical 



