360 Natural History of Molluscous A?iimal$ • — 



same rock, I 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 conceive a 

 fact, short of absolute demonstration, which could give a more 

 decisive support to the theory of a solvent. A peculiar pro- 

 vision is given to the animal to preserve it from destruction 

 by an injury to which it is particularly exposed. The sup- 

 position 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 strong, perhaps conclusive, yet 

 has a solvent never been detected; and every experiment 

 which Mr. Osier has made for this purpose has been quite 

 unsuccessful. Had the question been previously balanced, 

 our 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 a strong and con- 

 sistent support to the theory of a solvent, and are opposed 

 as decidedly to the supposition of penetration by a mechanical 

 force, the failure of the experiment cannot be considered to 

 militate very strongly against the only inference to be drawn 

 from the facts. And it may be observed, that, where the 

 Lithophaga happen to be lodged in situations which affi^rd 

 them sufficient room and shelter, they make no attempt to 

 enlarge their habitation. Thus Saxicava praecisa * {/ig, 64<. b) 

 is more frequently found among groups of Serpulae, or in the 

 roots of sea-weed, than in a hole excavated by its own effiDrts ; 

 and Mr. Osier has obtained full grown specimens of Hiatella 

 arctica]* {Jig.64'.c) attached by the byssusto aPecten. It may 

 therefore be presumed that the solvent is secreted only when 

 its agency is required ; and this would sufficiently explain why 

 a free acid cannot be detected in the animal by any chemical 

 tests. 



The boring species of Mollusca are few in number when 

 compared with the hosts which burrow in sand, mud, or 

 gravel ; but, as the resistance they have to overcome does not 

 seem disproportioned to their powers, less interest is taken 

 in their proceedings. The <Solenes and Tellinida^ prefer 



* These are merely varieties of Saxicava rugosa ; and they are common 

 on most of our shores. 



