358 Natural History of Molluscous Animals : — 



particles washed out of it will be deposited before they reach 

 the mouth of the hole, the passage along which the Pholas 

 projects its siphon is constantly found to be lined with a soft 

 mud. 



The Teredo, which, as a British animal, is probably now 

 quite extinct, is also a mechanical borer ; and it does its work 

 much in the same manner, and by means of a structure very 

 analogous to that of the Pholas. The muscles indeed vary 

 in their relative size, because their size is proportioned to the 

 force they are required to exert, which differs in the two 

 genera ; but their arrangement and mode of action are so 

 similar that it is unnecessary to enter into the detail. The 

 Teredines, it would seem, however, do not eject as useless all 

 the debris worn down in their operations, but turn part of it 

 at least to their nourishment, for Mr. Hatchett found the 

 contents of the intestine to be " vegetable sawdust." It is 

 also worth remarking, that they bore across the grain of the 

 wood as seldom as possible ; for, after they have penetrated a 

 little way, they turn and continue with the grain tolerably 

 straight until they meet with another shell, or perhaps a knot, 

 which produces a flexure, the course and size of which depend 

 on the nature of the obstruction, and which, if considerable, 

 causes the individual to take a short turn back in form of a 

 siphon, rather than work any distance across the grain.* 



But the Lithophaga and Lithodomi {fg. 61, a) have no 

 structure for boring such as I have described in the Pholas, 

 and yet it were reasonable to suppose for them a structure 

 stronger and more fully developed for the purpose, did they 

 really operate mechanically, seeing that the substances they 

 dig into are harder than those selected by the Pholas or 

 Teredo. This anatomical argument might be deemed suffi- 

 cient of itself to prove that the Lithophaga must work by the 

 agency of other means. Moreover, the texture of the shell 



is so soft, that it could make no 

 impression upon the stone without 

 being itself acted on ; and the effect 

 of this would be permanent, because 

 superficial injuries of the shell are 

 never repaired. But nothing of 

 this kind is met with. Mr. Osier 

 has even found a Saxicava rugosa (fg, 64. a), the species on 

 which his observations were made, fixed between two others, 

 which was so compressed that it was quite flat, and little more 

 than a third of its proper thickness ; yet neither of the three 



* Montagu, Test. Brit., p. 529. 



