PHOLADID^E. 99 



minute holes in the rock for the reception of its progeny? 

 It does not appear that he has ever witnessed so extra- 

 ordinary a proceeding ; and even if it is offered as a pro- 

 bable explanation of the method by which the fry effects 

 its entrance into the stone, this would only apply to the 

 case of chalk or limestone, which being calcareous can be 

 dissolved by an acid, and not to that of gneiss, sandstone, 

 peat, or wood, which are not liable to be thus acted upon, 

 and in which the Pholades more frequently take up their 

 abode. If the foot is to be considered merely as a point 

 d'appui, the motive power would be altogether wanting 

 while the young Pholas, encased in its tender shell, 

 remains outside the much harder material which it has 

 to penetrate. The same remark holds good with regard 

 to other mollusca which excavate stone and wood. I 

 am more than ever convinced that the modus operandi 

 is similar in all these cases, and that the laws of Nature 

 are more simple and uniform than those which direct 

 human actions ; nor do I infer from the case now before 

 us, 



" That many things, having full reference 

 To one consent, may work contrariously." 



When the Pholas has to make its habitation in clay 

 or sand, instead of in stone, no great amount of force 

 seems requisite. Reaumur in 1712 stated, from his own 

 observation, that P. Candida uses its lozenge-shaped and 

 comparatively large foot for this purpose. He took 

 several of them out of their holes, and placed them on 

 a clay as soft as mud ; each soon put out its foot, and 

 in a few hours made a fresh hole deep enough to contain 

 the Pholas, which met with so little resistance and was 

 evidently anxious to conceal and shelter itself without 

 delay. Is it likely that the Pholas uses its foot or shell 

 according to the nature of the material which it seeks 



