74 THE UNIVERSE. 



another problem remains to be solved, and that is to know 

 how these animals execute a task which seems so much be- 

 yond their powers. 



Some naturalists have fancied that the Pholades are only 

 a kind of living files, mechanically boring their habitations 

 by rasping the rock with the aid of the sharp points on 

 their shells. But this opinion is quite untenable, for before 

 they could pierce the hard stone these delicate projections 

 themselves would be completely worn away. 1 



Other naturalists think these molluscs make use of some 

 chemical process, and hollow out their abodes by distilling 

 an acid which acts upon the stone. This theory is not more 

 admissible than the other, for it is certain that, the calca- 

 reous outer skeleton of the animal being of a composition 

 analogous to that of the rock, it would itself be the first 

 victim of the corroding agent, and would be dissolved long 

 before the hole was formed. 



It is, however, clear that among the Pholades living in 



28. Stone-Eating Modiolus : Modiolus lithophayus. From nature. 



the calcareous rocks of our shores the strongly made foot 

 undertakes the task in question. By its ceaseless move- 

 ments this fleshy sole, little by little, wears away the rock 



1 As to the pholads of our shores, I have proved the fact by demonstrating 

 that all the interior of the hole, on a level with the shell, is covered with a layer 

 of mud, which would prevent the action of the sharp points upon the irregulari- 

 ties of the stone. 



