120 HOMES WITHOUT HAITDS. 



rowed into such substances, are dwarfed, abraded, and often mis- 

 shapen, and are valueless except to the physiologist. 



tVe naturally ask ourselves why the Pholas should bury itself 

 in the rocks instead of passing its life in the open sea, like the 

 generality of bivalves. The creature does not feed upon the sub- 

 stances in which it forms its curious tunnel, and to all appearance 

 would obtain as much food without as within, the burrow. One 

 obvious answer to this question is, that the creature buries itself 

 in the rock for the sake of safety, its. shell being, as has already 

 been remarked, of a soft and fragile texture. This opinion is far- 

 ther corroborated by the fact that one of the British species, the 

 Paper Pholas {Pholas papyracea), has a peculiarly thin and deli- 

 cate shell, so as to earn for it the name by which it is popularly 

 known. Yet, although this may be a reason, it is not the reason ; 

 for there are many well-known shells which are far more fragile 

 than those of the Pholas, yet which need no such protection, 

 and instead of concealing themselves in any way, roam the ocean 

 freely. 



In my own opinion, the burrowing instincts of this a!nd many 

 other marine creatures of similar habits are implanted in them for 

 other than mere individual purposes. Judging by the effects 

 which these animals must have produced upon the line of coast 

 throughout a succession of centuries, I can not but think that 

 they are, at all events partially, intended as instruments which 

 aid in producing those mighty changes that are continually tak- 

 ing place over the whole face of the globe. 



"We know that although the general proportions of sea and 

 land are maintained, a continual change is being worked in their 

 relative positions. Even within the memory of man, fields now 

 blooming with corn were once covered with the salt water, and 

 buildings that were once a mile from the shore are now in hourly 

 danger of falling into the sea. And I have very little doubt but 

 that the Pholas plays a very important part in these changes. If 

 the reader will examine any of our chalk-bound coasts, and will 

 walk from the foot of the existing cliff to the extreme of low- 

 water mark, he will see that the Pholas has every where left the 

 tokens of its industry. Even at the verge of low water, the spot 

 whereon he stands was once the base of the cliff which has now 

 receded so far from the waves, and continues to recede yearly. 

 And as he looks out to sea, and watches the breakers fling their 

 white foam over the sunken rocks, and notes the dark masses of 



