BELOW THE TIDEMARK 269 



frailer and hardly larger bore their way into many kinds of 

 rock. Lumps of chalk and limestone on the shore are often 

 seen pitted with numerous holes into which one can push the 

 little finger ; and sometimes the gallery is still occupied by 

 the borer a frail though rough white shell, like a large 

 almond. This is the shell of the pholas, and it is frailer than 

 most other marine shells because it is nested in these safe 

 burrows in the stones. The secret by which the pholas 

 melts the rocks is the same as the legendary method by 

 which Hannibal split the crags of the Alps, except that it 

 uses an acid secreted by itself instead of vinegar. It melts 

 its way into the calcareous rock by dissolving it, just as the 

 rivers of limestone districts fret out a subterranean path by 

 means of the carbonic acid from the atmosphere. The most 

 famous example of the work of these rock-boring shells is in 

 the columns of the Temple of Serapis on the shore of the 

 Bay of Naples, which sank till the shell-fish made their 

 homes in the marble, and now stands dry again. The borers 

 in this case were date-shells, which are near relatives of the 

 common mussel. 



Shells occupying exposed positions need to anchor them- 

 selves tightly to their hold. Mussels, which abound on 

 almost every rock uncovered at low water, are bound to their 

 place with strands of byssus spun by the animal. Limpets 

 and barnacles are living synonyms of adhesiveness, and 

 there is a magnificent simplicity about the limpet's method of 

 attachment. Unlike the whorled sea-snails and the tightly 

 closed bivalves, the limpet spreads its largest fleshy area on 

 the rock, and sits under a conical shell like a bell-tent. 

 The instant it is alarmed it draws the edge of the shell to 

 the rock with the strong muscular band round its circum- 

 ference, and is protected by its hard roof and by the forces 

 of suction. Even if we were to bore a hole in the top of 



