408 ON THE STRUCTURE AND HABITS 



are found introducing themselves into the substance of the hardest rocks, and 

 are called Rock-borers (Pkolades). 



Of the other borers the Stone-eater, the Common Cockle, and the Razor- shell 

 (Solen) are examples. The Common Cockle buries itself in the sand, and is 

 always found in this position on the margins of our rivers, waiting for the return 

 of the tide, when it opens its shell for the purpose of admitting the water, which 

 is charged with matters necessary to its subsistence. 



Amongst the Bivalve shells that creep about with the aid of a foot is the 

 Clamp-shell, which is sometimes four or five feet long, and weighs as much as 

 4001bs. From its weight some idea may be formed of the powerful nature of 

 the muscular power by .which this animal perambulates the mountains and vallies 

 at the bottom of the sea. 



The best-known and most useful of this group of animals is the Oyster ; and it 

 is the only one that is cultivated by Man. They are found at the bottoms of the 

 ocean and rivers. When cultivated they are placed in what are called " beds" in 

 rivers, where the salt-water has access only during the flow of the tide. 



But of all shell-fish that which perhaps has excited greatest attention is the one 

 that produces pearls. This is a kind of Muscle (Mytilus margaraliferus), and 

 the pearl is produced on the inside of its shell. The pearls are not a natural but a 

 diseased production, so to speak, for it is only after the shell of the Muscle has been 

 pierced by some external intruder, or when it has been irritated by some annoying 

 substance, that this costly jewel is produced. Sometimes some of the boring animals 

 of the preceding section are the cause of this ; in attempting to penetrate the shell 

 of the Pearl Muscle, that animal has the power of secreting from its mantle this 

 peculiar, hard and precious substance,* which thus excludes the entrance of the 

 borer, and saves the Muscle from destruction. Off the coasts of Ceylon, the 

 Persian Gulf, and the Sea of New Holland, this animal is found in abundance. 

 At Condatchy, in Ceylon, is the largest fishery, where, for thirty days in the 

 year, not less than 6,000 boatmen and attendants are engaged in the business. 



On looking at these shells we should hardly suppose that they were intended 

 to swim, much less to sail ; yet many of them have the power of elevating 

 themselves to the surface of the ocean, and there opening themselves ; one of their 

 shells serves for a boat, and the other is hoisted for a sail, and thus they glide 

 merrily over the waves of the sunny ocean. — This is said to be the case with the 

 Cockle and the Venus-shell. 



Most persons who know the Common Oyster will probably have observed on 



* On the question of the manner in which Pearl is formed, in reply to Mr. Lankester's 

 remarks, see Mr. Leviso.n's letter in The Naturalist, Vol. III., p. 378 — En. 



