256 THE MICKOSCOPE. 



drawn into the shell when it is open by means of cilia. Oysters 

 attach one of their valves to rocky ground, or some fixed substance, by 

 a mucilaginous liquid, which soon becomes as hard as the shell. They 

 generally spawn in May ; and their growth is so rapid, that in three 

 days after the deposition of the spawn, the shell of the young oyster is 

 nearly a quarter of an inch broad ; and in three months it is larger 

 than a shilling. The spawn is a very interesting object for micro- 

 scopic examination, especially when viewed under polarised light. The 

 young fry is shown in fig. 121, nearly ready to escape from the shell. 



The well-known ciliary currents in the fringes of the oyster in- 

 duced me to examine the contents of the stomach, under the expecta- 

 tion of finding some minute forms of Infusoria ; for it seemed but 

 reasonable to infer that the absence of locomotive power, and the con- 

 sequent inability of seeking for food, might be compensated by so 

 beautiful a contrivance for ensuring constant nourishment. My expec- 

 tations were fulfilled and surpassed. In the stomach of every oyster 

 I examined, and in the alimentary canal, I found myriads of living 

 Monads, the Yibrio also in great abundance and activity, and swarms 

 of a conglomerate and ciliated living organism, which may be 'named 

 Volvox ostrearius, somewhat resembling the Volvox globator, but of so 

 extremely delicate a structure, that it must be slightly charred to be 

 rendered permanently visible. 



The Pearl Oyster. At one time a most extravagant value was set 

 upon pearls : for one of these molluscous secretions, it is recorded that 

 Tavernier paid the sum of 110,000?. ; it was found at the Catifa fishery, 

 off the coast of Arabia. At the Bahrein Islands, Persian Gulf, the 

 produce of the two months' fishing is said to average 90,000?. 



Pearls are usually found in the Meleagrina Margaritifera, or Pearl 

 Oyster; also in a mussel termed Mya Margaritifera. An inferior 

 kind of pearl is also found in many mussels of the rivers of Great 

 Britain ; and, at one time, the pearl-fishery of Ireland was greatly 

 celebrated. The oysters on our coasts have frequently a dull, common 

 kind of pearl within their shells. 



Naturalists somewhat differ in their opinions as to the mode in 

 which pearls are formed. Some think that they are caused by par- 

 ticles of sand having got into the stomach ; the animal, to prevent the 

 roughness of these particles from injuring its delicate structure, covers 

 them over with a secretion from its body, and by continual additions, 

 they are gradually increased in size. It is now, however, pretty gene- 

 rally admitted to be a disease ; and it is quite certain that the pearls 

 are matured on a nucleus, consisting of the same matter as that from 



