256 THE MICROSCOPE. 



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

 attach one of their yalves 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 Vibrio 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 glohator, but of so 

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

 rendered permanently visible. 



The Pea/rl 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 110,000Z. ; 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 thirtk 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 

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



