MOLLUSCA. 533 



The Oyster is the type of the tribe Ostracea, all of 

 which are Acephalus, that is, animals without a distinct 

 head. The gills, or breathing apparatus, form what is 

 commonly called the beard of the oyster. The creature 

 is attached by strong muscles to its shell. The mouth of 

 the oyster is a mere opening in the body, without jaws or 

 teeth ; its food consists of nourishing substances suspended 

 in the water, and which are 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 mu- 

 cilaginous liquid, which soon becomes as hard as the shell 

 itself. They spawn some time 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 ; in three months it is larger than a shil- 

 ling. The spawn is a very interesting object for micro- 

 scopic examination, especially with polarised light. The 

 young fry is represented in fig. 254 ; some with cilia pro- 

 truded. 



In the stomach of the Oyster, and in the alimentary 

 canal, myriads of living Paramcecium and other Infusoria 

 are found swimming in great activity ] swarms of a con- 

 glomerate and ciliated li.ving organism, somewhat resem- 

 bling the Volvox globator, and so extremely dehcate in 

 their structure that they require a good objective to define 

 them. 



Pearls are usually met with in the Meleagrina Marga- 

 ritifera, " Pearl Oyster," which, however, does not belong 

 to the family Ostracea. They are likewise found in the 

 Mussel known as Mya Margaritifera, and an inferior kind 

 in many Mussels of the rivers of Great Britain ; and, at 

 one time, the pearl-fishery of Ireland was justly cele- 

 brated. ^Naturalists somewhat difi'er in their opinions as 

 to the mode in which pearls are formed. Some think that 

 they are produced by particles of sand getting into the 

 stomach ; the animal, to prevent the roughness of these 

 particles from injuring its delicate structure, covers them 

 over with a secretion from a gland, and, by continual ad- 

 ditions, they gradually increase in size. Mussels, in which 

 artificial pearls were said to have been formed by tho 

 Chinese, have frequently found their way to this country. 



