3378 



THE MOLLUSKS OR SHELLFISH 



ft'. 



are sometimes long and slender. In the pearl oysters, Meleagrina, there are, how- 

 ever, no wings, and in the case of M. margaritifera the shells become very thick 

 and heavy. This species is one of the principal pearl producers, and is largely 

 collected by divers off the north coasts of Australia and other places, not only for 

 the pearls which they may contain, but also for the shells themselves, which 

 are valuable as mother-of-pearl. Until recently, the fishery was carried on by na- 

 tive divers, but now the diving dress is largely employed. The Ceylon pearl oyster, 

 M. fucata, is much smaller than the Australian species. This fishery has been 

 carried on for over two thousand years, and the accumulation of shells is so enor- 

 mous as to extend for miles several feet deep. The shells are thin and of little use 

 as mother-of-pearl, consequently they are thrown away, after being- examined for 

 pearls. Pearl fishing is also carried on in the Persian Gulf, the South Sea is- 

 lands, Panama, West Indies, and a few other localities. The spherical pearls, 

 like the shell itself, are produced by the mantle, and probably other parts of the 

 animal also. They consist of layers of pearl, deposited round some foreign sub- 

 stance which has intruded itself within the shell. This may be a grain of sand, but 

 is said to be usually an egg of the mollusk itself, which has not properly developed. 



The hammer oyster 

 (Malleus}, Vulsella, 

 Crenatula, and Melina 

 are other interesting 

 existing forms of Avi- 

 culidce, and many ex- 

 tinct genera have been 

 referred to this family. 

 The shells of the 

 oysters (Ostreidce} are 



c so familiar that no 



description is neces- 

 sary. The animal has 

 no foot, and the mantle 

 lobes are free nearly 

 all round, the borders 

 being fnnged with 

 short papillae. The 

 shells, excepting in the 

 very early stages, are 

 closed by a single ad- 

 ductor. The sexes are 

 separate in the Ameri- 

 can oyster (Ostrea vir- 

 giniana], but united in 

 the British O. edulis. 

 In a gastronomic point 

 of view the oyster 



ONE VALVE AND SOFT PARTS OF COMMON OYSTER. 

 (Natural size.) 



a. Position of mouth ; b. Mantle ; c. Adductor ; d. Junction of mantle lobes ; 



e. Gills. 



