MOLLUSCA 



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Japan some are said to attain a length of nearly a meter. The 

 scallops, whose single large adductor muscle is the only part 

 eaten, usually have numerous tentacles and variously colored 

 eves at the margins of the unfused mantle folds. They have a 

 small foot, often with a byssus gland, and can swim rapidly by 

 opening and closing the valves of the shell. 



Of the mussels, the pearl mussel or oyster is the most valuable 

 because of the pearls which its mantle secretes. The real pearl 



FlG. 256. Pholas, a boring mussel in a cavity which it has excavated in a rock. (From 



Brehm's Thierleben.) 



mussel, Meleagrina margaritifcra, is found in almost all tropical 

 waters ; in the Orient it attains a length of from twenty to thirty 

 centimeters, but in the West Indies and Gulf of Mexico it is 

 smaller. The finest pearls come from the Gulf of Persia, while 

 the most fishing is done off Ceylon. The divers go out in 

 February, March, and April to the beds, which are usually at a 

 depth of from eighteen to twenty meters ; they fill nets with the 

 oysters and then come to the surface, rarely remaining under 



