A LAND SNAIL AND OTHER MOLLUSKS 167 



Pearls. — Pearls are interesting and valuable products of 

 bivalves. The most famous pearl fisheries are those of Ceylon. 

 The pearl oysters (really mussels) are taken by the thousand 

 and allowed to decay. Their shells are then washed out and 

 thrown away and any pearls that may be present are picked out 

 of the slimy debris. Pearls are built up around some foreign 

 substance within the shell such as a grain of sand or more prob- 

 ably around the remains of a parasitic worm. The mantle of 

 the mussel secretes the pearl substance in layers, just as the 

 shell is formed. Only a small proportion of the pearls formed by 

 mussels are ever taken, since many of them drop out of the shell 

 and are lost in the bottom and others disappear when the 

 mussels die. 



Characteristics and Classification of Mollusks. — Mollusks 

 are soft-bodied animals usually protected by a shell of calcium 

 carbonate. They are unsegmented. The locomotor organ is 

 in most of them a muscular foot. The main part of the body 

 lies in a cavity, the mantle cavity, and is covered by an envelope, 

 the mantle. Three of the five classes contain common and well- 

 known species. 



Class 1. Gastropoda. — Snails, Slugs, and Nudibranchs. 



Class 2. Pelecypoda. — Bivalves, such as Clams, Mussels, 

 Oysters, and Scallops. 



Class 3. Cephalopoda. — Squids, Cuttlefishes, Octopods, 

 and Nautili. 



REFERENCES 



College Zoology, by R. W. Hegner. — The MacmiUan Co., N. Y. City. 

 See references to Chapter XVII. 



