THE CLAM AND OTHER BIVALVES 



173 



FIG. 89. Larva of Mussel. Much 

 enlarged. (After Balfour) 



1, shell; 2, adductor muscle; 3, lar- 

 val hook ; 4, byssus-thread 



(Fig. 89, 4) clings to the surface of the fish's gill or skin, and 

 the little hook (Fig. 89, 3) on the edge of either valve sinks 

 into the flesh. For several weeks the glochidium is trans- 

 ported on the fish, during which 

 time it may be carried into an- 

 other river, even by way of the 

 sea. When the end of fixed 

 development comes, the protect- 

 ing coat is dissolved and the 

 little mussel falls to the bottom 

 again. If this happens in a 

 favorable place, it burrows into 

 the mud and begins the life of 

 its adult kin. 



Economic Importance. The dis- 

 tribution of fresh-water mussels has become a matter of con- 

 siderable economic importance, especially in the states of 

 Iowa and Illinois. Factories have been established there for 

 the purpose of making pearl buttons from the valves. True 

 pearls of fine quality have been found in many species of 

 mussels in all parts of the Mississippi valley. 



The opinion has been held by many that if grains of sand 

 find lodgment between the mantle and the valve of a mussel 

 or an oyster, the mantle will surround the particle with 

 layers of pearl, like the substance normally secreted by the 

 species to line its valves, and thus form pearls. We have heard 

 stories of these valuable products being formed by pearl-oyster 

 fishermen, who inserted grains of sand between the valve 

 and the mantle of the famous pearl-oyster of the Arabian 

 Gulf, but we have no evidence that a grain of sand has ever 

 been found in a true pearl. Grains of sand only cause rough 

 spots in the layer of mother-of-pearl. 



The investigations of Professor Jameson, of South Africa, in 

 1902, afford the first conclusive evidence regarding the origin 



