^6 ZOOLOGY DIEECTIONS 



the mantle, gills, visceral mass, intestine, foot, rectum and its 

 typhlosole, and ureter. Draw and label all the parts. 



2. Make a similar section through the region of the pos- 

 terior adductor. Determine the relation of the suprabranchial 

 chamber. Draw. 



IV. Economic Importance and Reproduction 

 Freshwater mussels are of especial importance because of 

 the use of many varieties in the manufacture of pearl buttons 

 and novelties. In many localities in North America, the species 

 of most value in the industries have become practically extermin- 

 ated as a consequence of continued fishing operations. 



Practically all of the freshwater mussels (excepting the 

 minute finger-nail shells) undergo a complicated development. 

 The fertilized eggs are retained by the parents in a modified 

 portion of the gills termed the marsupium. There they un- 

 dergo development resulting in the formation of a larval stage 

 known as a glochidium. This larva is unable to develop inde- 

 pendently upon leaving the parent. Completion of the life 

 cycle is possible only for the individuals which become at- 

 tached to the fins or gills of a suitable fish. Not all fish may 

 serve in this capacity. At the close of the parasitic stage the 

 young mussel drops off from its host and undergoes its final 

 growth to adult condition as a free animal at the bottom of the 

 stream or lake. 



REFERENCES 



Brooks, W. K., 1882. Handbook of Invertebrate Zoology. 

 S. E. Cassino, Boston. 



Coker, Robert E., 1919. Fresh-water Mussels and Mussel In- 

 dustries of the United States. Bull. Bureau of Fisheries, 

 36:13-89. 



Lefevre, G. and Curtis, W. C, 1910. Studies of the Reproduc- 

 tion and Artificial Propogation of Fresh- Water Mussels. 

 Bull. U. S. Bureau Fisheries, 30. 



Simpson, C. T., 1899. The Pearly Fresh- Water Mussels of the 

 United States. Bull. U. S. Fish Comm. for 1898, pp. 279- 

 288. 



