220 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



knowledge of this relation of the fish to the mussel is of prime 

 importance in the attempts that are being made to restock 

 some of the mussel beds that have been depleted on account 

 of he increased demand for the shells for the making of 

 pearl buttons. 



Mussels and Buttons. Until about 1891 no use was made 

 of the shells of fresh-water mussels. But at this time it was 

 found that excellent pearl buttons could be made from these 

 shells, and so there has sprung up an industry spread through- 

 out the central part of the United States that has a value of 

 more than $6,000,000 annually and gives employment to 

 hundreds of people. The Government has established a station 

 for the propagation of mussels in order that depleted streams 

 may be restocked and new areas made productive. Mussels 

 containing the developing glochidia are teased up in water and 

 this water is poured into tanks containing fish. When their fins 

 and gills are well covered with the glochidia the fish are liber- 

 ated in the streams that are to be stocked with the mussels. 



Mussels and Pearls. In an earlier chapter reference has 

 been made to the fact that pearls are produced in many mol- 

 luscs by the pearly nacreous substance, of which the shell is 

 formed, being deposited around certain parasitic worms that 

 are found in the body of the animal. When such secretions 

 are irregular in shape they are usually called baroques, or slugs. 

 When round or pear-shaped, or of some other regular shape, 

 they are called pearls. 



It is not an uncommon thing to find baroques or slugs in the 

 fresh-water mussels, some of them very beautiful. These are 

 usually formed around certain parasitic flat worms, a 

 Distomid having the muskrat or otter as one of its hosts, being 

 a common form. 



Perfect round pearls of delicate luster and great value are 

 also often found. Recent investigations have shown that the 

 egg or the dead body of a small water-mite may form the nu- 

 cleus around which the pearl is formed, the most perfect 

 pearls probably being formed around the eggs. These mites, 

 Unionicola (Atax), liveparasitically in the gills or mouth cavity 

 of the mussel, and when they lodge in the tissues in such a way 



