206 SYSTEMATIC ZOOLOGY. 



luscs, the precious pearls. These pearls are really the 

 shell-forming secretions of the mollusc around some 

 foreign body and they receive their beauty from the way 

 in which the shell is deposited around the centre. Fresh- 

 water mussels, to be referred to a few lines below, also 

 form pearls of value. The shell of the pearl-oyster also 

 has its value, for it furnishes the mother-of-pearl used 

 for buttons, knife-handles, for inlaying, etc. The pearl- 

 oysters occur in the Indian Ocean, and also in the Bay 

 of Panama. 



The salt-water mussels (fig. 44) so abundant on the 



FIG. 44. Salt-water mussel (Mytilus edulis). 



mud flats all along northern shores have a peculiar gland 

 in the foot which secretes strong silky threads (byssus) by 

 which these animals anchor themselves. The common 

 species, which occurs both in Europe and New England, 

 is called the edible mussel; but not infrequently severe 

 sickness follows its use as food. The fresh-water mussels 

 or Unios are especially abundant in America, the Mis- 

 sissippi basin being their centre. They are useless as 

 food, owing to their strong taste. There are possibly a 

 hundred species of these forms in America; over six 

 hundred so-called species have been described. In their 

 siphonal structure they form a transition to the next 

 group. 



