262 ELEMENTS OF COMPARATIVE ZOOLOGY. 



of-pearl used for knife-handles, for inlaying, etc. The 

 pearl-oysters occur in the Indian Ocean, and also in the 

 Bay of Panama. 



The salt-water mussels, so abundant on the 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 



FIG. 121. Quahog (Venus mercenaria) , with foot and siphons extended. 



mussel; but not infrequently severe sickness follows its 

 use as food. The fresh-water mussels are especially abun- 

 dant in America, the Mississippi 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 transi- 

 tion to the next group, 



