144 



ANARTHROPODA 



FIG. 168. Oyster of t 

 (Ostrea virginiana). 

 half natural size. 



Photograph one 



differs from the clams in having no siphon and a foot so 

 rudimentary as to be unable to move. The oystermen are 

 therefore accustomed to plant the young in shallow water 

 where they remain three or four years to mature. Our 



Atlantic coast furnishes 

 twenty-five millions of dol- 

 lars' worth of oysters an- 

 nually. The Japan oysters 

 are the largest, some species 

 attaining a length of three 

 feet. 



The pearl oysters (Melea- 

 grina), along the Persian 



Q u j and coagts Q f C ey l ol ^ 



are the source of most 

 pearls, but some pearls have been found in the fresh water 

 clams and oysters of our own country. Pearls are depos- 

 its of nacre formed around foreign bodies by the mantle, 

 and are of the 

 same composition 

 as the inner lining 

 of the shell so 

 much valued as 

 the mother of 

 pearl. The ship- 

 worm (Teredo 

 navalis) is a pecu- 

 liar bivalve, causing much damage by boring into ships 

 and piles supporting the wharves along the ocean. 



Anatomy of the Fresh Water Mussel. Any clam may 

 be used for dissection, but the following description applies 



FIG. 169. Photograph of the shipworm slightly en- 

 larged. 



