MOLLUSKS 



105 



through the burrow to the surface when the tide is in, 

 and draws into its shell the water containing animalcules 

 and oxygen. 



Oysters to the value of many millions of dollars are gath- 

 ered and sold every year. The most valuable oyster fish- 

 eries of the United States are in Chesapeake Bay. The 

 young oysters, or " spat," after they attach themselves to 

 the bottom in shallow water, are transplanted. New oyster 

 beds are formed in this way. The beds are sometimes 

 strewn with pieces of rock, broken pottery, etc., to encourage 

 the oysters to attach themselves. The dark spot in the 

 fleshy body of the oyster is the digestive gland, or liver. 

 The cut ends of the tough adductor muscles are noticeable 

 in raw oysters. The starfish is very destructive in oyster 

 beds. 



Pearls are deposited by bivalves around some irritating 

 particle that gets between the shell and the mantle. The 

 pearl oyster furnishes most of the pearls ; sometimes 

 pearls of great value are obtained from fresh-water mussels 

 in the United 

 States. Name 

 articles that are 

 made partly or 

 wholly of mother- 

 of-pearl. 



Study of a Live 

 Snail or Slug. — Is 

 its body dry or 

 moist ? Do land 



snails and slugs have lungs or gills? Why? How many pairs 

 of tentacles has it? What is their relative length and position? 

 The eyes are dark spots at bases of tentacles of snail and at the 

 tips of the rear tentacles of slug. Touch the tentacles. What 

 happens? Do the tentacles simply stretch, or do they turn inside 



Fig. 202. — A Snail. 



/, mouth: vf, hf, feelers; e, opening of egg duct; fit, foot; 

 via, mantle; In, opening to lung; a, vent. 



