THE FRESH WATER MUSSEL 143 



joins the body a little back from the mouth. A nerve may 

 also be traced to the visceral or posterior ganglion, on the 

 under surface of the posterior adductor. Note the irregu- 

 larity in the plan of the nervous system. Draw nervous 

 system. 



(4) The reproductive organs are found in the posterior 

 region on each side, and open into the gill cavity. The 

 ^gg^ of the mussel are hatched in the gills and the young 

 mussels may be found there if the animal is studied at 

 the proper time. After awhile they pass out into the water 

 through the siphon. 



Summary. — (i) What are the advantages and disad- 

 vantages of a shell like that of the mussel? Can it be suc- 

 cessfully attacked by any animal that you know? What 

 is the origin of the pearl in the shell? The pearl of com- 

 merce is usually made by this or some other species of 

 bivalve when something of an irritating nature gets between 

 the shell and the mantle. Why does the animal deposit 

 pearl under such circumstances? Formerly pearls were all 

 obtained from pearl oyster, but recently many of con- 

 siderable value have been obtained from the mussels of 

 the rivers in the northern part of the United States. 



(2) Compare an oyster shell with that of the mussel just 

 studied. Are the valves of the oyster alike? Does either 

 valve show signs of being attached to something? Since 

 it lives in the ocean, would the oyster need such support 

 more than the mussel? Why? 



Oyster raising is a great industry in certain sheltered 

 places along the eastern coast of the United States. The 

 greatest menace to the oyster beds is the starfish, which 

 devours the young oysters. 



