MOLLUSKS IN GENERAL 233 



the Mississippi River Valley and Great Lakes region. Shells of mollusks 

 are crushed and sold as "grit" to be fed to fowls, or used in liming the 

 soil, while cuttlefish bone is given to captive songbirds as a source of 

 lime. Pearls are also a product of mollusks, being developed in the shells 

 of a number of bivalve forms. Whenever any irritating particle gets in 

 between the mantle and the shell, a nacreous covering called mother-of- 

 pearl is secreted around the particle, thus forming the pearl. Parasitic 

 worms or infective organisms may also cause the formation of pearls. 



Some mollusks are injurious. In addition to those which attack 

 oysters and other bivalves of value may be mentioned the shipworm, 

 Teredo navalis Linnaeus. This is a bivalve with an elongate wormlike 

 body (Fig. 142) ending posteriorly in two long slender siphons which 

 reach the outer end of the tube in which the animal fives. At the anterior 

 end are two small valves, not hinged but separate and movable. With 

 the sharp anterior edges of these valves the animal burrows into the 

 wood of ships and pifing, sometimes to a depth of 4 feet, its siphons being 

 extended into the water for the purpose of feeding and breathing. It 

 has long been disputed whether or not the animal eats the wood which 

 it removes and the particles of which are passed back through the body 

 and out of the siphon, but it now seems definitely proved that it does 

 and that it also gets food in small particles from the water as do other 

 bivalves. The tube which serves to lodge and protect the soft body is 

 smallest at the outer end, largest at the inner, and becomes lined with 

 nacre secreted by the mantle. Some snails secrete an acid which dissolves 

 limestone rock and thus they excavate cavities in which they live. 



The defenceless condition of most mollusks makes them the prey of 

 many other animals, including all classes of vertebrates. They are also 

 the intermediate hosts of many parasitic worms. 



