PHYLUM MOLLUSCA 219 



deposited after storms or floods. The ship worm, Teredo 

 navaliSj bores in piles, wharves, and even in wooden ships; 

 hence is very destructive to timber. It is soft and worm- 

 like, the shell being restricted to one end of the body and 

 serving as an auger for boring. 



The breeding habits of lamellibranchs are quite variable. 

 Most of them are dioecious and some species show sexual 

 dimorphism, the females usually being larger than the males. 

 Marine species usually have a free-swimming larva (veliger) 

 which is trochophore-like (page 190), but those in freshwater 

 generally bring forth living young. The life history of the 

 fresh-water mussel (Fig. 87) shows great specialization 

 and is well-adjusted to conditions in rivers, ponds, and lakes. 

 The eggs are fertilized within the mantle cavity of the 

 female; then pass into the gills, which serve as brood- 

 pouches, or marsupia, and become greatly swollen. The 

 larvae developed within the marsupia are known as glo- 

 chidia, and there are two types, hooked (Fig. 87, B') and 

 hookless. Both are usually set free in the water and lie 

 on the bottom until a fish comes into contact with them. 

 A hooked glochidium becomes attached to the fish's 

 fins (C) and grows for a time as a parasite within a cyst. 

 The hookless glochidia live as parasites on the gills of 

 fishes. Both types mature within cysts, then break out, 

 ready to take up the usual activities of young mussels (D). 



Lamellibranchs are of great economic value in the United 

 States. Along the seacoast oysters, clams, mussels, and 

 scallops are collected for food. The national and state 

 governments maintain laboratories for the propagation 

 and study of these important natural resources. The ship 

 worm, on the other hand, causes great losses and the govern- 

 ment is trying to find better means to control or exter- 

 minate it. Pearls are formed when small foreign bodies or 

 parasites lodge in the mantle of a clam and become covered 

 with layers of lime. In the United States the chief source 

 of these jewels is from the fresh- water mussels which are 

 dredged from the bottoms of rivers and lakes. The shells 



