The Pearly Fresh-water Mussels 



grand divisions of the Naiades are based upon differences in the 

 structure and development of the embryo, which widely separate 

 the two families. The young of the Mustelidae begin with a three- 

 segmented body hatched from the egg, the central segment alone 

 having a single shell. The Unionidae hatch from the eggs as 

 creatures with bivalve shells. This universal family trait deserves 

 some definite description. 



The gills of the parent clam are modified to form brood 

 pouches which are swollen with eggs, and retain the young after 

 they are hatched until they reach the stage called glochidium. 

 The young are submerged during this waiting period in a nutri- 

 tious mucus in the outer gill of the parent. A bivalve shell, loosely 

 hinged, and joined by a muscular band, contains the soft parts, 

 undiiferentiated into organs until the mollusk is six months or a 

 year old. Two strong, inward-pointing, saw-toothed hooks stand 

 at the lips of the gaping valves. A thread-like byssus is coiled 

 between them. 



When the mucus is all absorbed the glochidia are discharged 

 from the brood pouch into the water. On suitable bottom they 

 lie on their backs with the byssal cords floating upward. Here 

 each creature perishes unless a fish comes near enough for it to 

 catch hold of with the prehensile cord. The successful indi- 

 vidual is not slow to secure his hold by clamping the edge of a 

 scale or a fleshy gill filament of the fish with the two hooks of the 

 bivalve shell. A slimy exudation of the skin of the fish covers 

 the minute clam that is catching a ride on fin or gill or elsewhere. 

 The nutriment absorbed while thus encysted is sufficient to per- 

 fect the development of the vital organs. The young clam bursts 

 the cyst, and falls to the bottom, where subsequent growth con- 

 sists of gradual increase in size, through feeding upon microscopic 

 organisms from the water. 



Genus QUADRULA, Agassiz 



Shell triangular, or quadrate, solid, swollen, with distinct 

 posterior ridge crossed by irregular, coarse, concentric ridges, 

 beaks prominent, with deep cavities inside; hinge plate heavy, 

 wide, flattened; teeth ragged, solid, under beaks; laterals double, 

 blade-like in left valve, single in right; epidermis dark, dull, some- 

 times feebly rayed; male and female shells alike. Inner gills 



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