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The commercially valuable razor clam of the Ameri- 

 can Pacific coast, Siliqtia pattila, lives on sandy 

 beaches and can completely bury itself within about 

 seven seconds. The one in the foreground has just 

 done so, leaving only the siphons protruding. (Ore- 

 gon. Ralph Buchsbaum) 



and lesser width — greater where the foot pulled the 

 shell deeper into the bottom. 



The two size ranges of fresh-water clams represent 

 two unlike families. The small ones (family Sphaeri- 

 dae) lack a mother-of-pearl lining to the shell and 

 are dull white inside. These clams retain the fertilized 

 eggs in a brood pouch formed of the inner gill on 

 each side of the foot, and release shelled clamlets. A 

 viviparous parent may produce from two to twenty 

 young at a time, but they breed repeatedly through- 

 out the year. 



The larger fresh-water clams (family Unionidae) 

 are often called mussels. Their shells have been 

 much sought in tributaries of the Mississippi River as 

 material from which pearl buttons can be cut, for 

 the inner surface of the shell is coated with mother- 

 of-pearl. From fifteen to twenty buttons of different 

 sizes can be made from one 5-inch valve. In Ano- 

 donta the shell lacks hinge teeth and the two valves 

 are held in alignment only by the hinge ligament. In 

 Unio and related genera, hinge teeth inside the shell 

 give extra support. 



Unionid clams have become strangely adapted to 

 life in flowing fresh water. Instead of retaining the 

 eggs in a viviparous habit, they discharge the young 

 as larvae, called glochidia. Each glochidium has a 

 pair of small, widely-spread valves, each terminat- 

 ing in a tonglike hook. It uses these to pinch into the 

 skin of a passing fish or to hold to its gills if taken 

 into the mouth. The glochidium thereupon becomes 

 a temporary parasite, absorbing nourishment from 

 the fish's blood stream. After a few weeks the larval 

 stage ends with the glochidium freeing itself, drop- 

 ping to the bottom wherever the fish happens to be, 

 and changing to the adult form of clam. In this way 



192' 



the unionids make use of fish to distribute their off- 

 spring, and depend upon the ability of the fish in re- 

 sisting the current — staying in fresh water and not 

 being carried to the sea. 



A still stranger way in which a pelecypod depends 

 for success upon the activities of another type of life 

 was revealed recently in the giant bear's-paw clam 

 Tridacna of the South Pacific. These mollusks lie 

 hinge downward in coral reefs, usually in quite shal- 

 low water, and display a thick purple mantle over the 

 coarsely fluted, exposed edge of the heavy shell. If a 



A European razor shell, Ensis siliqua, with the foot 

 extended. It is common on many sandy beaches, but 

 can be found only bv digging for it at low tide. ( Eng- 

 land. D. P. Wilson) 



