CHAPTER XVI 



(Left) octopus; (below octopus) sea cradle or chiton; 

 (center top) scallop; (center bottom) tooth shell; 

 slug and clam. 



The Mollusks 



{PJiyhun MoUnsca) 



To 



.O many people the mollusks are "shellfish." 

 Clams and oysters, perhaps snails and squids, are the 

 most familiar kinds. Yet squids have no obvious 

 shell, and no one seriously would consider a plate of 

 cooked snails as fish except on "fish day." Nor did 

 those who gave the phylum Mollusca its name (from 

 the Latin mollis, soft ) have in mind the giant squids 

 of the open oceans, creatures that wrestle — some- 

 times successfully — with the great sperm whales. 



Better than forty thousand different kinds of living 

 mollusks are known, a total exceeded among inverte- 

 brate phyla only by the arthropods. These mollusks 

 include representatives above the snow line in the 

 Himalayas at an altitude of 16,400 feet, and deep 

 blue sea slugs creeping on the underside of the sur- 

 face film in the open ocean, and clams plowing the 

 sea bottom at a depth of at least 17,400 feet where 

 the hydrostatic pressure is almost four tons to the 

 square inch. Some snails manage to survive freezing 

 in the ice over ponds, and others tolerate thermal 

 springs at a temperature of 112 degrees Fahrenheit. 

 A few desert snails live where the air above them at 

 noon is in the same temperature range. 



All of this versatility demonstrates the possibilities 



in an unsegmented body whose dorsal and lateral 

 surfaces bear a fleshy tissue ( the mantle ) capable of 

 secreting an external limy shell. Ordinarily the ven- 

 tral surface is a flat, creeping foot. Features of the 

 foot, mantle, and shell are particularly helpful in 

 identifying each different kind of mollusk. 



Some features of mollusk anatomy are peculiar to 

 this phylum. A rasping organ (radula) is found in the 

 mouth of most mollusks as a ribbon-shaped tool that 

 can be slid back and forth while its sharp teeth act 

 like those on a file, scraping free small particles of 

 food. All mollusks, even the largest and most active, 

 have a nervous system consisting of only three paired 

 ganglia. One lies above or beside the mouth, a sec- 

 ond below the gullet as a center for nerves to the foot 

 region, and a third still more ventrally with connec- 

 tions to mantle, gills, heart, and other visceral organs. 



This way of life is very old. Clams appear early in 

 the fossil record, along with uncoiled snails and the 

 ancestors of today's splendid pearly nautilus. Alto- 

 gether, more than forty thousand extinct kinds of 

 mollusks have been found, showing that modern 

 shelled types are but the living heirs to a spectacu- 

 larly diverse heritage. 



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