person examines the mantle surface closely, he sees 

 little bright spots suggesting eyes. These are actually 

 "skylights" admitting energy from the sun into 

 "greenhouses" within the mantle. In these spaces, 

 microscopic green algae grow and carry on photo- 

 synthesis. Triclacna uses its white blood cells to har- 

 vest the algae and, as an adult animal, seems not to 

 use its digestive tract. It depends instead upon the 

 plant food raised in the exposed parts of its body. 

 The bear"s-paw clam has become world famous 

 as a trap for unwary persons walking on a coral reef, 

 for the shell valves will surely close tightly on a leg 

 or arm accidentally thrust past the soft mantle folds 

 into the interior cavity. Native people actually fear 

 these clams very little, and often use a hammer to 

 break a portion of the shell edge so that a hand 

 wielding a knife can be slid into the interior to cut 

 free as food the big muscle that clamps the shells to- 

 gether. Tridacmi shells of small size are often used 

 elsewhere in the world as bird baths and baptismal 

 fonts. Large ones, weighing half a ton or more, are 

 usually left in the reef, where the live animal con- 

 tinues to demonstrate how the giant of the clam 

 world succeeds in getting enough to eat. 



The Elephant-Tusk 



OliellS {Class Scaphopoda) 



When white men first reached the west coast of 

 America, they found Indians there wearing necklaces 

 of almost cylindrical shells, each slightly curved and 

 open at both ends. The Indians esteemed the shells 

 according to their length, and used them for barter. 

 A 1-inch shell had only slight value, but a 2-inch 

 shell was equivalent in purchasing power to a shilling 

 (then twenty-five cents) — equal to a dollar at pres- 

 ent-day prices. Rare 3-inch shells were owned only by 

 the wealthy chiefs. 



An elephant-tusk shell or tooth shell seems made 

 to order as a piece for a necklace, since it is open at 

 both the larger end and the smaller. While occupied 

 by the mollusk that secretes it, the large end is usu- 

 ally below the surface of the sandy sea bottom and 

 the shell slants upward, exposing the smaller end. 

 This latter is the place where water enters and leaves 

 the shell, attending to the respiratory needs of the 

 scaphopod merely through the walls of the mantle 

 cavity, for no gills of any kind are present. 



From the larger end of the shell, the elongated and 

 bilaterally symmetrical animal extends a foot resem- 

 bling a horse's hoof, using this as a digging organ. 

 Behind the foot, at the edge of the almost cylindrical 

 mantle, a cluster of ciliated tentacles arises around 

 the mouth. They bring food particles to the mouth, 



The jackknife clam, Taf^elus gibbus, sometimes erron- 

 eously called a razor clam, occurs from New England 

 to north Patagonia, and is often eaten where no 

 better clam is available. { North Carolina. Ralph 

 Buchsbaum) 



the small ones to be swallowed whole, the larger 

 ones to be rasped into fragments by use of the radula. 

 The digestive tract bends in a U and opens at an 

 anus, exposed from the larger end of the shell when 

 the scaphopod is fully extended. 



The sexes of elephant-tusk shells are separate, and 

 the eggs are laid singly, the sperm discharged as a 

 cloud into the sand through the pair of excretory 

 tubes opening beside the anus. 



The most widespread genus of these exclusively 

 marine animals is Dentaliwn. The common tooth 

 shell D. eiitale reaches a length of 2 inches in quiet 

 sandy bays on both sides of the North Atlantic. The 



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