additions are made discontinuously throughout the 

 Hfe of the animal, is of "mother-of-pearl," in which 

 extremely thin layers of limy material alternate with 

 equally thin films of horny material. These provide 

 the diffraction of light that produces the iridescence 

 for which these creatures are famous. 



If a grain of sand or a small animal gets between 

 the mantle and the mother-of-pearl layer of the 

 shell, the soft tissues may become irritated enough to 

 wall off the foreign object and then continue to de- 

 posit mother-of-pearl over its surface. This creates a 

 pearl. It may be sperical, but more frequently it is ir- 

 regular. "Cultured pearls" are the products of pearl 

 oysters between whose shells a foreign object has 

 been placed deliberately. 



From season to season the abundance of food par- 

 ticles in the water varies markedly. When food is 

 easily available, pelecypods usually grow rapidly; 

 whereas during adverse times the mantle adds little 

 to the rim and inner surface of the shell valves. Often 

 these changes in the rate of shell production show as 

 ridges on the outside of the shell. If growth is slow at 

 only one period in each year, the ridges may indicate 

 how many years the clam has been enlarging its 

 shell. Great care must be used in making these esti- 

 mates, however, for a storm can stir up the bottom 

 sediments and induce pelecypods to cease feeding, 

 simulating the malnutrition of a winter season and 

 adding "growth rings" several times a year. 



SWIMMING CLAMS 



Over much of the world, the word "Shell" has 

 come to be a familiar trademark, an emblem taken 

 for a big corporation from the valve of the scallop 

 Pecten. Scallops are swimming clams ( Plates 41-43 ) 

 whose shells bear "ears" at each end of the hinge. 

 They are handsomely fluted in a radiating pattern as 

 well as regularly wavy ("scalloped") along the outer 

 edge. 



Scallops swim by a gobbling movement of the 

 shell valves, taking water in around the scalloped 

 margin and expelling it in little jets through the 

 "ears" at the hinge line. The edges of the mantle 

 serve as valves in controlling this flow of water. 

 They also bear many bright little eyes with which a 

 scallop can keep informed of moving objects nearby. 



When scallops complete a bout of swimming and 

 settle to the bottom, they come to rest on the right 

 valve. If they fall on the left valve, they immediately 

 turn themselves over. Only a single large muscle con- 

 trols these shell movements. It is the short, cylindrical 

 morsel that is so delectable when fried in deep fat 

 or served in cream sauce. 



The flavor of an approaching sea star in the water 

 or of an octopus is enough to start scallops into a dif- 

 ferent type of swimming — a flight reaction at much 

 higher speed, with the hinge in advance. In this emer- 



Matiiig slugs (Limax) have lowered themselves from 

 a tree trunk by means of a mucus cord. While thus 

 suspended their bodies entwine. ( Massachusetts. Lyn- 

 wood M. Chace) 



