THE CLAM S SHELL. 



33 



mollusk is hatched from an egg, and is surrounded by 

 its mantle. This mantle has the wonderful power of 

 taking or secreting lime from the water and turning it 

 into hard shell. 



The shell begins to grow at the hinge, and increases 

 as fast as the animal grows. The manner of this 

 growth is like that of shingling a roof backwards from 

 the ridge to the eaves. First, one little circular layer 

 of shell is made at the hinge. Then as the animal 

 grows, another still larger layer or shingle pushes out 

 from under the first. And so on, as show r n in Fig. 3, 

 each bit of growth leaving its definite line on the out- 

 side of the shell, while the inside is spread over with 

 a polished coat of pearl. 



This is the place to speak of the way in which the 

 clam opens and closes its shell. Suppose, as in the 

 figures, two shells cut 

 through both valves 

 so as to take a cross 

 slice or section from 

 each. The pieces will 

 then show the way in 

 which the layers of 

 growth have been add- 

 ed, each one extend- 

 ing from under the 

 preceding one. They also show the contrivances for 

 opening and closing the shell in the salt-water clam 

 and in the fresh-water clam, or mussel. 



If you put a piece of rubber inside a book close 

 to its hinge, the book may be closed by pressing the 

 covers together, but it will not stay shut, because the 



L. C.-3. 



Fig. 2. 

 Section of Clam. 



Fig-. 3. 

 Section of Mussel. 



