128 



Descriptive Zoology. 



at right angles to the plane of the flat spiral. By pushing 

 out the center, first to the right and then to the left, we may 

 illustrate both the right-handed and the left-handed shells. 

 A very good substitute may be made by winding a narrow 

 strip of paper around a lead pencil at one end. This, 

 unwound, forms- a flat spiral, representing the discoid shell. 

 By pushing the center, first to one side and then the other, 

 illustrate the right- and left-hand shells. Lay snail shells 

 alongside a common wood screw ; those having the whorls 

 run the same way as the threads of the screw are right- 



Left-hand shell Flat spiral or Right-hand shell 



FIG. 80. COMPARISON OF KINDS OF SNAIL SHELLS. 



hand shells ; those with the whorls twisting in the opposite 

 direction are left-hand shells. 



The structure and composition of the shell are essentially 

 the same as in clams, the lines of growth usually showing 

 plainly parallel to the border of the lip. 



The Operculum. Nearly all sea snails, and many fresh- 

 water snails, have a trap door attached to the hinder part 

 of the foot, with which they close the aperture of the shell 

 when the body is drawn in. This covering is the opercu- 



