143 



THE STUDY OF SHELLS. 



SPIRAL SHELLS. 



Take a common shell, e.g. the shell of a garden snail. 

 Make a drawing of it so as to show the various parts 

 mouth, spire, apex. Note that the spiral is right-handed. 



To determine this, look 

 down upon the apex of 

 the spire, and follow the 

 coils downwards with the 

 finger. The finger will 

 move in the same direction 

 as do the hands of a watch. 

 Fig. si.-spirais; that to the left is left- Some shells have 



handed, to the right is right-handed. 

 (In following the spiral, commence at 

 the inner end.) 



Spiral, 



a con- 



i.e. they are 



T>mw <i 



JJiaW a 



and a right- 



trai'V 

 ] ,, 

 leit- 



left-handed 

 handed spiral side by side (Fig. 51). 



When the outer wall of a shell is broken, we can see a 

 pillar inside. To this pillar the snail is attached, so that 

 it cannot be pulled 

 out of the shell. 

 The pillar is some- 

 times hollow in 

 part at any rate 

 and has an open- 

 ing to the outside. 

 Find the pillar 

 opening on the 

 shell of the gar- 

 den snail. (It lies 

 on the inner mar- 

 gin of the mouth 

 opening.) This 

 opening is some- 

 times covered over 

 in part by the folded edge of the shell. 



The shell of the snail is a turreted spiral. Some shells 

 have long turrets and others have very short ones. Com- 

 pare such shells as the whelk, a common sea-shore form, 



Fig. 52. Shell cut to show pillar (C). 

 M, mouth ; A, apex. 



