114 GROWTH OF SHELLS. 



A common snail has a shell of this form. And here 

 another circumstance is worthy of remark : in conse- 

 quence of the situation of the heart and great blood- 

 vessels relatively to the shell, if we consider the animal 

 as resting on its foot, the head being in front, the left 

 side of the mantle is more active than the right, so that 

 the lateral turns of the spiral are made in the contrary 

 direction, that is, to the right. There are, however, a 

 few species in which, in consequence of the heart being 

 placed on the right side, the turns of the spiral are made 

 to the left. Such shells have been termed sinistral, or 

 reversed ; but this form seldom occurs in the shell of 

 fresh- water or land melius ca. 



Already instances have been mentioned of the bene- 

 volence of the universal Father, and here may be added 

 another. From the mode of forming shells, just de- 

 scribed, it follows that the apex, both of the simple and 

 of the spiral cone, is the part which was formed the 

 earliest, and which protected the young animal at the 

 moment of its exclusion from the egg. This portion 

 may generally be distinguished by its colour and ap- 

 pearance from that which is afterwards formed. The 

 succeeding turns made by the shell, in the progress 

 of its growth, enlarging its diameter as they descend 

 from the apex, form, by degrees, a wider base. During 



