FORMATION OF SHELLS. 243 



as is well known, has a shell of this form. Fig. 

 108, which represents the shell of the Achatina 

 zebra, and of which Fig. 109 shows a longitudi- 

 nal section, may serve as an example of a shell 

 of this kind. The axis of revolution is termed 

 the Columella^ and the turns of the spiral are de- 

 nominated ivhorls. In consequence of the situa- 

 tion of the heart and great blood-vessels rela- 

 tively to the shell, the left side of the mantle is 

 more active than the right side, so that the lateral 

 turns are made in the contrary direction, that is, 

 towards the right.* There are a few species, 

 however, where, 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 shells : but this 

 left-handed convolution seldom occurs among the 

 shells of land or fresh-water mollusca. 



It results from this mode of formation 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 gene- 

 rally be distinguished by its colour and appear- 

 ance from that which is afterwards formed. The 

 succeeding turns made by the shell in the pro- 

 gress of its growth, enlarging in diameter as they 



* The terms right and left have reference to the position of the 

 animal when resting on its foot ; the head being of course in front. 

 See Gray, Zool. Journal, i. 207. 



