176 



THE MECHANICAL FUNCTIONS. 



of this form. Fig. 108, which represents the shell of the 

 t/ichaiina zebra,, and of which Fig. 109 shows a longitudinal 

 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 denominated lohorls. In consequence 

 of the situation of the heart and great blood vessels relative- 



ly to the shell, the left side of the mantle Is more active than 

 the risht side, so that the lateral turns are made in the con- 

 trary 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, 

 ov reversed shoVi'. but this left-handed convolution seldom 

 occurs amons; 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 generally be distinguished by its colour and appearance 

 from that which is afterwards formed. The succeeding turns 

 made by the shell in the progress of its growth, enlarging in 

 diameter as they descend from the apex, form by degrees a 

 wider base. During the growth of the animal, as the body 

 extends toward the mouth of the shell, its posterior end 

 often quits the first turn of the spire, and occupies a situa- 



* The terms rig-ht 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. 



