Shell of Edible Snail. 47 



14. Shell of Edible Snail {Helix Pomatia). 



When the shell has its apex directed upwards, and its aperture 

 downwards, and towards the describer, its spire will be seen to 

 ascend obliquely towards the right, and this is the case in the 

 great majority of spiral Gasteropodous shells, which are in con- 

 sequence ordinarily called 'dextral/ In such molluscs the heart 

 is placed on the left, and the generative, respiratory, and anal 

 orifices on the right of the animaFs body. The columella is seen 

 in the angle formed by the left border of the peristome, and the 

 first whorl of the shell ; its umbilicus is partly, but not wholly, 

 as in the common garden snail. Helix Asjoersa, concealed by the 

 reflection over it of the peristome, corresponding to which structure 

 in this situation, there was developed in the mantle of the living 

 animal, a process of the collar known as the ^columellar lobule.' 

 The smooth rounding off of the peristome shows the animal to 

 have been adult. The shell is coarsely striated in a longitudinal 

 direction, that is to say, in a direction corresponding with the 

 lines of growth; and more delicately in a spiral direction, that 

 is to say, in a direction at right angles to these lines, and parallel 

 to the five coloured bands with which it is marked. The ' apex ' 

 or ' nucleus ' of the shell, which was the earliest developed or 

 embryonal shell-whorl, differs, as is so often the case, from the 

 rest of the shell, and in this instance, by possessing a smooth and 

 semi-porcellanous appearance. The way in which the injuries 

 which the shell had received during the life of the animal, have 

 been repaired, without any attempt at a restoration of the damaged 

 exterior layer, shows that it is only along the peristomal margin 

 that such repair can be effected. Injuries received posteriorly to 

 the free rim of the shell, which is in relation with the collar of 

 the mantle, call forth the secretion of protecting layers, by the 

 thin and transparent portion of the mantle ; and these layers by 

 successive super-imposition from the mantle outwards, produce the 

 thickenings visible on the interior, in correspondence to the in- 

 juries on the exterior of the shell which is left unrepaired. 



The thickness of the Gasteropodous shell diminishes from its 

 free rim upwards, and it is probable, consequently, that, except 

 under the stimulus of injury, little accession of growth takes place 



