GASTEROPODA. 251 



placed, it will be easily seen that the convolution of the whole takes 

 place on the side which enlarges the least. 



This part, on which the cone is rolled, is termed the columella; 

 it is sometimes solid, and sometimes hollow. When hollow, its 

 aperture is called the umbilicus. 



The whorls of the shell may either remain in one plane, or incline 

 towards the base of the columella. 



In this last case the preceding whorls rise above each other, form- 

 ing the spire, which is so much the more acute, as the whorls 

 descend more rapidly, and the less they increase in width. These 

 shells with a salient spine are said to be turbinated. 



When, on the contrary, the whorls remain nearly in the same 

 place, and do not envelope each other, the spine is flat , or even con- 

 cave. These shells are said to be discoidal. 



When the top of each whorl envelopes the preceding ones, the 

 spire is hidden. 



The part through which the animal appears to come out is named 

 the aperture. 



When the whorls remain nearly in the same plane, while the ani- 

 mal crawls, its shell is vertical, the columella crosswise on the 

 hind part of its back, and its head passes under the edge of the 

 opening opposite to the columella. 



When the spire is salient, it inclines from the right side in almost 

 every species; in a very few only does it project from the left when 

 they are in motion; these are said to be reversed. 



It is observed that the head is always on the side opposite to that 

 to which the spire is directed. Thus it is usually on the left, and 

 in the reversed on the right. 



The organs of respiration, which are always situated in the last 

 whorl of the shell, receive the ambient element from under its edge, 

 sometimes because the mantle is entirely detached from the body 

 along this edge, and sometimes because it is perforated there. 



It sometimes happens that the margin of the mantle is prolonged 

 in a canal, in order to allow the animal to seek the ambient element 

 without protruding its head and foot from its shell. 



Most of the aquatic Gasteropoda, with a spiral shell, have an 

 operculum, a part sometimes horny, sometimes calcareous, attached 

 to the posterior part of the foot, which closes the shell when its oc- 

 cupant is withdrawn into it and folded up. 



