GASTEROPODA. 239 



of a vertical lamina. The back is furnished with a mantle which is 

 more or less extended, takes various forms, and in the greater number 

 of genera, produces a shell. Their head, placed anteriorly, is more or 

 less visible, as it is the more or less involved under the mantle ; its 

 tentacula are very small, situated above the mouth and do not surround 

 it, varying in number from two to six ; sometimes they are wanting ; 

 their function is that of touch, or at most that of smell. The eyes 

 are very small, here adhering to the head, and there to the base, side, 

 or point of the tentaculum : sometimes they are wanting. The po- 

 sition, structure, and nature of their respiratory organs vary, and afford 

 the means of dividing them into several families ; they never, however, 

 have more than a single aortic heart, that is to say, one placed between 

 the pulmonary vein and the aorta. 



Several are entirely naked ; others have merely a concealed shell, 

 but most of them are furnished with one that is large enough to receive 

 and shelter them. 



The shell is formed in the thickness of the mantle. Some of them 

 are symmetrical and consist of a single piece ; others are non-syin- 

 metrical, which, in those species where they are very concave, and 

 where they continue to grow for a long time, become obliquely spiral. 



If we figure to ourselves an oblique cone, in which other cones, 

 always wider in one direction than in the others, are successively 

 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, forming 

 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 concave. 

 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 animal 



