4 CLASS GASTEROPODA. 
generation come forth, and that of the anus, vary. Neverthe- 
less, they are almost always on the right side of the body. 
Several are absolutely naked. Others have but a concealed 
shell; but the greater number have a shell which can receive 
and shelter them. _ 
These shells are produced in the thickness of the mantle: 
some of them are symmetrical, with several pieces; some 
symmetrical, with a single piece; and some non-symmetrical, 
which, in the species where they are concave, and grow for 
a long time, necessarily produce an oblong spiral form. 
Let us, in fact, figure to ourselves an oblique cone, in 
which other cones are successively placed, always broader in 
a certain direction than in others: it will be necessary that the 
whole shall be rolled towards the side which is the smallest. 
The part on which the cone is rolled is named the columella, 
and it is sometimes full, sometimes hollow. When it is hol- 
low, its aperture is named wmbilicus. 
The whorls of the shell may remain pretty nearly in the 
same plane, or tend always towards the base of the colu- 
mella. 
In this last case, the preceding whorls rise one above the 
other, and form what is called a spire, which is so much the 
sharper as the whorls descend more rapidly and are less wide. 
These shells, with projecting spires, are named turbinated. 
When, on the contrary, the whorls remain pretty nearly in 
the same plane, and do not envelope each other, the spire is 
flat, or even concave. These shells are called discoid. 
When the top of each whorl envelopes the preceding, the 
spire is concealed. 
The part from which the animal appears to come forth is 
named the aperture. 
When the whorls remain pretty nearly in the same plane 
when the animal creeps, it has its shell placed vertically, the 
columella crosswise over the hinder part of the back, and the 
