24 
MOLLUSC A. 
The ivhorls 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 ftpire, which is so much the more acute, as the whorls de- 
scend 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 \s jlat, or even co7i- 
cave. These shells are said to be discoidal. 
AA^’lien 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 
crawls, it has its shell placed vertically, the columella crosswise on 
the hind part of its back, and its head passes under the edge of the 
opening opposite to the columella. 
AA^’lien the spire is salient, it inclines from the right side in almost 
cverv species ; in a very few only does it project from the left Avhen 
they are in motion ; these are said to be reversed. 
It is observed that the heart 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. This relation is exactly inverted Avith re- 
spect of the organs of generation. 
The organs of respiration, Avhich are always sitiiated 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 by a 
hole. 
It sometimes happens that the margin of the mantle is prolonged 
in the form of a canal, in order to allow the animal to seek the am- 
bient element without exposing its head and foot beyond its shell. In 
sucli a case as this, the shell has also in its margin, near the extremity 
of the columella, opposite that to which the spire inclines, a fissure or 
canal, for the purpose of lodging that of the mantle. The canal, 
consequently, in ordinary species, is on the left; and in the reversed, 
cn the right. 
The animal, however, being very flexible, can vary the direction 
of the shell, and most commonly when there is a fissure or'canal, it 
directs the latter foiuvards, which throws the spine behind, the colu- 
mella to the left, and the opposite margin to the right. It is the 
contrary in the reversed, for which reason their shell is said to be 
contorted to the left. 
