CLASS GASTEROPODA. 23 
head passes under the edge of the aperture opposite to the 
columella. 
When the spire is projecting, it is directed obliquely from 
the right side in almost all the species. A small number only 
have their spire projecting at the left side when they walk, 
and are named reversed. 
It is remarked that the heart is always on the side opposite 
to that on which the spire is directed. ‘Thus it is usually on 
the left, but in the reversed it is on the right. ‘The contrary 
is the case with the organs of generation. 
The organs of respiration, which are always 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 that edge, sometimes because it is pierced there 
with a hole. 
Sometimes the edge of the mantle is prolonged into a canal, 
that the animal may be able to seek the ambient element 
without putting forth its head and foot from the shell. Then 
the shell has also in its edge, near the end of the columella, 
opposite to that towards which the spire tends, a notch, or a 
canal, to lodge that of the mantle. Consequently, the canal is 
on the left in the ordinary species, on the right in the 
reversed. | 
For the rest, the animal, being very flexible, causes the 
direction of the shell to vary, and most frequently, when there 
is anotch or a canal, it directs the canal forward, which causes 
the spire to be behind, the columella towards the left, and the 
opposite edge towards the right. ‘The contrary takes place in 
the reversed. ‘This is the reason why it is said that their shell 
turns to the left. 
The aperture of the shell, and consequently, also, the last 
whorl, are more or less large, in relation to the other whorls, 
according as the head or foot of the animal, which are con- 
stantly to pass out and in there, are more or less large in rela- 
