110 
NATURAL HISTORY. [EAST. ZOOL. 
digestive organs, &c., are generally inclosed in a more or 
less conical bag on the back, which is covered with the 
mantle, which is itself usually covered with a single large, 
conical, often spiral valve, and they sometimes have a 
small horny opercular valve, which is occasionally thick¬ 
ened by a shelly coat. The shell being formed on the bag 
which contains the digestive organs, agrees with it in 
shape; if the bag is only a little prominent, the shell is 
simply conical; but if it is very long, it is then generally, 
for the purpose of being out of the animal’s way when it 
walks, coiled up, and then the shell which covers it is 
spiral or discoidal, according as the body is coiled up on 
itself, or in a more or less oblique manner on a central 
axis. The foot is sometimes contracted to a narrow 
groove, and at others compressed into a vertical fin. 
They are divided into orders, according to the form of 
their respiratory organs. 
The Ctenobranchiata, or pectinated-gilled Mollusca, 
are so called from their respiratory organs consisting of 
one or more comb-like gills, placed on the inner surface 
of the mantle, which forms an open bag in the last whorl 
of the shell, over the back of the neck. There is a con¬ 
stant current of water passing over the gills, which enters 
at the front and makes its way out near the inner hinder 
angle of the gill-cavity and mouth of the shell. This order 
contains the greater part of the Gasteropodous Mollusca 
which are furnished with large and well developed shells. 
They are divided into sections, according to the food they 
live upon and the position of their eyes. 
1. The Flesh-eating Gasteropodes, ( Zoophaga ,) or those 
which prey almost exclusively on dead or living animal 
food, as they require the blood to be more perfectly aerated, 
have a syphon attached to the inner side of the front 
part of the mantle, to enable the water more freely to arrive 
at their respiratory organs. Their shells are always pro¬ 
vided with a canal, placed in the front of the pillar which 
is formed to protect this syphon. They are furnished with 
a retractile proboscis, armed with teeth, which enables them 
to form round holes in bivalve and other shells, for the 
purpose of extracting the inhabitant; their operculum is 
always horny, and formed of irregular concentric plates. 
They are unisexual, and the females are generally much 
