82 
NATURAL HISTORY. [NKW BUILDING. 
light univalve shell; and the Cephalopoda , so called be¬ 
cause they walk head downwards on the large tentacles 
that surround their mouth. 
The more typical Mollusca have a single more or less 
distinct muscular foot, on which they walk, or rather glide, 
placed under their stomach, as the Gasteropoda and the 
Conchifera. 
The first class of Belly-walking or Gasteropo- 
dous Mollusca (Cases 1 to 26) walk on a broad, flat, 
fleshy disk under the stomach; they have a distinct head, 
furnished with two or more pairs of feelers, and are ge¬ 
nerally provided with a pair of eyes and other organs of 
sense; and their digestive organs, &c., are generally in¬ 
closed 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 thickened 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 pro¬ 
minent, 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 nar¬ 
row groove, and at others compressed into a vertical fin. 
They are divided into two sections, according to the form 
of their respiratory organs, into Ctenobranchiata , and 
Heterobranchiata , (p. 95.) 
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 Mol¬ 
lusca which are furnished with large and well developed 
shells. 
