NATURAL HISTORY. 
51 
GALLERY. ] 
called because 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 Gasteropodous 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 generally provided with a pair of eyes and other organs of 
sense ; and their 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 oc¬ 
casionally 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 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 some¬ 
times contracted to a narrow 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. 64). 
The Ctenobranchiata, or comb-shaped-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 constant 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 orders, according to the food they live upon, 
as Zoophagous and Phytophagous (p. 56). 
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 wdth a retractile proboscis, 
armed with teeth, which enables them to form round holes in shells, for 
the purpose of extracting the inhabitant; they have this faculty in 
common with the JVaticce, which are said chiefly to attack the animals 
of bivalve shells; their operculum is alw r ays horny, and formed of ir¬ 
regular concentric plates. They are unisexual, and the females are 
generally much larger, and have much more ventricose whorls to their 
shells than the males. The eggs of these animals are contained in 
coriaceous cases, each holding many eggs, of which only a few gradually 
enlarge and come to perfection. The cases of Buccinum undatum , 
and Fusus despectus have been mistaken for the eggs of the oyster, 
