GALLERY.] NATURAL HISTORY. 109 
acts as a lid to close the mouth of the larger, when the ani¬ 
mal is retracted into it; hence it has been called an oper¬ 
culum. This smaller valve, or operculum, is generally 
cartilaginous, either wholly formed of animal matter, or 
strengthened by a quantity of calcareous matter deposited 
on one or both of its surfaces. Sometimes this valve is 
altogether wanting, especially in those genera which have 
an expanded mouth compared with the size of the remain¬ 
ing shell. 
Most of the Gasteropodous animals, especially the lung¬ 
breathing kinds, which are not provided with a second 
valve, prevent the evaporation of their juices, during the 
very hot or cold weather, when they are in a torpid state, 
by secreting a layer of mucus, which forms a temporary 
lid, called an epiphragma , and is sometimes hardened by 
a quantity of calcareous particles. 
Linnaeus referred all the animals inhabiting shells to five 
different genera, viz., Bimax, Ascidia , Anomia, Clio, and 
Sepia . These genera have become the types of five very 
distinct classes, viz., first, the Gasteropoda, which have a 
distinct head and a univalve spiral shell; secondly, the 
Conchifera, which have the mouth placed at the bottom of 
the bag-like mantle, and are covered with a bivalve shell. 
The three other classes are remarkable for being desti¬ 
tute of any foot, as they are attached to other bodies, 
float about in the sea, or walk on the tentacles which 
surround their head. They consist of the Brachiopoda, 
which have an attached bivalve shell and a pair of spiral 
arms on the sides of the mouth; the Pteropoda , which 
have a pair of fins on the sides of the head, and a very 
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 Belly-walking, or Gasteropodous Mollusoa 
(Cases 1 to ) 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 
