CLASS GASTEROPODA 
HE class Gasteropoda is the largest and most comprehensive 
subdivision of the Mollusca, and within its wide range of 
families many differing details of organization are to be found. 
As a class it possesses all the most characteristic features of the 
phylum, though it is frequently modified in a high degree. This 
class includes all the univalve mollusks (except Nautilus and 
Dentalia), such as the snails, the whelks, and the host of spirally 
coiled land, fresh-water, and marine shells. 
The gasteropod foot is, as has been remarked, primarily the 
game as in our schematic mollusk—a flat, muscular disk caused 
by the thickening of the ventral body-surface. The neck, head, 
and tentacles are also quite the same, but considerable modifica- 
tion of these organs will be found when we come to examine 
some specimens. The mantle is always present, excépt in the 
nudibranch or non-shell-bearing forms; but in few of the Gas- 
teropoda is the mantle so regularly simple as in our ideal mollusk. 
Perhaps the most striking feature of the Gasteropoda, and one 
that will at first surprise him who has in mind the simple struc- 
ture of the ideal mollusk, is the fact that they are always asym- 
metrical—that is to say, a median line drawn longitudinally 
through a gasteropod will not divide it into halves of similar 
anatomical structure. 
The quality of symmetry is an important one throughout the 
lower orders of animal life. In nearly all phyletic or class de- 
scriptions the word “symmetry” occurs, and its exact meaning 
must be understood. Take, for example, a human being; a median 
line drawn vertically would divide him into two similar halves— 
upon each side would be an eye, an arm, a leg, ete., of similar 
shape and construction. So far at least as the external features 
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