CLASS GASTEROPODA 



THE 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 

 same 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, except 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 moll ask, is the fact that they are always asym- 

 metricalthat 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, etc., of similar 

 shape and construction. So far at least as the external features. 



