GASTEBOPODOtIS MOLLFSCA. 



245 



The heart is composed of an auricle and ventricle. Its 

 position, with that of the respiratory tube, is always on 

 the side opposite ,to that towards which the shell is 

 coiled. 



As the genera of this class are terrestrial, lacustrine, and 

 marine, the organs of respiration are modified in accordance 

 with the habitat of the animal. 



The terrestrial and some lacustrine species breathe by a 

 lung, situated at the anterior and superior part of the body, 

 which opens and shuts, at the will of the animal, by the 

 action of a muscular sphincter. 



On the walls of this air-sac a beautiful net-work of pul- 

 monary vessels ramify, whereby the blood is aerated. The 

 species organised for an aquatic life possess branchiae, the 

 form, structure, and arrangement of which vary much in 

 this class. So permanent are the characters afforded by 

 these organs, that they form the basis of their methodical 

 distribution into orders, as shown in the following table : 



THE GASTEROPODA, 



Naked 

 and 

 disposed 



fwith a shelll 



under the free I C m P 8ed f I 

 border of the 



mantle 



I one or many f 



1 P 5 f ces J 

 without a 1 

 t shell . . J 

 In symmetrical tufts on the back 



CYCLOBRANCHIATA 

 INFEROBRANCHIATA. 



NuDIBRANCHIATA. 



The gasteropoda are found in the rocks of the Silurian and 

 of all subsequent periods ; their numbers gradually augment, 

 and their forms become more and more varied in the second- 

 ary and tertiary strata ; but their greatest development was 

 reserved for the seas of the modern epoch. 



The remains of this class are highly important to the 

 geologist, as they afford him unequivocal evidence of the 

 fluviatile, lacustrine, and marine conditions under which 



