GASTEROPODA. 395 



(Gould), represented in Figs. 185, 186, and 187, are also well-known 

 species. 



We take leave of our little friends the Headless Mollusca or 

 Acephalte, and direct our attention to those molluscs to which Nature 

 has been more generous, and furnished with a head. This head, how- 

 ever, is still carried humbly ; it is not yet os sublime dedit ; it is drawn 

 along an inch or so from the ground, and in no respect resembles the 

 proud and magnificent organ which crowns and adorns the body of the 

 greater and more perfectly organized animals. 



The organization of the Cephalous Mollusca present three principal 

 types, which has led to their being divided into three classes, after 

 their more salient characteristics of form and locomotive apparatus ; 

 namely, Gasteropoda, Pteropoda, and Cephalopoda. 



In the class Gasteropoda (from jao-rrjp, belly, 7701)9, gen.7roo9, foot) 

 the locomotive apparatus consists of a flattened muscular disk, placed 

 under the belly of the animal, aided by which it creeps. The Snail 

 (Helix), the Slug (Limax), and the Cowrie, (Cyprea), are types of 

 this class. 



In the Pteropoda, from irrepov, wing, and TTOVS, foot, the locomotive 

 apparatus assumes the form of wings, or membranous swimming- 

 fins, placed on each side of the neck. The Hyalea and Clio are types 

 of this class. 



In the Cephalopoda, from Ke^aXrj, head, and TroO?, foot, the locomo- 

 tive apparatus consists of arms, or tentacles, which surround the mouth 

 in numbers more or less considerable. The Cuttle-fish (Sepia), and 

 the Poulpes (Octopoda) are types of this last class. 



The Molluscous Gasteropoda have the organs of respiration formed 

 for aerial respiration, or for respiration under water. 



This physiological arrangement involves important differences in 

 internal organization in these molluscs, and renders it convenient to 

 divide them inta two secondary groups ; namely, Pulmonary Gastero- 

 pods, which breathe in .the air, and by a species of lung, and Non- 

 pulmonary Gasteropods, which breathe in the water, by means of 

 branchia3 or gills. 



