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CHAPTER XIV. 



CEPHALOUS MOLLUSC A. 

 GASTEROPODA. 



We take leave of our little friends the Headless MoUusca or 

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

 has been more generous, and which are furnished with a head. This 

 head, however, is still carried humbly ; it is not yet os sublime dedit ; 

 it is dra\vn 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 organised animals. 



The organisation of the Cephalous Mollusca present three prin- 

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

 after their more salient characteristics of form and locomotive appa- 

 ratus ; namely, Gasteropoda, Pteropoda, and Cephalopoda. 



In the class Gasteropoda (from yao-r^p, belly, irods, gen. ToShs, foot), 

 locomotion is effected by means of a flattened muscular disk, placed 

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

 Snail {Helix), the Slug {Limax), and the CowTie {Cyprea), are tyjjes 

 of this class. 



In the Pteropoda (from impbx], wing, and -n-oDs, foot), locomotion is 

 effected by appendages in the form of wings, or membranous swim- 

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

 are types of this class. 



In the Cephalopoda (from Kf<pa\7i, head, and ttoDs, foot), locomotion 

 is effected partly by means of a set of arms, or tentacles, which 

 surround the mouth in numbers more or less considerable. The 

 Cuttle-fish {Sepia), and the Poulpes {Octopus) 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 



