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CHAPTER XIV. 
CEPHALOUS MOLLUSCA. 
GASTEROPODA. 
WE take leave of our little friends the Headless Mollusca 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 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 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 -yaorhp, belly, nots, gen. rodds, 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 
of this class. 
In the Preropoda (from wrepbv, wing, and rots, 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 Cho 
are types of this class. 
In the Cephalopoda (from neparh, head, and pois, foot), locomotion 
is effected partly by means of a set of arms, or tentacles, which 
surround the-mouth in numbers more or iess: considerable. The 
Cuttle-fish (Sepia), and the Poulpes (Octopus) are types of this last 
class. 
The Mottuscous GasTERopopA have the organs of respiration 
formed for _aérial respiration, or for respiration under water. 
This physiological arrangement involves important differences in 
