THE CHITON. 145 



Numerous are the modifications displayed by the 

 digestive organs of the gasteropoda ; we shall, therefore, 

 not attempt a description, which, if applicable to one 

 group, is not so to another. Suffice it to say, that a 

 large liver, sometimes consisting of distinct portions, 

 and salivary glands are always found ; the stomach 

 is sometimes a simple cavity, and sometimes furnished 

 with a crop at its commencement. 



The gasteropoda have been denominated univalve 

 mollusks, in contradistinction to others, from their being 

 enclosed in a shell consisting of two valves, united by 

 a hinge. One genus, however, among the gasteropoda, 

 scarcely merits the title of univalve ; it is the genus 

 Chiton, which is protected above by a series of shelly 

 plates, overlaying each other, and resembling plates of 

 armour. On examining these animals, we find the back 

 covered with a tough leathery mantle of 

 an oval figure, extending considerably be- 

 yond the body of the animal beneath. 

 Along the central part of this mantle 

 are arranged the plates, usually eight in 

 number, and partially imbedded in the 

 substance of the mantle, by which they are 

 secreted : they do not, however, extend Chiton. 

 to the edge of the mantle, which is left free and 



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