GASTEROPODA. 



49 



The organs of digestion also manifest much diversity ; the 

 mouth is formed by contractile lips, which by many can be 

 protruded in the manner of a proboscis ; the tongue is a long 

 band (with a few exceptions) beset with numerous little 

 horny hooks disposed in rows, and directed backwards ; 

 these reduce the food to a state to be swallowed, and it is 

 then received into the stomach, which frequently possesses 

 rows of calcareous teeth which help the further commi- 

 nution. 



In most Mollusca the mantle forms a shell, which con- 

 sists of animal matter, in which is incorporated more or 

 less carbonate of lime. In that genus of land snails 

 which Ferussac has called Arion, there are only uncon- 

 nected morsels of chalk to be found ; in most cases, how- 

 ever, the shell obtains considerable consistence, strength, 

 and often great weight. It is seldom covered so entirely 

 by the mantle as to be called internal, but is usually ex- 

 terior and of sufficient size to admit of the animal with- 

 drawing within it completely. A second piece of horny 

 or calcareous shell adheres in many Gasteropoda to the 

 foot, and when the Mollusc retires into its house, it closes 

 the entrance more or less completely, whence it is called 

 the operculum. Of all the Gasteropoda the shell is formed 

 of one single piece, except the genus Chiton, which forms 

 its shell of eight portions overlapping each other, forming 

 a very curious exception. 



All the Gasteropoda are propagated by eggs, either 

 hatched in the body of the parent, or attached by it to 

 plants, &c. The young in some cases, on leaving the 

 egg, perfectly resemble the parent, in other cases, they 

 undergo a remarkable metamorphosis. 



Proportionally few live on land, and then chiefly in 

 moist places ; yet fewer live in fresh water ; the greater 

 portion of Gasteropoda being inhabitants of the ocean. 



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