Ill 



THE GASTROPODA 



THE Gastropoda, or Snails, are much more active ani- 

 mals than the Lamellibranchia, most of them crawling 

 about in search of their food, which they take into the 

 mouth by means of the radula a rasping tongue set 

 with chitinous teeth. The head and foot are perma- 

 nently outside the mantle-chamber, which is consequently 

 much reduced in size, with a small opening, and in nearly 

 all cases lies on one side of the body. This last feature 

 is part of a profound asymmetry which affects all parts 

 of the body except the head and foot. In the mantle- 

 chamber of primitive genera are a pair of gills much like 

 those of Nucula ; these are reduced to one as a part of 

 the asymmetry of higher forms, and disappear alto- 

 gether in the air-breathing forms, the whole mantle- 

 chamber then serving as a lung-sac. Except in these 

 last, a ciliary mechanism exists for the purpose of respi- 

 ration, and in many it also serves to remove the excre- 

 ment from the mantle-chamber, while in some it even 

 collects microscopic food (though it cannot convey it to 

 the mouth, which lies outside the mantle-chamber). The 

 gastropod shell is distinguished from that of lamelli- 

 branchs by not being divided into right^ and left valves : 



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