CHAPTER VII. 



GASTEROPODS — THE STRUCTURE OP SHELLS VARIOUS EXAMPLES. 



The gasteropoda next demand our notice ; these ani- 

 mals constitute a numerous class, comprising groups 

 both aquatic and terrestrial, and therefore constructed, 

 some for the respiration of air, others of water. But 

 it is interesting to find that many aquatic species, as 

 the limneus and the planorbis, are organized for the 

 respiration of air, for which purpose they come to the 

 surface of the ponds or stagnant waters in which they 

 habitually dwell. The term gasteropoda, which means 

 animals having the under surface of the body organized 

 as a foot, is given to this class, because, as we see in 

 the common snail, they creep along upon a fleshy disc, 

 placed under the body, by means of the expansive and 

 contractile movements of the muscular fibres of which 

 it is composed. 



The limnese, or slug, too, as it pursues a similar 

 course, emits a viscous matter, which serves to attach it 

 to the bodies over which it moves. By means of this sort 



H 



