236 ELEMENTS OF SCIENCE 



breathe air, but not by lungs. Instead of one windpipe, 

 they have many, which, opening on different parts of the 

 external surface of the body, ramify inwards, and carry 

 air (for respiration) to all parts of the frame. Scorpions, 

 however, do not thus breathe, but by means of a series 

 of small sacs, which open in the under surface of the 

 body and admit air within them. The jaws of arthro- 

 pods are quite different from those of vertebrates, and 



a 



THE SNAIL (Helix pomatia). 



a The hinder termination of the 



intestine. 



ga Generative aperture. 

 I Lip. 



p ' Shell margin. 



pi" Respiratory aperture. 



t' So-called olfactory tentacle. 



t Eye tentacle. 



Shell. 



(After Howes.} 



bite laterally, and neither from above, downwards, nor 

 from before backwards. 



The other sub-kingdom to be noticed here, is that of 

 the molluscs (Mollusca), and embraces all snails, whelks, 

 cuttle-fishes, oysters, mussels, &c. Almost all of them 

 breathe in water by means of gills, but a few, as the 

 snails, perform aerial respiration by the aid of a small 

 sac which admits air within it. 



The snail is an organism which contrasts greatly with 



