THE FRESH WATERS 177 



the toad buries itself among withered leaves, 

 or in some dry spot, while the frog returns 

 to the pond or ditch and hides itself in a hole, 

 perhaps in a drain-pipe, it may even be in 

 the damp mud. 



But the frog has long ago lost its gills, and 

 its lungs are closed in winter by the shutting ' 

 of the nostrils. How then does it breathe, for 

 breathe it must, even though the fires of life 

 are very low? The skin is exceedingly thin 

 and delicate, and there is a network of very 

 fine blood-vessels all over it, and the exchange 

 of gases used carbonic acid gas for fresh 

 oxygen, which is the essential part of breath- 

 ing takes place directly from these blood- 

 vessels through the skin. 



WATER INSECTS 



One more group we must think of for a 

 little the aquatic insects. 



All fully developed insects breathe air 

 through little openings on the surface lead- 

 ing into fine tubes, called tracheae, which 

 carry the air to all parts of the body. Even 

 insects which spend their whole lives on and 

 in the water breathe in this way. They may 



