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 trachee, 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 
