112 A COURSE ON ZOOLOGY. 



truly amphibious. In order thus to live in water like a 

 fish, the frog, the toad, and the salamander must possess 

 respiratory organs adapted to aquatic life ; and in order to 

 live in the air like man, the dog, or the lizard, they must 

 also possess the organs necessary for aerial respiration. 



The frog, the toad, and the salamander have indeed 

 lungs. We can satisfy ourselves on this point by watch- 

 ing them for a moment, when we will observe the move- 

 ments of swallowing, by which they replace the respira- 

 tory movements, as we have already seen. But we would 

 seek in vain in these animals any respiratory organs 

 adapted for aquatic life, such as we find in the fish. 



The aquatic respiration of the frog is not effected by 

 any special organ : it takes place over the entire surface 

 of the skin. The latter is thin, always moist, very porous 

 to the air, and the blood which circulates under it comes 

 into almost immediate contact with the atmosphere. 

 The cutaneous respiration is thus so active that it is able 

 to maintain the life of the animal under water. Frogs 

 from which the lungs had been entirely removed have 

 continued to live several months under water by cuta- 

 neous respiration only, while others, whose skin had 

 been covered with an impermeable varnish, preventing 

 entirely the passage of air, soon died, and apparently of 

 suffocation. If the skin of a frog be dried, the animal 

 quickly dies, because the drying prevents the passage of 

 air to the blood beneath. These facts explain why am- 

 phibians always select for their abodes cool and moist 

 places, such as the banks of streams and ponds. 



An important peculiarity of frogs and other animals 

 of the same group is the metamorphosis that accompanies 

 their development. The eggs are laid in considerable 

 numbers in marshes and ponds, and are agglutinated 



