194 Descriptive Zoology. 



young being hatched in the form of the adult. A few forms bring forth 

 the young alive. 



CHARACTERISTICS OF AMPHIBIA. 



The Amphibia breathe by gills in the larval state, but 

 generally develop lungs in becoming adult. They have no 

 fin rays as with the fishes, but usually have paired limbs 

 with distinct fingers and toes. 



Aquatic Life vs. Terrestrial Life. Several lines of investigation 

 converge to prove that at one time the globe was entirely covered by 

 water. Of course until there was land there could be no land life. 

 What were the first forms of life on the land ? Did some of the 

 forms of aquatic animals gradually become fitted for living on land and 

 then desert the water ? 



The study of the amphibia seems to throw some light on these ques- 

 tions. In the ganoid fishes we saw that the swim bladder had some cir- 

 culation of blood in its walls and that there was an opening from the. 

 gullet into the swim bladder, which is, in action at least, a sort of rudi- 

 mentary lung. The lungfishes have the air bladder still more com- 

 pletely developed as a lung. The lungfishes came to the very door of 

 land life, but remained aquatic. The Amphibia boldly stepped over the 

 threshold and were, probably, among the first of the animal kingdom to 

 emerge from the water to live upon the land. This transition from life 

 in the water to life upon the land marks a great step upward. Perhaps it 

 would be hard to believe that any group of animals had made such a 

 profound change, if it were not for the fact that we see this same change 

 in the individual life of every amphibian. 



Gradation among the Amphibia. It is interesting and instructive to 

 observe the successive progress in the various groups of amphibians. 

 If we consider in succession the siren, mud puppy, salamander, and frog, 

 we see that they all have traveled along the same road, only some have 

 gone farther than others. They all start as limbless tadpoles, with gills, 

 practically in the same stage of development as the fishes. The siren 

 develops one pair of legs, retains the legs and gills, although developing 

 lungs, and goes no farther. The mud puppy makes a slight advance 

 by developing another pair of legs. It retains the two pairs of legs and 

 the gills, at the same time breathing, in part, by means of lungs. The 

 salamander takes a decided step upward when it sheds the gills and 



