264 



AN INTRODUCTION TO ZOOLOGY. 



had these fish-like vertebree, and also an external 

 ai'mour ; and they have been considered to afford a 

 link between Amphibians and Ganoid Fishes. The 

 Dipnoi also are, as already stated, links between Fishes 

 and Amphibia. 



Fig. 97.— Successive stages of the development of the Frog, from the Fish- 

 stage to the Amphibian. 



Characters of Amphibia. The possession of a 

 larval stage is the most marked peculiarity of Am- 

 phibians among vertebrata. The adult stage, being 

 chiefly adapted for terrestrial life, has limbs with 

 finger-like digits, and lungs adapted for breathing 

 air. Amphibia are distinguished from Reptiles by 

 having two occipital condyles, whereas Reptiles have 

 only one. Amphibians have a characteristic kind of 

 skin. It is almost invariably without scales, but 

 provided with glands, which secrete a fluid that keeps 

 the skin moist, and is sometimes of an acrid character; 

 in which case it doubtless acts as a protection against 

 enemies. 



