252 THE SCIENCE OF LIFE. 



tadpoles are like the Urodelans till the gill and tail arc 

 absorbed. Frogs (Rand) have teeth in the upper jaw, 

 and webbed feet. Toads (Bufd) have neither teeth nor 

 webbed feet. 



4. Class III. REPTILIA, or Reptiles. These are air- 

 breathing, cold-blooded Vertebrates, differing from 

 Fishes and Amphibians by never having gills, and from 

 Birds by being covered with horny scales, or bony plates. 

 The skeleton is ossified, and never cartilaginous. Most 

 are carnivorous, and teeth are present, except in Turtles, 

 where a horny sheath covers the jaws. The lungs are 

 imperfectly cellular, and the heart is three-chambered, 

 containing two auricles and one ventricle, which is some- 

 times divided by a partition. In all cases a mixture of 

 arterial and venous blood is circulated. The limbs, when 

 present, have three or more fingers as well as toes. 



There are four orders of living and five of extinct 

 Reptiles. The living orders are Snakes, Lizards, Tur- 

 tles, and Crocodiles. 



i.) Opliidia, or Snakes. (Fig. 150.) These have no 

 visible limbs, but a vast number of vertebrae. The Py- 

 thon has two hundred and ninety-one, the Rattlesnake 

 one hundred and ninety-four, and the Boa Constrictor 

 three hundred and five. They have immovable trans- 

 parent eyelids. The tongue is bifid (cleft) and extensile. 

 The mouth is very dilatable, from the number of joints 

 in the lower jaw united only by ligament. The skin is 

 shed in one piece by reversing it. Snakes move well 

 either on land or in water. 



Poisonous snakes, as Vipers and Rattlesnakes, usually 

 have a triangular head covered with small scales, a con- 



