ORDER IV. _The Dipnoi, or Lung Fishes._ This is a very small group. In many respects they are more like amphibians than fishes, the swim bladder being used as a lung. They live in tropical Africa, South America, and Australia, inhabiting the rivers and lakes there. Characteristics of Amphibia.--The frog belongs to the class of vertebrates known as Amphibia. As the name indicates (_amphi_, both, and _bia_, life), members of this group live both in water and on land. In the earlier stages of their development they take oxygen into the blood by means of gills. When adult, however, they breathe by means of lungs. At all times, but especially during the winter, the skin serves as a breathing organ. The skin is soft and unprotected by bony plates or scales. The heart has three chambers, two auricles and one ventricle. Most amphibians undergo a complete metamorphosis, or change of form, the young being unlike the adults. [Illustration: Newt. (From a photograph loaned by the American Museum of Natural History.) About natural size.] [Illustration: The leopard frog, an amphibian.] CLASSIFICATION OF AMPHIBIA ORDER I. _Urodela._ Amphibia having usually poorly developed appendages. Tail persistent through life. Examples: mud puppy, newt, salamander. ORDER II. _Anura._ Tailless Amphibia, which undergo a metamorphosis, breathing by gills in larval state, by lungs in adult state. Examples: toad and frog. Characteristics of Reptilia.--These animals are characterized by having scales developed from the skin. In the turtle they have become bony and are connected with the internal skeleton. Reptiles always breathe by means of lungs, differing in this respect from the amphibians. They show their distant relationship to birds in that their large eggs are incased in a leathery, limy shell. [Illustration: Box tortoise, a land reptile. (From photograph loaned by the American Museum of Natural History.) About one fourth natural size.] [Illustration: The gila monster, a poisonous lizard. About one twelfth natural size.] CLASSIFICATION OF REPTILES ORDER I. _Chelonia_ (turtles and tortoises). Flattened reptiles with body inclosed in bony case. No teeth or sternum (breastbone). Examples: snapping turtle, box tortoise. ORDER II. _Lacertilia_ (lizards). Body covered with scales, usually having two-paired appendages. Breathe by lungs. Examples: fence lizard, horned toad. [Illustration: The common garter snake. Reduced to about one tenth natural size.] ORDER III. _Ophidia_ (snakes). Body elongated, covered with scales. No limbs present. Examples: garter snake, rattlesnake. ORDER IV. _Crocodilia._ Fresh-water reptiles with elongated body and bony scales on skin. Two-paired limbs. Examples: alligator, crocodile.