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Class Amphibia

(frogs, toads, and salamanders)

Class Reptilia

(lizards, snakes, and turtles)

Class Aves (birds)

Class Mammalia

(mammals)

Class Myxini

(hagfishes)

Class

Cephalaspidomorphi

(lampreys)

Class

Chondrichthyes

(sharks, skates, rays, and ratfishes)

Class

Actinopterygii

(ray-finned fishes)

Class Sarcopterygii

(lobe-finned fishes)

Class Amphibia

(frogs, toads, and salamanders)

Class Reptilia

(lizards, snakes, and turtles)

Class Aves (birds)

Class Mammalia

(mammals)

• Lobe-finned Fishes (class Sarcopterygii)—These fishes have fins that are supported by a main axis of bone. There are two living groups of lobe-finned fishes: lungfishes and the coelacanth. Extinct lobe-finned fishes are thought to be the ancestors of amphibians.

• Amphibians (class Amphibia)—About 4,880 species of frogs, toads, and salamanders belong to this group. Their skin is thin and is permeable to gases and water. Most species lay their eggs in water and pass through an aquatic larval stage.

• Reptiles (class Reptilia)—This group includes turtles, crocodiles, alligators, lizards, and snakes. The skin of reptiles is dry and scaly. The eggs of reptiles protect the embryo from drying out and can be laid on land. There are about 8,000 species.

• Birds (class Aves)—Birds are characterized by adaptations that enable flight, including feathers, hollow bones, and a unique respiratory system. There are over 10,000 species.

• Mammals (class Mammalia)—Humans, cats, mice, and horses are among the members of this group. All mammals have hair and nurse their young with milk. There are about 4,400 species.

Figure 39-1 shows the relationships among the nine classes of living vertebrates.

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