Vertebrate

B

Cartilaginous Fish

Cartilaginous fish do have jaws and use them to deadly effect. Numbering about 1,000 species, they include sharks, skates, and rays, as well as chimaeras, also known as ratfish. Cartilaginous fish are widespread throughout the world's oceans. Most skates and rays feed on or near the seabed, but sharks typically hunt in open water.

C

Bony Fish

Bony fish are some of the most successful vertebrates alive today. These animals can be found in a vast variety of habitats, from coral reefs and the deep-sea bed to lakes hidden away in caves. As their name indicates, bony fish have a skeleton made of bone, and most also have an air-filled sac called a swim bladder that keeps them buoyant. At least 24,000 species of bony fish have been identified, and many more probably await discovery. Common bony fish include salmon, sturgeon, and cod.

D

Amphibians

Amphibians make up the smallest of the four groups of tetrapods, with about 4,000 species. Most amphibians, such as frogs and toads, live in damp habitats. Like fish, the majority of amphibians reproduce by laying eggs. Amphibians usually lay their eggs in water, because they dry out quickly in air. The eggs produce swimming, fishlike young called tadpoles, which develop limbs and lungs as they mature.

E

Reptiles

Compared to amphibians, reptiles are much more fully adapted to life on land. They have scaly, waterproof skin, and they either give birth to live young or lay eggs with waterproof shells. There are about 7,000 species alive today, including snakes, alligators, and turtles. During the age of the dinosaurs, about 230 million to 65 million years ago, reptiles outnumbered all other land vertebrates put together.

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F

Birds

Birds evolved from flightless reptiles but underwent some major changes in body form during their evolution. Of the roughly 10,000 species alive today, most have lightweight, air-filled bones, and all have a unique and highly efficient respiratory system that is found in no other group of vertebrates.

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