Chordata: Class Aves

The birds evolved from reptiles in the Jurassic period. Their front limbs are modified into wings, and the breastbone is greatly enlarged to support flight muscles. They have an insulating covering of feathers, which has been an important factor in their ability to regulate body temperature. The other advance that enabled birds to become warm-blooded was the evolution of a four-chambered heart, making the circulatory system a complete double circuit: oxygenated blood is pumped from the lungs to the tissues, and deoxygenated blood is pumped from the tissues to the lungs. The only major group besides insects to invade the air, birds are much less restricted by external temperature requirements than cold-blooded animals, and they have spread throughout every part of the world. They live in many kinds of habitat and have evolved a diversity of forms. Some have become flightless terrestrial animals, while others are aquatic, using their wings for swimming instead of or in addition to flying. Fertilization is internal. The eggs of birds are similar to those of reptiles, but parental care of the eggs and young is highly developed.

Sections in this article:

• Introduction
• Class Mammalia
• Class Aves
• Class Reptilia
• Class Amphibia
• Class Sacropterygii
• Class Actinopterygii
• Class Chondrichthyes
• Class Placodermi
• Classes Myxini and Hyperoartia
• Subphylum Vertebrata
• Subphylum Cephalochordata
• Subphylum Urochordata
• Bibliography

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