All vertebrates are further classified into 5 classes as follows:
- Pisces: These are typically aquatic vertebrates. Their skin is covered with scales or plates. They obtain oxygen dissolved in water by using gills. They have a streamlined body and a muscular tail for movement. They are cold-blooded and their hearts are 2-chambered, They lay eggs.
- Amphibia: These animals can live both, on land and in water. The animals breathe through the skin when in water and through the lungs when on land. Animals are cold-blooded and have 3-chambered hearts. Skin lacks scales but has mucus glands. They lay eggs.
- Reptilia: They are crawling vertebrates. They are cold-blooded, have scales, and breathe through the lungs. They have a 3-chambered heart with crocodiles being an exception (heart is 4-chambered). They lay eggs with tough shells.
- Aves: These vertebrates are adapted for an aerial mode of life. Their body is covered with feathers and fore-limbs are modified into wings. They are warm-blooded and possess a 4-chambered heart. Their bones are hollow.
- Mammalia: These vertebrates are warm-blooded animals with 4-chambered hearts. They have hairs on the skin as well as sweat and oil glands. They have mammary glands for nourishing the young ones. Most mammals are viviparous while some mammals lay eggs or give birth to poorly developed young ones.