- Fish breathe through gills, and live in water; most are cold-blooded and lay eggs (although sharks give birth to live young).
- Amphibians are cold-blooded and live both on land (breathing with lungs) and in water (breathing through gills) at different times. Three types of amphibians are frogs and toads, salamanders, and caecilians. Caecilians are primitive amphibians that resemble earthworms. They are found in the tropics.
- Reptiles are cold-blooded and breathe with lungs. They have scales, and most lay eggs. Reptiles include snakes, turtles and tortoises, crocodiles and alligators, and lizards. Dinosaurs were reptiles, although some scientists believe that some were warm blooded.
- Birds are warm-blooded animals with feathers and wings. They lay eggs, and most can fly (although many, including penguins and ostriches, cannot).
- Mammals are warm-blooded, and are nourished by their mothers' milk; most are born live (however, the platypus lays eggs). Most mammals also have body hair.