Vertebrates are animals with backbones. They can be classified according to their features, and include bony fish, amphibians, reptiles, birds and mammals. 

Bony fish 

Bony fish absorb oxygen through their gills and they have wet scales. Their body temperature varies according to their surroundings (organisms that do this are called ectothermic animals). Bony fish reproduce by external fertilization and lay eggs (they are oviparous). 

Amphibians 

Amphibians usually have lungs but they can also absorb oxygen through their moist skin. Their body temperature also varies according to their surroundings (ectothermic animals). Amphibians also reproduce by external fertilization and lay eggs. 

Reptiles 

Reptiles have lungs and dry scales. As with bony fish and amphibians, their body temperature varies according to their surroundings. However, reptiles reproduce by internal fertilization and lay eggs. 

Birds 

Birds have lungs, feathers and beaks. They can maintain their body temperature (organisms that do this are called endothermic animals). Like reptiles, birds reproduce by internal fertilization and lay eggs. 

Mammals 

Mammals have lungs and fur. Like birds, they can maintain their body temperature. Mammals reproduce by internal fertilization and they give birth to live young (they are viviparous).