The five main groups of vertebrates, known as classes, are: fish, amphibians, reptiles, birds and mammals. Vertebrate species are assigned groups based on shared traits, such as body covering, the ability to maintain a steady temperature and the number of chambers in the heart, among others. Some groups of birds and mammals, called clades, are grouped according to a shared ancestry.
Fish are the largest and most diverse group of vertebrates. Fish are the oldest of the vertebrate classes, breathe through gills, and both live in and lay eggs in the water. Fish are ectothermic, and vary their body temperature to match the surrounding environment.
Amphibians are also ectothermic and, like fish, lay their eggs in water. Larval amphibians have gills, but the adult stages of almost every species breathes air through lungs. Amphibians have moist, permeable skin that cannot be allowed to dry out.
Reptiles are ectothermic, air-breathing vertebrates that breed on land. Their skin is usually scaly, and different groups of reptiles either lay a water-resistant egg or give birth to live young.
Mammals are derived from ancestral synapsid reptiles. They maintain a high body temperature, generally give birth to live young and are usually covered with hair. Birds are descended from a different line of reptiles, lay eggs and are covered with feathers. Birds are also capable of maintaining a constant temperature.