They live in the aquatic ecosystem. They breathe oxygen from the water through their gills. Most of them live in the salt water. The majority of them do not care for their eggs or their offspring
They have a body adapted for acuatic or terrestrial life. They are oviparous but do not incubate eggs. Their body is covered with scales.
They are the most advanced species among vertebrates. They have hairy body. Sea mammals, such as the whale, have hair follicles They breathe through lungs. They are viviparous.
