Explain, how animals in Vertebrata are classified into further sub-groups?

Vertebrates are divided into five groups, i.e. Pisces, Amphibia, Reptilia, Ayes and Mammalia on the basis of certain characteristics as follows : (a) Pisces : Exoskeleton of scales, endoskeleton of bone/cartilage, breathing through gills. (b) Amphibia : Slimmy skin, larvae with gills, lungs in adults. (c) Reptilia : Exoskeleton in the form of scales, lay eggs outside water. (d) Ayes : Exoskeleton of feathers, lay eggs outside water, most of them fly in air. (e) Mammalia : Exoskeleton of air, external ears present, most of them give birth to young ones.