Explain how animals in vertebrata are classified into further subgroups.

Vertebrates classified into further subgroups on the basis of simple to complex form , the structure of the organs like gills/lungs,  heart and function of the organisms .

For example, fishes have two chambered heart, amphibians and reptiles have three chambered heart where as aves and mammals have four chambered heart to keep oxygenated and deoxygenated blood separately.