This document classifies animals according to their structure, diet, and reproduction methods. It divides animals into two main groups: vertebrates, which have backbones, and invertebrates, which do not. Vertebrates are further divided into mammals, birds, reptiles, amphibians, and fish. Mammals are distinguished by traits like nursing their young with milk and being warm-blooded. Reptiles lay eggs but some give live birth. Amphibians undergo metamorphosis from aquatic young to terrestrial adults. Invertebrates make up over 90% of animal species but have no backbone.