This document classifies animals into two main groups: vertebrates and invertebrates. Vertebrates are further divided into five subclasses based on physical characteristics: fish, amphibians, reptiles, birds, and mammals. Each subclass shares defining traits such as how they breathe, whether they have scales or feathers, how they reproduce, and whether their body temperature changes with the environment or remains constant. Invertebrates either have a hard outer shell like insects and crabs or a softer body without a shell like jellyfish and worms, with examples given like spiders, shrimp, and snails.