Think of the Animal Kingdom (scientifically known as Kingdom Animalia) as the world’s most diverse family tree. It includes everything from the tiny flea to the massive blue whale.
To be part of this “club,” an organism generally needs to meet three criteria: it must be multicellular, it must consume other life for energy (rather than making food from sunlight like plants), and it must be able to move at some point in its life.
The Major Branches
Scientists group animals based on their body structure and ancestry. The biggest divide is between those with backbones and those without.