Every animal begins life as a hollow sphere of cells called a blastula

The defining characteristic of the kingdom Animalia is the blastula, a hollow ball of cells formed during the early stages of an embryo's life. This structure, which appeared roughly 650 million years ago, allows for gastrulation—the process where cells migrate inward to form the distinct tissue layers that eventually become skin, muscle, and organs. Even sponges, the most primitive animals, produce a swimming blastula larva before settling into their adult forms.