An invertebrate is any animal without a backbone. While you have a spine made of connected bones running down your back, many invertebrates have no bones inside their bodies at all. This enormous group includes insects like beetles and butterflies, sea creatures like jellyfish and octopuses, worms, spiders, snails, and countless others.

In fact, about 97% of all animal species on Earth are invertebrates. They vastly outnumber animals with backbones, called vertebrates (like mammals, birds, reptiles, amphibians, and fish). Some invertebrates have hard outer shells or exoskeletons for protection, like crabs and lobsters. Others, like slugs and octopuses, have soft bodies with no hard parts at all.

Invertebrates come in astonishing variety. A tiny ant, a giant squid, a garden snail, and a tarantula are all invertebrates, even though they look completely different and live in different environments. What they share is the absence of an internal skeleton made of bone.

Scientists use this term to group together an incredibly diverse collection of animals based on what they lack, rather than what they have in common otherwise.