Sea stars, snails, spiders, and earthworms -- what do they have in common? No backbone! They're all invertebrates, animals without backbones.
Invertebrates come in a variety of shapes and sizes, from microscopic organisms to giant octopuses. They can be found in virtually every habitat on earth, from oceans to rainforests. They may float, swim, fly, crawl, wiggle, or stay firmly attached to a surface (like sponges). The only thing all invertebrates have in common is their lack of a backbone.
Scientists estimate that as many as 99% of all animal species are invertebrates. This means that vertebrates -- animals with a backbone, including fish, amphibians, reptiles, birds, and mammals -- make up only a small fraction of life on Earth.
Scientists divide invertebrates into about 30 different groups, or phyla. These phyla include coelenterates (jellies and anemones), echinoderms (urchins and stars), mollusks (snails and octopuses), annelids (segmented worms), and arthropods (insects, spiders, and crustaceans such as crabs).