Invertebrates are animals without a backbone and make up over 95 percent of known animal species. Major groups include insects, spiders, worms, and mollusks.
Insects have segmented bodies and six legs; spiders have eight legs; worms have soft, segmented bodies; and mollusks such as clams and octopuses have soft bodies, often with shells.
Invertebrates are vital to ecosystems as pollinators, decomposers, and food for other animals.