More than 98 percent of all animal species have no bony vertebrae or skeletons and are collectively classified as invertebrates. Jellyfish, octopuses, worms, snails, crabs and lobsters, insects, squid and sea stars are some common examples. The invertebrate designation has no bearing on the biological traits or characteristics of the species. More than 30 animal groups are included in the invertebrate classification.
Some of the most commonly known examples of invertebrates are arthropods, including spiders, mites, insects and crustaceans; molluscs, including snails, oysters, clams and octopuses; cnidarians, including jellyfish, corals and sea anemones; and echinoderms, including starfish, sea cucumbers and sea urchins.
Because they have no bones, invertebrate organisms have developed alternate methods of movement, support and protection. Snails grow shells that are large enough to house their bodies. Insects, crabs, lobsters and other crustaceans have developed exoskeletons, which are hard outer coverings that encase and protect their soft tissues. Squid and octopuses have developed layers of muscles that allow them to squeeze and stretch their bodies to navigate ocean environments.
Invertebrates were the first animals to evolve from single-celled, food-eating organisms. Sponges are the most primitive invertebrates, as they obtain food and respiration through a simple water-filtering mechanism. Other animals have developed more intelligent methods of feeding: Octopuses are able to open jars that contain live shrimp and can solve similar problems to obtain food.
Many invertebrates survive by forming groups or colonies and remaining together most of their lives. Living in colonies provides protection in numbers, and sharing food resources and raising young as a group increase the chance of survival. Coral and jellyfish are examples of marine invertebrates that live in colonies. Well-known colonial land invertebrates include honeybees, wasps and ants.