invertebrate, any animal lacking a backbone. The invertebrates include the tunicates and lancelets of phylum Chordata, as well as all animal phyla other than Chordata. The major invertebrate phyla include: the sponges (Porifera), coelenterates (Cnidaria), echinoderms (Echinodermata), flatworms (Platyhelminthes), roundworms (Nematoda), segmented worms (Annelida), mollusks (Mollusca), and arthropods (Arthropoda). Invertebrates are tremendously diverse, ranging from microscopic wormlike mezozoans (see Mezozoa) to very large animals such as the giant squid. Approximately 95% of all the earth's animal species are invertebrates; of these the vast majority are insects and other arthropods. Invertebrates are important as parasites and are essential elements of all ecological communities.

Any animal that lacks a vertebral column, or backbone. They include the protozoans, annelids, cnidarians, echinoderms, flatworms, nematodes, mollusks, and arthropods. More than 90percnt of living animals are invertebrates. Worldwide in distribution, they range in size from minute protozoans to giant squids. Apart from the absence of a vertebral column, invertebrates have little in common. They are generally soft-bodied and have an external skeleton for muscle attachment and protection. Seealso vertebrate.

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