Why Are Arthropods Invertebrates?

Of all the subsets of the animal kingdom, there is none more populous than the phylum Arthropoda. Arthropods are invertebrates with exoskeletons, which are bony coverings that protect and give shape to their soft internal organs. Termites that nest underground, dragonflies that skim over ponds and shrimp that scuttle across the seabed are all examples of arthropods. Arthropods are classified as invertebrates because they lack a backbone. Conversely, all animals with backbones are considered vertebrates.

  1. Invertebrates

    • Invertebrates like this one lack the segmentation, exoskeleton and jointed limbs of arthropods.

      Animal Planet's online encyclopedia of wildlife reports that about 98 percent of all animals lack a backbone and are therefore invertebrates. According to the same source, the most prolific invertebrates on earth are the arthropods, which make up about 75 percent of all animal species. Arthropods are the only invertebrates with a skeletal support system and joints. The rest are soft-bodied like worms and jellyfish.

    The History of Arthropods

    • Fossilized arthropods, like the one pictured, can resemble modern horseshoe crabs.

      According to the interactive guide "The Arthropod Story", arthropods got their start 530 to 570 million years ago in the Cambrian period. All animals of the Cambrian period were sea-dwelling invertebrates. Cambrian fossils often bear the imprint of bizarre and extinct species. However, all of the traits unique to arthropods, like segmented bodies and jointed legs, are present in some Cambrian fossils.

    Comparing Arthropods and Vertebrates

    • The mantis shrimp is a crustacean, an arthropod type found mainly in the sea.

      Although they lack a backbone, arthropods have something in common with vertebrates -- a skeleton. The skeletons of both vertebrates and arthropods have some of the same functions -- locomotion, protection and support. Three features that distinguish arthropods from vertebrates are their small size, segmented bodies, and compound eyes.

    Arthropod Anatomy

    • The small size of insects and other land dwelling arthropods reflects the limitations of the exoskeleton.

      All arthropods have compound eyes, segmented bodies, jointed limbsand an exoskeleton. The arthropod exoskeleton separates it from other invertebrates, but it limits it as well. The exoskeleton gives arthropods a protective armor that other invertebrates do not have, but it has also prevented arthropods from growing as large as vertebrates can. According to "The Arthropod Story", molting, breathing and moving are more difficult for large arthropods than for smaller ones.