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Biology LibreTexts

38.1B: Types of Skeletal Systems

  • Page ID
  • The hydrostatic skeleton, exoskeleton, and endoskeleton support, protect, and provide movement to the bodies of different types of animals.


    Differentiate among skeletal types: hydrostatic skeleton, exoskeleton, and endoskeleton


    Key Points

    • In organisms with hydrostatic skeletons, the muscles contract to change the shape of the coelom, which then produces movement due to the pressure of the fluid inside the fluid-filled cavity.
    • Exoskeletons are external skeletal systems that are made up of chitin and calcium carbonate.
    • Organisms with an endoskeleton are supported by a hard, mineralized skeletal system that resides inside the body.
    • In vertebrates, the endoskeleton system is further divided into the axial skeleton and appendicular skeleton.

    Key Terms

    • coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
    • peristalsis: the rhythmic, wave-like contraction and relaxation of muscles which propagates in a wave down a muscular tube
    • endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage
    • exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda

    Types of Skeletal Designs

    A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. There are three different skeleton designs that provide organisms these functions: hydrostatic skeleton, exoskeleton, and endoskeleton.

    Hydrostatic Skeleton

    A hydrostatic skeleton is one formed by a fluid-filled compartment within the body: the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism. This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates.


    Hydrostatic skeleton: The skeleton of the red-knobbed sea star (Protoreaster linckii) is an example of a hydrostatic skeleton.

    Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions (peristalsis) of the skeletal muscle of the body wall hydrostatic skeleton, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.


    An exoskeleton is an external, hard, encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons. This skeleton type provides defense against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods, such as crabs and lobsters, have exoskeletons that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a strong-but-flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular and does not grow as the organism grows, arthropods must periodically shed their exoskeletons.


    Exoskeletons: Muscles attached to the exoskeleton of the Halloween crab (Gecarcinus quadratus) allow it to move.


    An endoskeleton consists of hard, mineralized structures located within the soft tissue of organisms. An example of a primitive endoskeletal structure is the spicule of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues. Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton.


    Endoskeletons: The skeletons of humans and horses are examples of endoskeletons. They provide bodies with support, protection of organs, and aid in movement.

    The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle).