A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment in ~ the body, called the coelom. The offal of the coelom are supported by the aqueous fluid, which additionally resists outside compression. This compartment is under hydrostatic pressure because of the fluid and also supports the other organs the the organism. This form of skeletal system is uncovered in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and also other invertebrates (Figure 19.2).

You are watching: An insect has primarily _____ muscle attached to the exoskeleton.

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Figure 19.2. The skeleton the the red-knobbed sea star (Protoreaster linckii) is an example of a hydrostatic skeleton. (credit: “Amada44”/Wikimedia Commons)

an exoskeleton is an external skeleton that consists of a tough encasement on the surface of an organism. For example, the shells of crabs and insects space exoskeletons (Figure 19.3). This skeleton kind provides defence against predators, support the body, and allows for activity through the contraction of attached muscles. Just like vertebrates, muscles need to cross a joint inside the exoskeleton. Shortening that the muscle transforms the connection of the 2 segments of the exoskeleton. Arthropods such as crabs and also lobsters have actually exoskeletons the consist the 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 additional strengthened through the enhancement of calcium lead carbonate in organisms such together the lobster. Because the exoskeleton is acellular, arthropods need to periodically burned their exoskeletons because the exoskeleton does not thrive as the organism grows.

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Figure 19.3. Muscles attached to the exoskeleton the the Halloween crab (Gecarcinus quadratus) enable it to move.

an endoskeleton is a skeleton that consists of hard, mineralized structures situated within the soft organization of organisms. An example of a primitive endoskeletal structure is the spicules that sponges. The skeleton of vertebrates room composed of tissues, vice versa, sponges have actually no true organization (Figure 19.4). Endoskeletons provide support for the body, safeguard internal organs, and allow for activity through contraction of muscles attached come the skeleton.

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Figure 19.4. The skeletons the humans and horses are examples of endoskeletons. (credit: Ross Murphy)
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The axial skeleton develops the central axis that the body and also includes the skeleton of the skull, ossicles the the center ear, hyoid bone the the throat, vertebral column, and the thoracic cage (ribcage) (Figure 19.5). The function of the axial skeleton is to carry out support and protection for the brain, the spinal cord, and also the guts in the ventral human body cavity. It provides a surface ar for the attachments of muscles that move the head, neck, and trunk, performs respiratory movements, and stabilizes parts of the appendicular skeleton.

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Figure 19.5. The axial skeleton is composed of the skeletal of the skull, ossicles that the center ear, hyoid bone, vertebral column, and also rib cage. (credit: change of occupational by Mariana Ruiz Villareal)