A hydrostatic skeleton is a skeleton created by a fluid-filled compartment within the body, referred to as the coelom. The organs of the coelom are sustained by the aqueous fluid, which also resists outside compression. This compartment is under hydrostatic push bereason of the fluid and also supports the other organs of the organism. This form of skeletal mechanism is found in soft-bodied pets such as sea anemones, earthworms, Cnidaria, and other invertebrates (Figure 19.2).

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

An exoskeleton is an outside skeleton that is composed of a difficult encasement on the surconfront of an organism. For example, the shells of crabs and also insects are exoskeletons (Figure 19.3). This skeleton form provides defence against predators, supports the body, and enables for activity through the contraction of attached muscles. Just like vertebprices, muscles have to cross a joint inside the exoskeleton. Shortening of the muscle transforms the relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskelelots that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a solid but flexible product. Chitin is secreted by the epidermal cells. The exoskeleton is even more strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods should periodically shed their exoskelelots because the exoskeleton does not flourish as the organism grows.

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

An endoskeleton is a skeleton that is composed of hard, mineralized frameworks located within the soft tworry of organisms. An instance of a primitive endoskeletal structure is the spicules of sponges. The bones of vertebprices are created of tissues, whereas sponges have actually no true tissues (Figure 19.4). Endoskeletons carry out assistance for the body, defend inner organs, and also enable for activity via contractivity of muscles attached to the skeleton.

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Figure 19.4. The skeletons of human beings and also horses are examples of endoskeleloads. (credit: Ross Murphy)
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The axial skeleton forms the main axis of the body and also has the bones of the skull, ossicles of the middle ear, hyoid bone of the throat, vertebral column, and the thoracic cage (ribcage) (Figure 19.5). The attribute of the axial skeleton is to administer assistance and protection for the brain, the spinal cord, and the organs in the ventral body cavity. It offers a surface for the attachment of muscles that relocate the head, neck, and trunk, percreates respiratory movements, and also stabilizes parts of the appendicular skeleton.

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Figure 19.5. The axial skeleton consists of the bones of the skull, ossicles of the middle ear, hyoid bone, vertebral column, and rib cage. (credit: alteration of occupational by Mariana Ruiz Villareal)