The skeletal system is the framework of hard, articulated structures that provide physical support, attachment for muscles, and protection for the bodies of animals. The skeletal system of invertebrate animals, particularly those in the phylum Arthropoda, is on the outside of the body, and is referred to as an exoskeleton. That of vertebrate animals in the phylum Vertebrata is on the inside of the body, and is referred to as an endoskeleton.
The external skeleton of such arthropods as crustaceans, insects, and spiders consists of a large number of rigid plates and jointed appendages made of chitin (this is a complex polysaccharide carbohydrate, consisting of numerous linked monosaccharide units). The exoskeleton provides protection, and rigid structures to which muscles can attach, allowing the body parts to be moved in a directed fashion. The exoskeleton of arthropods is divided up into functional regions, such as the abdomen, thorax, and head of insects and crustaceans, and the abdomen and cephalothorax (fused head and thorax) of spiders. Each of these units typically has numerous jointed appendages, such as legs, antennae, or mouth parts. The numbers of structural units (such as appendages) and the size, shape, and color of parts of the exoskeleton all have functional significance in terms of the biology and ecology of arthropod species. These characteristics are commonly used as the principal characters to identify species of arthropods, and to study their evolutionary relationships.
The internal skeleton of vertebrate animals is composed of bone, a rigid, mineral-rich material laid down through the metabolism of the animal as part of its developmental biology. Bone is composed mostly of the mineral calcium phosphate, and to a much lesser degree, calcium carbonate. The structural elements of bone are: marrow, periosteum, and spongy bone. During the embryonic development of vertebrate animals, the skeleton is initially laid down as cartilage (a more flexible, elastic tissue), which later ossifies, or turns into bone. Even when fully mineralized, bones is not an "inert" tissue. Bones have a blood supply, are metabolically active, are capable of being remodeled throughout the life of an organism, and can be repaired if injured. Other materials are also part of the adult skeletal system, notably cartilage, which is generally present in the joints between bones. In addition, some vertebrate animals have elements of a so-called "dermal" skeleton, which provides protection to such animals as sturgeon, crocodiles, and turtles.Major elements of the skeletal system of vertebrate animals include the following (using a mammalian model):
the vertebral column, composed of individual bones known as vertebrae, and providing axial (or length-wise) support for the animal, while retaining a degree of lateral flexibility
the ribs, which provide support and protection for the front of the thoracic cavity, and are important in the mechanics of breathing
the skull, which includes the brain-case, enclosing and protecting the brain, the upper jaw, which is fused to the brain-case, and the separate lower jaw, opposite the upper jaw
the appendicular skeleton, consisting of the bones of the forelimbs (arms and hands), the scapular (or shoulder) apparatus, to which these attach, the hindlimbs (legs and feet), and the pelvic bones, to which the hindlimbs attach
the ligaments, consisting mainly of collagen, which keep the bones securely attached at the joints
The number, size, and shape of skeletal elements vary greatly within species during their development, and sometimes between sexes. They vary even more between species, and are commonly used as important characters when identifying species of vertebrates and studying their evolutionary relationships (this is particularly true of paleontology, or the study of fossil animals). Small species of fishes and salamanders have the tiniest bones, while large whales have the most massive ones. The human skeletal system contains 206 separate bones, the largest of which is the femur (upper leg bone), while the smallest are three bones of the inner ear known as the malleus, incus, and stapes (hammer, anvil, and stirrup; these are named after their presumed resemblance to these much-larger tools!).
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