SKELETON

the supportive framework of an animal body. In vertebrates the skeletal system is mainly internal, consisting for the most part of bone and cartilage; in invertebrates it is sometimes external and may consist of a variety of nonbony, noncartilaginous materials. Animal skeletons are composed of six basic types of structures, and often the same chemical compounds are present in different skeletal structures, although their mechanical characteristics may differ greatly. Moreover, elements of more than one type may make up a single skeletal system, especially in the more complex higher animals. The first type of skeleton is the hard structure. This ranges in complexity from the simple deposits excreted by the individual coral colony member, which harden and form a ridgy structure serving both as a base for growth and a place of refuge, to the more intricate bones of vertebrates. Other examples include the shells of clams, the spiky skeleton of the starfish, and the tough exoskeleton of the arthropods (insects, spiders, and crustaceans), which is composed of dead cells secreted by the outer skin and often is so hard that in order to grow, the animal must shed it several times during its lifetime. Calcium compounds contribute to the rigidity of many of these structures, as silica often does in marine skeletons; in arthropods dehydration can also be an important factor. Semirigid skeletal structures are a second type. In the arthropod exoskeleton, for example, some softening is needed at the joints to permit movement, which semirigid tissue accomplishes, while still protecting the animal's body. Cartilage in vertebrates is another type of semirigid skeletal material. A third skeletal element is connective tissue, which in more complex multicellular animals exists as layers of fibrous material, often composed of a protein called collagen. In many animals connective tissue serves as an anchor for muscles, as in the arthropods, where it grows on the inner layer of the exoskeleton and is not shed during molting. In animals with an inner cavity, or coelom (i.e., at the level of earthworm or higher), connective tissue holds inner organs in place, and in vertebrates it serves as the base for the formation of membranous bones. A hydrostatic skeleton is a fourth type of skeletal structure and can be the major skeletal element (as in the earthworm) or simply one of several. Fluid-filled cavities inside the animal exert pressure on the outer layers, much like water in a hose, and such pressure serves in some animals as a base against which muscles can work. (This is how the earthworm, for instance, moves along the ground.) Different cavities perform this function in different animals (the coelum, for instance, in earthworms, but the circulatory system in arthropods). But in more complex animalshigher arthropods and the vertebratesthe development of hard skeletons, whether internal or external, has all but supplanted the hydrostatic skeleton in determining movement. Elastic structures, a fifth skeletal element, permit animals to return to their normal body shape after the movement of muscles. The jellyfish, for example, contracts its entire bell-shaped body, propelling itself by a jet stream; its elastic framework then refills itself with water. Elastic elements in vertebrate musculature are responsible for their ability to stretch. The sixth skeletal element present in animals consists of buoyancy devices, which enable many marine animals to sense water pressure and to rise and fall to different water levels. In land vertebrates, the ear is a remnant of these buoyancy devices in fish. In vertebrates the bony skeleton aids in the functions of support, movement, and protection. It is subdivided into the axial and appendicular skeletons. The former includes the skull and the vertebral column, the latter, the limbs and the girdles that anchor the limbs to the axial skeleton. Sometimes a third region, the visceral, is distinguished, which includes the lower jaw and portions of the upper jaw. Vertebrates, although comprising a wide variety of animals, have basic similarities in their skeletal structures. While a few vertebrate fishes retain their notochord (a flexible, longitudinal rod, serving as the structural axis of the body during early embryonic development) into adulthood, in most vertebrates the notochord is replaced quickly by a column of bony segments (vertebrae) sandwiched together by cartilaginous intervertebral disks and organizedmoving from the head down the columninto the cervical (neck), thoracic, lumbar (loin), sacral (pelvic), and caudal (tail) regions. The number of vertebrae in each region differs according to species. Attached near the top of the vertebral column is the skull, which protects the brain and houses the ears, eyes, nose, and mouth. The vertebral column plays an extremely important role in the body: above all, it protects the spinal cord; but it also helps anchor many of the body's muscles; serves as the attachment for the rib cage, a bony and cartilaginous structure protecting the heart, lungs, and other organs; anchors the appendicular skeleton; and generally contributes to the free movement of the entire animal. The human vertebral column has developed as a function of the erect posture and in adults forms the shape of an S. This curvature helps cushion the shock of movement; in its concave areas it provides more room for internal organs and helps bear their weight, and it makes the skeleton less vulnerable to fractures. The transition from water to land caused the vertebral column to become much more flexible, when the back-and-forth motion of the whole body, which fish use to move around, was replaced with the up-and-down movement of the limbs of terrestrial animals. The appendicular skeleton, consisting of the limbs and the limb girdles, has evolved from the paired fins of fishes. The pectoral girdle connects the forelimbs to the axial skeleton; the pelvic girdle connects the hind limbs. Each limb consists of three partsthe proximal (closest to the body), containing a large bone; the medial, containing one large and one small bone; and the distal, containing the several small bones that make up the feet (or in primates, the hands and feet). The limb sections are connected to the girdles at joints, which give the vertebrate skeleton an extreme degree of manoeuvrability. the supportive framework of an animal body. The skeleton of invertebrates, which may be either external or internal, is composed of a variety of hard nonbony substances. The more complex skeletal system of vertebrates is internal and is composed of several different types of tissues that are known collectively as connective tissues. This designation includes bone and the various fibrous substances that form the joints, connect bone to bone and bone to muscle, enclose muscle bundles, and attach the internal organs to the supporting structure. Additional reading R.B. Clark, Dynamics in Metazoan Evolution (1964), deals with some aspects of the coelomate condition, hydrostatic skeletons, metamerism, and their evolution. J. Gray, Animal Locomotion (1968), a comprehensive, comparative account of the coordination and mechanisms of vertebrate locomotion, with some chapters on invertebrates (the approach is for the nonspecialist, but the treatment is mathematical and neurological); A.S. Romer, The Vertebrate Body, 4th ed. (1970), an excellent account of the evolution of the skeleton in vertebrates; and J.Z. Young, The Life of Vertebrates, 2nd ed. (1962).