Skeleton

• 1. Introduction

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  A skeleton is the structural frame that supports the body of most animals, and the forms it takes are stranger than the word suggests. A turtle wears its ribcage on the outside. A shark carries no bone at all, only cartilage. An earthworm holds its shape with nothing but pressurized fluid. The body of a jellyfish stays soft yet structured because fluid pressure does the work that bone does in you. So what counts as a skeleton, when one animal's frame is a rigid outer shell, another's is a hidden internal scaffold, and a third's is liquid held under pressure? This documentary follows that question across the animal world. It moves from armor worn outside the body to the bone hidden within, from frames that bend and spring back to frames built from glass-like mineral spicules. Along the way it asks why a bird's bones are hollow, why a snake carries hundreds of vertebrae, and why an adult human skeleton is only about an eighth of total body weight, half of that weight water.

• 2. Armor Worn Outside

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  An exoskeleton covers the body of an animal and acts as armor against predators. Arthropods, the group that includes insects, crustaceans, and arachnids, encase their entire bodies in this kind of shell. Because the shell cannot grow, arthropods must molt, shedding the old exoskeleton through a process called ecdysis as they enlarge. The shells of molluscs are another form of exoskeleton.

  Weight is the great limitation of wearing a skeleton on the outside. An external skeleton can be heavy relative to an animal's overall mass, so on land, exoskeleton-bearing organisms tend to stay relatively small. Underwater, that constraint loosens. Buoyancy carries some of the load, which lets larger aquatic animals support heavier shells. The southern giant clam, an extremely large saltwater clam of the Pacific Ocean, grows a shell massive in both size and weight. The sea snail Syrinx aruanus carries a very large shell of its own.

  The exoskeleton does more than shield. It provides surfaces where muscles attach, and specialized appendages of the shell assist with movement and defense. In arthropods, the exoskeleton even contributes to sensory perception, helping the animal read its surroundings through the very wall that protects it. That sensing wall is built from chitin, a material the body must manufacture and renew with every molt.

• 3. The Frame Within

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  Endoskeletons are the internal support structure of an animal, built from mineralized tissues, and they hide their work inside the body. A true endoskeleton is derived from mesodermal tissue. Endoskeletons appear in three groups: chordates, echinoderms, and sponges. They are highly specialized and vary widely from one animal to the next.

  The range of jobs an endoskeleton performs is broad. In sponges, the internal frame functions purely for support and nothing more. In other animals it becomes an attachment site for muscles and a mechanism for transmitting muscular forces, turning the skeleton into part of the engine of movement rather than just its scaffold.

  Not every internal skeleton is made of bone. Echinoderms, the group that includes starfish and sea urchins, build endoskeletons from large sclerite plates that adjoin or overlap to cover the body. These plates are composed of stereom, a form of calcite with a monocrystal structure and a significant magnesium content reaching up to 15% of the skeleton. Stereom is porous, and as the animal ages, connective stromal tissue fills the pores. Sea urchins carry as many as ten variants of stereom structure, and their spines are the largest type of echinoderm skeletal element. Such skeletons are unique among living animals to echinoderms, though some Paleozoic animals once used similar ones.

• 4. Frames That Bend

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  Pliant skeletons deform under stress and then spring back to their original shape, a kind of frame built for flexibility rather than rigidity. The hinge of a bivalve shell works this way, as does the mesoglea of cnidarians such as jellyfish. The advantage is economy of effort. Only muscle contraction is needed to bend a pliant skeleton; when the muscle relaxes, the structure returns to form on its own.

  Most pliant skeletons are made from a mixture of proteins, polysaccharides, and water, though cartilage is one material they may use. For extra structure or protection, a pliant skeleton can be reinforced by a rigid one. Animals with pliant skeletons typically live in water, which supports the body in the absence of a rigid frame.

  Rigid skeletons, by contrast, do not move when stressed, which makes them a strong support system and the most common type among terrestrial animals. In water, rigid skeletons tend to serve protection, as in barnacle and snail shells, or to support the heavy musculature that fast swimmers require. These rigid frames are formed from chitin in arthropods, from calcium compounds such as calcium carbonate in stony corals and mollusks, and from silicate in diatoms and radiolarians.

• 5. Structure From Fluid

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  Hydrostatic skeletons are flexible cavities within an animal that provide structure through fluid pressure, and they are always internal. Soft-bodied organisms rely on them: jellyfish, flatworms, nematodes, and earthworms. The walls of these cavities are made of muscle and connective tissue, and pressurized body fluid does the work a hard frame would otherwise do.

  A hydrostatic skeleton is not only a support; it is a means of motion. It transmits the forces of muscle contraction, so an animal can move by alternating contractions and expansions of muscle along its length. The earthworm advancing through soil is using exactly this mechanism, its fluid-filled body lengthening and shortening in waves.

  The word skeleton even reaches inside individual cells. The cytoskeleton, where cyto- means cell, stabilizes and preserves the form of cells. It is a dynamic structure that maintains cell shape, protects the cell, enables motion through flagella, cilia, and lamellipodia, moves vesicles and organelles, and plays a role in cellular division. It is not a skeleton for the body of an animal, but it performs a similar function one level down, at the scale of a single cell.

• 6. What Bone Does

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  Bones are rigid organs that support the body, assist movement by opposing muscle contraction, and form a protective wall around internal organs. They are primarily made of inorganic minerals such as hydroxyapatite, with the remainder an organic matrix and water. Their hollow tubular structure resists compression strongly while staying lightweight, and most of the cells within them are osteoblasts, osteoclasts, or osteocytes.

  Bone tissue is a dense, mineralized connective tissue with a honeycomb-like three-dimensional internal structure that gives it rigidity. It is also a factory and a storehouse. Bones produce both red and white blood cells, and they hold the body's reserves of calcium and phosphate. Inside them sit other tissues too: bone marrow, endosteum and periosteum, nerves, blood vessels, and cartilage.

  Bone begins forming early. During embryonic development, bones grow individually from skeletogenic cells in the ectoderm and mesoderm, most of which become separate bone, cartilage, and joint cells that are then articulated together. Cartilage grows more quickly than bone, so it dominates earlier in an animal's life before bone overtakes it. Specialized skeletal tissues of this kind are unique to vertebrates. Cartilage continues to resist stress at points of articulation, usually encased in perichondrium, while ligaments connect bone to bone and tendons connect muscle to bone.

• 7. Bodies Built To Specification

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  The skeletons of birds are adapted for flight, and the adaptation reaches into the material of the bones themselves. Bird bones are hollow and lightweight, which reduces the metabolic cost of flight. Their shape is tuned for the physical stress of flying, with a round and thin humeral shaft and the fusion of skeletal elements into single ossifications. That fusion leaves birds with fewer bones than other terrestrial vertebrates. They also lack teeth and a true jaw, having evolved a far lighter beak; many hatchlings carry an egg tooth on the beak to help them break out of the amniotic egg.

  Snakes solve a different problem with a different count. Snakes and caecilians have far more vertebrae than other animals, snakes often exceeding 300 compared with the 65 typical in lizards. Turtles took yet another path: their shells evolved from the ribcage, turning part of the internal skeleton into an exoskeleton.

  Marine mammals reshaped the skeleton for water. The hind legs were lost entirely in whales and manatees, or united into a single tail fin as in the pinnipeds, the seals. In the whale, the cervical vertebrae are typically fused, an adaptation that trades flexibility for stability during swimming. Fish meanwhile build their frame around a vertebral column of lightweight yet strong articulating vertebrae, with ribs attaching to the spine, no limbs or limb girdles, and fins of bony or soft rays that, except for the tail fin, have no direct connection to the spine. The cartilaginous fishes, including sharks, rays, skates, and chimeras, go without bone entirely, and the lighter weight of their all-cartilage skeletons lets them spend less energy swimming.

• 8. The Human Count

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  There are 206 bones in the adult human skeleton, though that figure depends on how you count. The pelvic bones on each side may be tallied as one or as three, the ilium, ischium, and pubis. The coccyx may be one bone or four. The sacrum is usually counted as a single bone rather than five fused vertebrae, and the figure excludes the variable wormian bones between skull sutures. A newborn has over 270 bones, some of which later fuse, and the human skeleton takes 20 years to fully develop.

  Not every bone connects to its neighbors. The three ossicles in each middle ear articulate only with each other. The hyoid bone in the neck, the anchor point for the tongue, articulates with no other bone at all, held in place by muscles and ligaments. The largest bone in the body is the femur in the upper leg, the smallest the stapes in the middle ear. The patella, or kneecap, is a large sesamoid bone, and because the patellae are constant they are counted in the total. In an adult, the skeleton makes up around 13.1% of total body weight, and half of that weight is water.

  Male and female skeletons differ in measurable ways. The male skeleton is generally larger and heavier; the female skull is generally less angular, with a wider and shorter breastbone and slimmer wrists. The sharpest contrasts lie in the pelvis, shaped by pregnancy and childbirth. The female pelvis is wider and shallower, with an enlarged pelvic outlet and a wider, more circular pelvic inlet, while a sharper angle between the pubic bones gives the male pelvis a narrower, near heart-shaped form. The same architecture that distinguishes one person's frame from another's traces back to a single material, cartilage, a connective tissue with no blood vessels, fed only by diffusion, which is why it grows and repairs more slowly than the bone that eventually surrounds it.