What if you had no bones?

Squeeze your arm. Feel something hard inside? That is a bone.

You have lots of bones. What if you had no bones at all? What would your body be like?

Press your fingers on your forearm, your knee or the top of your head and you can feel something hard under the skin. Those are bones, part of your skeleton.

We hardly think about our bones, but they are busy all the time. So what would happen if you had no bones at all? We can't take anyone's bones away, but we can follow the clues: what bones do for us, and how some animals manage without them.

Vertebrates are built around an endoskeleton: hard, mineralized structures inside the soft tissues. In humans it does several jobs at once, from holding the body's shape to anchoring the muscles that move it.

This thought experiment asks what a boneless human would lose, job by job. It then looks at the alternatives that real animals use, because soft-bodied animals do move, eat and thrive without a single bone. Each consequence is labeled as established fact or as a reasoned idea.

A grown-up has 206 bones. A baby has about 300!

Baby bones join together as they grow. Bones hold you up and give your body its shape.

A grown-up's skeleton has 206 bones. A newborn baby has about 300, and some of them fuse, or grow together, as the child gets older, according to KidsHealth.

Bones give your body its shape and hold it up. Your skull guards your brain, and your ribs make a cage around your heart, lungs and liver. Your spine, made of 33 bones called vertebrae, lets you bend and twist while it protects your spinal cord. Inside many bones, soft marrow makes blood cells.

The adult human skeleton has 206 bones. OpenStax lists its five main functions: supporting the body, storing minerals and lipids, producing blood cells, protecting internal organs and allowing movement. It is divided into the axial skeleton (skull, vertebral column and rib cage) and the appendicular skeleton (shoulders, limbs and the pectoral and pelvic girdles).

KidsHealth notes that a newborn has about 300 bones, some partly or entirely cartilage, which fuse and ossify into the adult 206 during childhood. Bone marrow inside many bones makes most of the body's red blood cells, white blood cells and platelets.

Now pretend your bones are gone. Could you stand up? Could you hold a cup?

Your muscles would still be there. But they would have nothing hard to pull on.

Muscles move you by pulling. Tough cords called tendons tie each muscle to a bone, and when the muscle tightens it tugs the bone and bends a joint. KidsHealth explains that muscles can pull but can't push, so they work in pairs: one bends a joint and the other straightens it.

Without bones, your muscles might pull, but there would be nothing firm to move. You might slump like a beanbag, your brain and heart would lose their armor, and your body would lose a place to make blood cells. These are our ideas about a pretend body, not things anyone has seen.

Skeletal muscle generates force only by contracting. Each muscle crosses a joint and attaches by tendons to bones on either side, so contraction rotates one bone relative to the other; antagonistic pairs, flexors and extensors, return the joint, because a muscle cannot push.

Remove the bones and that lever system has no rigid arms or pivots. It is reasonable to think a boneless human could not hold a posture against gravity or move its limbs in controlled ways, and would lose skeletal protection of the brain and thorax and its main site of blood-cell production. Those consequences are reasoned, not observed; the next section shows how other animals solve the same problems.

Some animals have bones inside, like us.

Some have a hard shell outside, like a crab. Some have soft bodies full of water, like a worm!

Animals have three main kinds of skeleton. We have an endoskeleton, a skeleton inside the body. Crabs and insects have an exoskeleton, a hard case on the outside; they have to shed it as they grow.

Earthworms and sea anemones have a hydrostatic skeleton: a space inside the body filled with fluid. Their muscles squeeze the fluid to change the body's shape, and that is how an earthworm moves, in waves of squeezing and stretching. A water-filled skeleton works well in water and in soil, but OpenStax notes that it is not an efficient skeleton for animals living on land.

OpenStax describes three skeleton designs. A hydrostatic skeleton is a fluid-filled compartment, the coelom, kept under pressure; muscles around it change its shape, as in the peristaltic waves that alternately shorten and lengthen an earthworm. It suits soft-bodied invertebrates such as sea anemones and earthworms, but is not efficient for terrestrial animals.

An exoskeleton is an external case, such as the 30–50 percent chitin shell of crabs and lobsters; because it is acellular and cannot grow, arthropods must shed it. An endoskeleton, like ours, is internal and grows with the body. Each design supports, protects and transmits muscle force in a different way.

An octopus has no bones at all!

It can squeeze through any gap bigger than its beak, which is its mouth. Some gaps are only as wide as a few fingers.

Some animals really do live without bones. The Aquarium of the Pacific says a giant Pacific octopus has no bones and is so flexible that it can fit through any space larger than its beak, which is its mouth. It can squeeze its whole body through a gap only a few inches across.

Its eight arms carry thousands of suction cups that grip food and even taste it. So bones are not the only way to build a body. But OpenStax, a free biology textbook, notes that a soft, fluid-filled body is not an efficient design for animals living on land.

Soft-bodied animals show what a boneless body can and cannot do. The Aquarium of the Pacific describes the giant Pacific octopus as an invertebrate with no bones that can pass through any opening larger than its beak, squeezing into spaces only a few inches in diameter; its arms carry about 2,140 to 2,240 suction cups, each with thousands of chemical receptors.

OpenStax notes that a fluid skeleton suits invertebrates such as earthworms and some aquatic organisms but is not efficient on land, and land animals rely instead on rigid exoskeletons or endoskeletons. A boneless human might face the same problem on land, which is why this lab stays a thought experiment.