Facts for Kids

Exoskeletons are hard coverings on the outside of an animal’s body. You can see them on crabs, insects, and snails. This outer shell works like armor: it protects the animal and gives a surface for muscles to pull against so the animal can move.

Arthropods, such as beetles and spiders, have jointed exoskeletons and must shed or molt their shell to grow. Molluscs, like snails, have a single hard shell that grows at the opening. Exoskeletons can help sense the world too—tiny hairs on an insect’s shell can feel touch and air movements.

Skeleton means the frame that holds an animal’s body up, much like the frame of a house. It gives shape, helps with movement, and can protect soft parts inside. Without a skeleton, many animals would be floppy and could not stand, swim, crawl, or grab things.

Animals use different designs for their skeletons. Some have hard coverings on the outside, some have inside bones, and some use body fluids to keep their shape. Each design fits how an animal lives and moves—where it lives, how big it grows, and what it needs to do every day.

Endoskeletons are support structures that sit inside an animal’s body. Humans, birds, and fish have endoskeletons made of bones and softer parts called cartilage. Bones give strength and shape, and muscles attach to bones so the animal can move arms, legs, fins, or wings.

An important part of many endoskeletons is the backbone, which keeps the body straight and protects the spinal cord. Some simple animals, like sponges, also have internal supports, but vertebrates have the most familiar kinds of endoskeletons that grow as the animal grows.

Hydrostatic skeletons use fluid inside a body cavity to hold shape. Imagine a water balloon: push on one side and the balloon changes shape but stays soft. Many soft animals, such as earthworms, jellyfish, and sea anemones, use this trick to move and bend.

Muscles around the fluid cavity squeeze and relax in different directions. When circular muscles tighten, the body gets longer; when lengthwise muscles tighten, it gets shorter or fatter. This system is great for squeezing into small places and for smooth, flowing motion, though it does not make a hard, protective shell.

Human skeleton is a good example of a mammal skeleton. An adult has about 206 bones, but babies are born with more than 270 bones that slowly fuse together as they grow. The skeleton is split into the axial part (skull, spine, ribs) and the appendicular part (arms, legs, shoulder, and hip bones). Bones hold marrow inside, and marrow makes many of the blood cells your body needs.

The human skeleton keeps growing until the late teens or about 20 years old. The femur (thigh bone) is the biggest bone, while the tiny stapes in the middle ear is the smallest. The hyoid bone sits in the throat and supports the tongue without touching other bones. Male and female skeletons can differ a bit in size and in the shape of the pelvis; the pelvis shape helps women carry and give birth to babies, so it is usually a little wider.

Exoskeletons are hard outer coverings that many invertebrates wear like a suit of armor. Arthropods—such as insects, spiders, and crabs—make an exoskeleton from a material called chitin. The exoskeleton covers the body, helps muscles move the legs, and protects the animal. Because it does not grow with the animal, arthropods must molt (called ecdysis): they make a new exoskeleton under the old one and then shed the old shell.

Other invertebrates use different kinds of support. Echinoderms (sea stars and sea urchins) have an internal skeleton of many small calcified plates that fit together, and sea urchin spines are one visible part. Sponges have tiny stiff pieces called spicules made of silica, calcium, or flexible spongin fibers that help hold their shape. These skeletons are made in different ways but all help animals move, feed, or stay safe.

Vertebrate animals all have a backbone made of many small bones called vertebrae. In amphibians and reptiles this backbone supports ribs and limbs when present. Some reptiles show big changes: turtles have ribs and shoulder bones that grew into a hard shell, and snakes and caecilians have many more vertebrae than most animals—some snakes have over 300, which helps them bend and move in narrow spaces.

Birds and fish show other special designs. Birds have light, hollow bones and some bones are fused to make flying easier; they do not have teeth and use beaks instead, and baby birds have a tiny egg tooth to help them hatch. Fish may have skeletons of cartilage (like sharks) or true bone (like most fish). Their backbone is the main support, and fins are built from bony or soft rays rather than limbs attached to the spine.