Skeleton facts for kids

Kids Encyclopedia Facts

This page is about skeletons in general. For the human skeleton, see Human skeleton. For other uses, see Skeleton (disambiguation).

Quick facts for kids Skeleton

A horse and human skeleton placed in a display at Australian Museum in Sydney

A skeleton is like the natural framework that holds up the body of most animals. Think of it as the main support system! Skeletons come in different types. Some animals have an exoskeleton, which is a hard outer shell, like a suit of armor. Others have an endoskeleton, which is an internal frame made of bones or cartilage, like humans. There are also hydroskeletons, which use fluid pressure to keep soft-bodied animals in shape.

Vertebrates are animals with backbones. They have endoskeletons made of bones and cartilage. Invertebrates are animals without backbones. Their skeletons vary greatly. They can have hard outer shells, internal plates, or flexible fluid-filled structures.

What Does the Word "Skeleton" Mean?

The word skeleton comes from an ancient Greek word, skeletos, which means "dried up."

Different Kinds of Skeletons

Skeletons can be described in several ways. Some are solid and made of hard materials like bone or cartilage. Others are flexible. Some are inside the body, and some are outside.

Exoskeletons: Your Outer Armor

The exoskeleton of an ant

An exoskeleton is a hard outer shell that covers an animal's body. It acts like armor, protecting the animal from danger. Arthropods, like insects and crabs, have exoskeletons. As these animals grow, they must shed their old exoskeleton in a process called moulting. The shells of molluscs, such as snails, are also a type of exoskeleton.

Exoskeletons provide places for muscles to attach. They also help with movement and defense. In arthropods, the exoskeleton can even help with sensing the world around them. An external skeleton can be quite heavy. This is why land animals with exoskeletons are usually small. Larger aquatic animals can have heavier exoskeletons because water helps support their weight.

Endoskeletons: Your Inner Support System

The endoskeleton of a bat

Endoskeletons are the internal support structures of an animal. They are often made of strong, mineralized tissues like the bones in humans. Endoskeletons are very specialized and differ a lot between animals. They can simply provide support, like in sponges. Or they can be places for muscles to attach, helping the animal move. Animals like chordates (which include vertebrates), echinoderms, and sponges have endoskeletons.

Hydrostatic Skeletons: Fluid Power

Hydrostatic skeletons are flexible, fluid-filled spaces inside an animal. They provide structure using fluid pressure. You can find them in some soft-bodied organisms, like jellyfish, flatworms, nematodes, and earthworms. The walls of these spaces are made of muscle and connective tissue. Besides giving the animal shape, hydrostatic skeletons help transmit muscle forces. This allows the animal to move by squeezing and expanding its muscles.

Vertebrate Skeletons: Bones and Cartilage

Pithecometra: From Thomas Huxley's 1863 Evidence as to Man's Place in Nature, comparing the skeletons of apes to humans.

Vertebrates have endoskeletons. The main parts are bones and cartilage. Each animal has a unique skeletal system. Cartilage is a strong, flexible tissue found in many places. In mammals, it's mainly in joints. Some animals, like cartilaginous fishes (sharks), have skeletons made entirely of cartilage. The skeleton often shows a repeating pattern, like the vertebral column (backbone) and the ribcage.

Bones are rigid organs that support the body. They help with movement by working with muscles. They also protect internal organs like the brain and heart. Bones are mostly made of minerals, which makes them strong. Their hollow structure makes them light but very resistant to pressure. Bones also produce red and white blood cells. They store important minerals like calcium.

During development, bones grow from special cells. Cartilage grows faster than bone. This means young animals have more cartilage, which is later replaced by bone. Cartilage also helps absorb stress in joints. Ligaments are elastic tissues that connect bones to other bones. Tendons are elastic tissues that connect muscles to bones.

Amphibians and Reptiles: Unique Adaptations

The skeletons of turtles have a special shell that grew from their ribcage. This shell acts like an exoskeleton. Snakes and caecilians have many more vertebrae (bones in their backbone) than other animals. Snakes can have over 300 vertebrae!

Birds: Built for Flight

Bird anatomy#Skeletal system Bird skeletons are specially designed for flight. Their bones are hollow and lightweight. This helps them save energy when flying. The shape and structure of their bones are strong enough to handle the stress of flight. Birds also have fewer bones than other land vertebrates. They don't have teeth or a true jaw. Instead, they have a lightweight beak. Many baby birds have a small projection called an egg tooth to help them hatch.

Fish: Swimming Skeletons

Fish anatomy#Skeleton|Fish bone The skeleton inside a fish is either made of cartilage, like in Chondrichthyes (sharks), or bones, like in Osteichthyes (bony fish). The main part is the vertebral column, which is strong but light. Ribs attach to the spine. Fish don't have limbs like arms or legs. Their fins are made of bony or soft spines called rays. These fins are supported by muscles.

Cartilaginous fish, such as sharks and rays, have skeletons made entirely of cartilage. This lighter skeleton helps them use less energy when swimming.

Mammals: Diverse Forms

Marine Mammals: Water Adaptations

Californian sea lion

To help marine mammals move easily in water, their hind legs have changed. Whales and manatees lost them completely. Pinnipeds (seals) have them joined into a single tail fin. In whales, the neck bones (cervical vertebrae) are often fused together. This makes their neck less flexible but more stable for swimming.

The Human Skeleton

Study of Skeletons, c. 1510, by Leonardo da Vinci

The human skeleton has both fused and individual bones. It is supported by ligaments, tendons, muscles, and cartilage. It acts as a scaffold, holding up organs and anchoring muscles. It also protects vital organs like the brain, lungs, heart, and spinal cord. The biggest bone in your body is the femur (thigh bone). The smallest is the stapes bone in your middle ear. In an adult, the skeleton makes up about 13% of total body weight.

Some bones are fused together, like those in the pelvis and cranium. Not all bones connect directly. The three tiny bones in each middle ear (ossicles) only connect to each other. The hyoid bone in the neck, which supports the tongue, doesn't connect to any other bones. It's held in place by muscles and ligaments.

An adult human skeleton usually has 206 bones. However, this number can vary slightly. For example, some small bones in the skull or tendons might not always be counted. Newborn babies have over 270 bones, but some of these fuse together as they grow. The human skeleton takes about 20 years to fully develop. Bones contain marrow, which produces blood cells.

There are some general differences between male and female skeletons. Male skeletons are usually larger and heavier. Female skeletons often have a wider and shallower pelvis. This difference is related to the female's ability to carry and give birth to babies.

Invertebrate Skeletons: No Backbones, Many Forms

Invertebrates do not have a vertebral column (backbone). They also don't have bone skeletons like vertebrates. Arthropods have exoskeletons. Echinoderms have internal skeletons. Some soft-bodied animals, like jellyfish and earthworms, have hydrostatic skeletons.

Arthropods: Hard Outer Shells

Arthropod exoskeleton The skeletons of arthropods, including insects, crustaceans, and arachnids, are cuticle exoskeletons. They are made of chitin, a tough material. This cuticle covers the animal's body and even lines some internal organs. As arthropods grow, they shed their old exoskeleton in a process called ecdysis. They then grow a new one. An arthropod's skeleton acts as a barrier, supports the body, and provides parts for movement and defense. It also helps with sensing the environment. Some arthropods, like crustaceans, use minerals from their surroundings to make their cuticle even stronger.

Echinoderms: Spiky Inner Frames

The skeletons of echinoderms, such as starfish and sea urchins, are endoskeletons. They have large, well-developed plates that cover their bodies. Sea cucumbers are an exception, with smaller skeletons to help them feed and move. Echinoderm skeletons are made of calcite, a type of calcium compound. These skeletons are porous, and the tiny holes fill with tissue as the animal gets older. The spines of sea urchins are the largest type of echinoderm skeletal structure.

Molluscs: Shells for Protection

Some molluscs, like conchs, scallops, and snails, have shells. These shells act as exoskeletons. They are made from proteins and minerals secreted by the animal's mantle.

Sponges: Tiny Support Structures

The skeleton of sponges is made of microscopic, needle-like structures called spicules. These spicules can be made of calcium or silica. Most sponges, called demosponges, have skeletons made of fibers of a protein called spongin, silica, or both.

Cartilage: Flexible Connective Tissue

Cartilage is a type of connective tissue in the skeleton. It is made of special cells called chondrocytes. These cells are surrounded by a strong, flexible material. This material is mostly made of collagen fibers, other proteins, and water. There are different types of cartilage, like elastic cartilage and hyaline cartilage. Unlike many other tissues, cartilage does not have blood vessels. Chondrocytes get their nutrients by diffusion. This process is helped by the squeezing and relaxing of the cartilage. Because of this, cartilage grows and repairs more slowly than other connective tissues.

Interesting Facts about Skeleton

It might seem like bones are just hard, lifeless structures, but they are actually very much alive! They are made of living cells, blood vessels, and nerves. This is why when you break a bone, it can heal itself over time.
The smallest bones in your body are found deep inside your ear! They are called the malleus, incus, and stapes, and they work together to help you hear all the sounds around you.
The longest and strongest bone in your whole body is your femur, which is in your thigh! It's incredibly strong and can handle a lot of pressure.
Babies have around 300 bones, but as you grow up, some of these bones join together. By the time you're an adult, you'll have 206 bones!
Bones are incredibly strong for their weight. A piece of bone the size of a matchbox can support about 18,000 pounds – that's the weight of two pickup trucks! While not quite stronger than steel in every way, their unique structure makes them incredibly resilient and flexible, allowing them to withstand a lot of stress without breaking.
Calcium is like the building blocks for your bones. You can get it from milk, cheese, yogurt, and leafy green vegetables.
Vitamin D helps your body absorb calcium. You can get it from sunlight (safely!) and foods like fatty fish and fortified cereals.
Activities like running, jumping, and playing sports put healthy stress on your bones, which encourages them to grow stronger and denser.