Skeleton

A skeleton is the frame that helps hold up the body of most animals. There are a few types of skeletons.

One type is the exoskeleton. This is a hard shell on the outside that keeps an animal's shape.

Another type is the endoskeleton. This is a hard frame on the inside where organs and soft tissues stick to.

There is also the hydroskeleton. This is a bendable frame inside that gets support from the hydrostatic pressure of body fluids.

Vertebrates are animals with an endoskeleton that has a central vertebral column. Their skeletons are usually made of bones and cartilages. Invertebrates are animals without a vertebral column. Their skeletons look very different. Some have hard outer shells, like arthropods and most molluscs. Others have inner plates, such as cuttlebones in some cephalopods, or small rods like ossicles in echinoderms. Many have body cavities that give them shape, and sponges have tiny spicules.

Cartilage is a strong connective tissue. It is found in the skeletons of both vertebrates and invertebrates. It helps give shape and support to many kinds of animals.

Etymology

The word skeleton comes from Ancient Greek. In Ancient Greek, it was spelled skeletós and means "dried up." An older spelling of the word is "sceleton."

Classification

Skeletons are the framework that supports the body of most animals. There are three main types: exoskeletons, which are hard outer shells that protect animals like insects and crabs; endoskeletons, which are internal frames made of bone or cartilage found in animals such as mammals and fish; and hydrostatic skeletons, which are flexible internal structures supported by fluid pressure, found in animals like jellyfish and earthworms.

Exoskeletons act as armor and must be shed when the animal grows. Endoskeletons, made mostly of minerals, support and protect internal organs and allow for movement. Hydrostatic skeletons use fluid pressure to give shape and help animals move. Each type of skeleton helps animals stay strong and move in their own special ways.

Main article: Exoskeleton

Main article: Endoskeleton

Main article: Hydrostatic skeleton

Main article: Cytoskeleton

Vertebrate skeletons

Vertebrate skeletons are internal frames called endoskeletons. The main part of these skeletons is bone. Bones give each animal its own special shape. Another important part is cartilage. In mammals, cartilage is mostly found where bones meet. In some animals, like sharks, the whole skeleton is made of cartilage.

Bones are strong and help support the body. They help with movement and protect important organs. Bones are made mostly of minerals. They have a special structure that makes them strong but not too heavy. Bones also help make blood cells and store important minerals.

During development, bones form from special cells. Cartilage, which is a softer tissue, appears first and is later replaced by bone in many animals. Cartilage helps bones move smoothly against each other. Ligaments connect bones to each other, and tendons connect muscles to bones.

Amphibians and reptiles

The skeletons of turtles have evolved to include a shell made from their ribcage. Snakes and caecilians have many more vertebrae than other animals, sometimes over 300.

Birds

Bird skeletons are designed for flight. Their bones are hollow and lightweight. Many bones are fused together to make them stronger but still light. Birds also have a beak instead of teeth, which is lighter.

Fish

Fish skeletons can be made of cartilage, like in sharks, or bone, like in other fish. The main part of the fish skeleton is the vertebral column. Fish have fins made of bony or soft rays that help them move, but these fins are not directly connected to the spine.

Mammals

In marine mammals like whales and manatees, the hind legs are either lost or changed into a tail fin. In whales, the neck bones are fused together for better stability while swimming.

Humans

The human skeleton includes both fused and separate bones. It supports organs, anchors muscles, and protects important parts like the brain, lungs, heart, and spinal cord. The largest bone is the femur in the upper leg, and the smallest is the stapes in the middle ear. An adult human has around 206 bones, but this number can vary. The skeleton takes about 20 years to fully develop and contains marrow that makes blood cells.

Main article: Human skeleton

Invertebrate skeletons

Invertebrates, such as arthropods, echinoderms, and sponges, do not have bones like vertebrates. Instead, they have different kinds of skeletons.

Arthropods, including insects, crustaceans, and arachnids, have exoskeletons made of a tough material called chitin. These exoskeletons protect their bodies and help them move.

Echinoderms, like starfish and sea urchins, have internal skeletons made of special plates. These plates are made mostly of calcite and help protect the animal's body.

Some molluscs, such as snails and clams, have shells that act like exoskeletons, made from proteins and minerals.

Sponges have tiny, needle-like structures called spicules that make up their skeletons.

Cartilage

Main article: Cartilage

Cartilage is a special kind of tissue that helps support and protect parts of the body. It is made of cells called chondrocytes surrounded by a substance that includes collagen fibers, proteoglycans, and water. There are different types of cartilage, such as elastic cartilage, hyaline cartilage, and fibrocartilage. Unlike other tissues, cartilage does not have blood vessels. Because of this, its cells get nutrients through movement. This means cartilage repairs and grows more slowly than other tissues.