Why Animals Need a Skeleton
A skeleton is a supporting framework that gives an animal shape, protects soft organs and provides a firm structure against which muscles can pull to produce movement. Animals show three main types of skeleton: hydrostatic skeleton, exoskeleton and endoskeleton.
The Three Types
- Hydrostatic skeleton: found in soft-bodied animals such as earthworms and jellyfish. It is a fluid-filled body cavity under pressure. Muscles squeeze against the trapped fluid (which cannot be compressed), and the animal changes shape and moves. It gives support but little protection.
- Exoskeleton: a hard outer casing on the outside of the body, made mainly of chitin in insects and other arthropods. It supports and protects well and provides attachment for muscles inside. Because it cannot grow, the animal must moult (shed) it to grow larger.
- Endoskeleton: an internal skeleton made of bone and cartilage, found in vertebrates such as humans, fish and birds. It grows with the body, supports large bodies, and works with muscles for movement.
Key idea
Hydrostatic = fluid pressure; exoskeleton = hard case outside; endoskeleton = hard framework inside.
Comparing Them
An endoskeleton can grow continuously and support a heavy body, which is why the largest land and sea animals are vertebrates. An exoskeleton protects very well but limits growth because of moulting. A hydrostatic skeleton is simple and light but offers no hard protection.
Example
An earthworm moves using its hydrostatic skeleton: circular and longitudinal muscles act on the fluid-filled segments so the body becomes long and thin, then short and fat, pushing it forward.
Remember
Arthropods (insects, crabs) have exoskeletons and must moult; vertebrates (fish, humans) have endoskeletons that grow with them.