Support in animals

Animals use one of three kinds of skeleton:

1. Hydrostatic skeleton — a fluid-filled body cavity surrounded by muscles (earthworm, jellyfish, Hydra). The fluid cannot be compressed, so contraction of muscles changes the body shape and produces movement.
2. Exoskeleton — a hard external covering (insects, crustaceans, snails). It protects the body but must be shed (moulted) to allow growth, and it limits body size.
3. Endoskeleton — a hard internal framework of bone and/or cartilage (all vertebrates, including man). It grows with the body, supports the weight, protects organs and provides large surfaces for muscle attachment. Cartilaginous fish (sharks) have a skeleton of cartilage only; bony fish, amphibians, reptiles, birds and mammals have a skeleton of bone with some cartilage.