Support and Movement in Animals
Animals need support to keep their body shape and to hold organs in place, and they need a system that allows movement from one place to another. The kind of support system depends on whether the animal is an invertebrate (no backbone) or a vertebrate (with a backbone).
Three Types of Skeleton
- Hydrostatic skeleton — support comes from fluid held under pressure inside the body. Found in soft-bodied animals such as earthworms and jellyfish.
- Exoskeleton — a hard outer casing made of chitin. Found in insects, prawns and crabs. It must be shed (moulting) for the animal to grow.
- Endoskeleton — an internal framework of bone and cartilage. Found in vertebrates such as fish, frogs, birds and mammals.
Key idea
Hydrostatic = fluid support (earthworm). Exoskeleton = hard outside (insect). Endoskeleton = bony inside (fish, human). Muscles pull on the skeleton to produce movement.
How Movement Happens
Movement is produced when muscles pull on parts of the skeleton. Muscles can only pull (contract), not push, so they work in antagonistic pairs — while one contracts, the other relaxes. In vertebrates the muscles are attached to bones by tough cords called tendons.
Example
A prawn, a common seafood in Malaysia, has an exoskeleton. As it grows, it moults its hard shell and forms a larger one. A fish, in contrast, has an endoskeleton of bone that grows with its body.
Each type of skeleton has advantages and limits. An exoskeleton protects the soft body and reduces water loss, which helps insects survive on dry land, but it restricts how large the animal can grow and must be shed to allow growth. An endoskeleton, being internal, grows steadily together with the body and can support the large size seen in mammals.
Remember
Muscles pull, they never push. Growth in animals means an increase in size and mass as cells divide and the body develops.