Most animals have a foundation from which their bodies are structured, a frame if you will. This frame is known as a skeleton. There are three types of skeletal structures found in animals. The exoskeleton found in insects and crustaceans. The endoskeleton is found in vertebrates. The hydrostatic skeletal system is found in invertebrates.
The exoskeleton is made up of plates. The endoskeleton is made up of bone. The hydrostatic skeletal system is made up of fluids inside tubes in the body.
Insects and crustaceans use their exoskeleton to protect the soft tissue of the body. The structure consists of plates made of chitin. As the animal grows it sheds its skeleton and within minutes the soft tissue of the new exoskeleton hardens. Lobsters, crayfish, insects and snails are examples of animals with exoskeletons.
All vertebrates have endoskeletons which support the body. It is consists of bone made of calcium giving the rigidity of the structure, cartilage adds flexibility and the hollow bone contains marrow. It is an interactive system working with other systems of the body to maintain function and health. Though this structure does not provide protection for most of the body, the rib cage does provide protection to the vital lungs and heart. Also the pelvis protects the reproductive and lower digestive systems.
The hydrostatic skeleton is found in invertebrates. Its function is movement. The contraction of muscles cause the fluid to flow through a tube system called a coelom thus extending appendages. It also allows these organisms to change shape enabling them to move through their habitat. The jellyfish uses this system to control its tentacles. The earth worm is a prime example of structure changes. This is the most prevalent of the three skeletal systems as invertebrates make up the majority of organisms on earth.