The Skeletal System
In the animal kingdom there are three types of skeletons -- hydrostatic, exoskeleton, and endoskeleton. The prefix "hydro-" implies liquid, water, or moisture. Hydrostatic skeletons are found in soft-bodied invertebrates and consist of muscle layers that surround a fluid-filled body cavity.  Earthworms and jellyfish are examples of animals with a hydrostatic skeleton.Exoskeleton
Exoskeletons are found in arthropods, such crabs and other crustaceans. Insects also have exoskeletons. Made of chitin, a strong, yet bendable complex carbohydrate, an exoskeleton provides the animal with protection and stability, and prevents if from drying out, enclosing the animal's tissues and organs in a hard case.