The kind of skeleton that results from muscle working against fluid in body cavities

is known as a hydrostatic skeleton. This type of skeleton is found in animals such as jellyfish, octopuses, and worms, and is made up of fluid-filled compartments surrounded by muscles. The muscles can contract and squeeze the fluid, allowing the animal to maintain its shape and move its body. Hydrostatic skeletons provide support and structure without the need for hard bones or a rigid exoskeleton.