Here's why:

* Hydrostatic Skeleton: This type of skeleton is found in many invertebrates, such as earthworms, jellyfish, and some sea anemones. It consists of a fluid-filled cavity, often called a coelom, surrounded by muscles.

* How it Works: The fluid inside the cavity is incompressible, meaning it cannot be squeezed into a smaller space. When muscles contract, they press against the fluid, creating pressure that extends the body. This pressure is then used for movement.

Example: An earthworm uses its hydrostatic skeleton to move. When its muscles contract, they squeeze the fluid in its coelom, pushing its body forward.

Let me know if you'd like more information on hydrostatic skeletons!