The fluid-filled cavity that aids movement in animals like earthworms hydrostatic skeleton. A hydrostatic skeleton is a structure in some soft-bodied animals where a fluid-filled cavity is used to support the body and help with movement.
Imagine trying to squeeze toothpaste out of a nearly empty tube. Just like how you need to apply pressure to specific parts of the tube to get the toothpaste to move forward, an earthworm relies on its hydrostatic skeleton—a fluid-filled cavity surrounded by muscles—to propel itself. The toothpaste tube represents the fluid-filled cavity in the earthworm, and your fingers are like the worm's muscles; by contracting and relaxing these muscles, the worm can push the fluid inside its body to facilitate movement, much like you manipulate the tube to get every last bit of toothpaste out.
