Which of the following would you classify as a hydrostatic skeleton?

A hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an organism's shape and produce movement, such as burrowing or swimming. Some examples are soft bodied animals such as sea anemones and earthworms. Hydrostatic skeletons have a role in the locomotion of echinoderms (starfish, sea urchins), cnidarians (jellyfish), annelids (earthworms), nematodes, and other invertebrates. They have some similarities to muscular hydrostats.

In flatworms, the interstitial fluid is kept under pressure and is the main hydrostatic skeleton.

C, maybe?