A
hydrostatic skeleton or
hydroskeleton is a structure found in many
ectothermic 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, longitudinal, or helical muscles are used to change an organism's shape and produce movement, such as burrowing or swimming. They alternately contract and expand their body segments along their length. Some examples are soft-bodied animals such as sea anemones and earthworms. Hydrostatic skeletons have a role in the locomotion of
echinoderms (starfish and sea urchins),
cnidarians (
jellyfish),
annelids (
earthworms),
nematodes, and other
invertebrates. They have some similarities to
muscular hydrostats.
