Hydrostatic skeleton

A support system in which pressurized fluid within a body cavity resists muscle contraction. Muscles change the cavity’s shape while its fluid volume remains nearly constant.

Hydrostatic pressure: Pressure exerted by a fluid at rest, increasing with depth in a gravitational field. Internal fluid pressure provides resistance against which muscles contract.
Incompressible fluid: A fluid whose volume changes negligibly under ordinary pressure changes. Nearly constant fluid volume lets contraction in one direction produce expansion elsewhere.
Earthworm locomotion: The crawling movement of earthworms through alternating contractions of body-wall muscles. Earthworms use fluid-filled segments as hydrostatic supports for crawling and burrowing.
Endoskeleton: An internal framework of rigid structures that supports and protects an animal’s body. Unlike a hydrostatic skeleton, it supports muscles through bones or other hard elements.
Muscular hydrostat: A muscular structure that changes shape without a rigid skeleton or fluid-filled body cavity. It generates support through muscle tissue rather than a pressurized cavity.
Body cavity: An internal space in an animal’s body that may contain organs or fluid. The cavity defines the space whose pressure and shape provide support.
Nematode locomotion: Movement in nematodes, produced by longitudinal muscles acting against a pressurized body cavity. Nematodes bend their bodies as muscles work against fluid pressure and a flexible cuticle.
Exoskeleton: A rigid external covering that supports and protects an animal’s body. An exoskeleton bears forces through a hard outer layer rather than pressurized fluid.
Antagonistic muscle: A muscle that produces an action opposite to that of another muscle. Opposing muscle groups bend or extend a hydrostatic body segment.
Circular muscle: A muscle arranged in a ring around a body region or opening. Contraction narrows a segment and lengthens it when its enclosed fluid volume is constrained.

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