Hydroskeleton characteristics and examples

Egbert Haynes

A hydroskeleton or hydrostatic skeleton consists of a fluid-filled cavity that surrounds the muscular structures and provides support to the animal's body. The hydrostatic skeleton participates in locomotion, giving the animal a wide range of movements.

It is common in invertebrates that lack rigid structures that allow body support, such as earthworms, some polyps, anemones, and starfish and other echinoderms. Instead, there are hydrostatic skeletons.

Source: By Rob Hille [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
Some specific structures of animals work through this mechanism, such as the penis of mammals and turtles, and the legs of spiders..

In contrast, there are structures that use the hydrostatic skeletal mechanism but lack the fluid-filled cavity, such as the limbs of cephalopods, the tongue of mammals, and the trunk of elephants..

Among the most outstanding functions of hydrostatic skeletons is support and locomotion, since it is a muscle antagonist and assists in the amplification of force in muscle contraction..

The functionality of a hydrostatic skeleton depends on maintaining a constant volume and the pressure it generates - that is, the fluid that fills the cavity is incompressible..

Article index

  • 1 Features
  • 2 Mechanism of hydrostatic skeletons
    • 2.1 Musculature
    • 2.2 Types of movements allowed
  • 3 Examples of hydrostatic skeletons
    • 3.1 Polyps
    • 3.2 Worm-shaped animals (vermiforms)
  • 4 References


Animals require specialized structures for support and movement. For this, there is a wide diversity of skeletons that provide an antagonist for the muscles, transmitting the force of contraction..

However, the term “skeleton” goes beyond the typical bone structures of vertebrates or the external skeletons of arthropods..

A fluid substance can also meet the support requirements using an internal pressure, forming the hydroskeleton, widely distributed in the invertebrate lineage..

The hydroskeleton consists of a cavity or closed cavities filled with fluids that use a hydraulic mechanism, where the contraction of the musculature translates into the movement of the fluid from one region to another, working in the mechanism of the transmission of the impulse - muscle antagonist.

The fundamental biomechanical characteristic of hydroskeletons is the constancy of the volume that they form. This must have the ability to compress when applying physiological pressures. This principle is the basis for the function of the system.

Mechanism of hydrostatic skeletons

The support system is spatially arranged as follows: the musculature surrounds a fluid-filled central cavity.

It can also be arranged in a three-dimensional way with a series of muscle fibers that form a solid mass of muscle, or in a muscle network that pass through spaces filled with fluid and connective tissue..

However, the limits between these arrangements are not well defined and we find hydrostatic skeletons that present intermediate characteristics. Although there is wide variability in the hydroskeletons of invertebrates, they all function according to the same physical principles..


The three general arrangements of muscles: circular, transverse, or radial. The circular musculature is a continuous layer that is arranged around the circumference of the body or the organ in question.

Transverse muscles include fibers that lie perpendicular to the longest axis of the structures and can be oriented horizontally or vertically - in bodies with a fixed orientation, conventionally vertical fibers are dorsoventral and horizontal fibers are transverse.

Radial muscles, on the other hand, include fibers located perpendicular to the longest axis from the central axis towards the periphery of the structure..

Most muscle fibers in hydrostatic skeletons are obliquely striated and have the ability to "super elongate".

Types of movements allowed

Hydrostatic skeletons support four types of movement: elongation, shortening, bending, and twisting. When a contraction in the muscle decreases, the area of ​​the volume constant, elongation of the structure occurs.

Elongation occurs when any of the muscles, vertical or horizontal, contracts just keeping the tone towards the orientation. In fact, the entire operation of the system depends on the pressure of the internal fluid.

Imagine a constant volume cylinder with an initial length. If we decrease the diameter by means of a contraction of the circular, transverse or radial muscles, the cylinder is stretched to the sides due to the increase in pressure that occurs inside the structure..

In contrast, if we increase the diameter the structure shortens. The shortening is related to the contraction of muscles with longitudinal arrangements. This mechanism is essential for hydrostatic organs, such as the tongue of most vertebrates..

For example, in the tentacles of a cephalopod (which uses a type of hydrostatic skeleton), it requires only a 25% decrease in diameter to increase 80% in length..

Examples of hydrostatic skeletons

Hydrostatic skeletons are widely distributed in the animal kingdom. Although common in invertebrates, some vertebrate organs work on the same principle. In fact, hydrostatic skeletons are not restricted to animals, certain herbaceous systems use this mechanism..

Examples range from the notochord characteristic of sea squirts, cephalochords, larvae and adult fish, to larvae of insects and crustaceans. Below we will describe the two best known examples: polyps and worms.


Anemones are the classic example of animals that have a hydrostatic skeleton. The body of this animal is formed by a hollow column closed at the base and with an oral disc at the upper portion surrounding the mouth opening. The musculature is basically the one described in the previous section.

The water enters through the cavity of the mouth, and when the animal closes it the internal volume remains constant. Thus, the contraction that decreases the diameter of the body, increases the height of the anemone. Similarly, when the anemone extends the circular muscles it widens and its height decreases.

Worm-shaped animals (vermiformes)

The same system applies to earthworms. This series of peristaltic movements (lengthening and shortening events) allows the animal to move.

These annelids are characterized by having the coelom divided into segments to prevent fluid from one segment from entering the other, and each one operates independently..


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