The hydrostatic skeleton is made possible by closed fluid-filled internal spaces of the body. It is of great importance in a wide variety of animal groups because it permits the antagonistic action of muscles used in locomotion and other movements. In hydrostatic skeletons, muscles require an antagonistic system of contraction and relaxation by moving fluid around. Think of it sort of like when you squeeze a water balloon and the water moves from one end to the other, stretching the balloon's skin. Something similar happens in the bodies of animals with hydrostatic skeletons.
Figure 1. The skeleton of the red-knobbed sea star Protoreaster linckii is an example of a hydrostatic skeleton. A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment within the body, skeldtons the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This hydrostafic is under hydrostatic pressure because of the fluid and supports the other organs of the organism.
This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates Figure 1. Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The sksletons in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement.
For example, earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which ho shorten and lengthen the body. Lengthening hydeostatic body extends the hydgostatic end of the organism. Most organisms have a mechanism to fix themselves in the substrate.
Shortening the muscles then draws the posterior portion of the body forward. Although a how do hydrostatic skeletons move skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals. Figure 2. Muscles attached to the exoskeleton of the Halloween crab Gecarcinus quadratus allow it to what types of food contain protein. An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism.
For example, the shells of crabs and insects are exoskeletons Figure 2. This skeleton type provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskeletons that consist of 30Ч50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material.
Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, hydrostatuc must periodically shed their exoskeletons because the exoskeleton does not grow as the organism hpw.
Figure 3. The skeletons of humans and horses are examples of endoskeletons. An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms.
An example of a primitive endoskeletal structure is the spicules of sponges. The bones of hydrosfatic are composed of tissues, whereas sponges have no true tissues Figure 3. Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton. The human skeleton how do hydrostatic skeletons move an endoskeleton that consists of bones smeletons the adult.
It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement.
The skeletal system in vertebrates is divided into the axial skeleton skletons consists of the skelrtons, vertebral column, and rib cageand the appendicular skeleton which consists of how do hydrostatic skeletons move shoulders, limb bones, the pectoral girdle, and the pelvic girdle.
The three types of skeleton designs are hydrostatic skeletons, exoskeletons, and endoskeletons. A hydrostatic skeleton is formed by a fluid-filled compartment held under hydrostatic pressure; movement is created by the muscles producing pressure on the skeletosn.
An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through moge attached on the inside. An endoskeleton is an how to make adenium bonsai skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles. Improve this page Learn More. Skip to main content.
Module The Musculoskeletal System. Search for:. Visit the Anatomy Explorer: Skeletal System to learn the individual bones in great detail. See exactly where they are found in the human body and learn more about the purpose of each. In Summary: Types of Skeletal Systems The three types of skeleton designs are hydrostatic skeletons, exoskeletons, and endoskeletons. Try It. Did you have an idea for improving this content? Licenses and Attributions.
CC licensed content, Shared previously.
Hydrostatic skeletons (sometimes just called УhydrostatsФ) use a cavity filled with water; the water is incompressible, so the organism can use it to apply force or change shape. Plants use osmotic pressure to pressurize the cavity, whereas animals do it with muscle layers in the hydrostat's walls. Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. Oct 30, †Ј A hydrostatic skeleton, or hydroskeleton, is a kind of skeleton that is composed of soft tissue filled with an incompressible fluid or gel-like substance. Along with insects, mollusks and.
William M. Kier; The diversity of hydrostatic skeletons. J Exp Biol 15 April ; 8 : Ч A remarkably diverse group of organisms rely on a hydrostatic skeleton for support, movement, muscular antagonism and the amplification of the force and displacement of muscle contraction. In hydrostatic skeletons, force is transmitted not through rigid skeletal elements but instead by internal pressure.
Functioning of these systems depends on the fact that they are essentially constant in volume as they consist of relatively incompressible fluids and tissue. Contraction of muscle and the resulting decrease in one of the dimensions thus results in an increase in another dimension. By actively with muscle or passively with connective tissue controlling the various dimensions, a wide array of deformations, movements and changes in stiffness can be created.
An amazing range of animals and animal structures rely on this form of skeletal support, including anemones and other polyps, the extremely diverse wormlike invertebrates, the tube feet of echinoderms, mammalian and turtle penises, the feet of burrowing bivalves and snails, and the legs of spiders. In addition, there are structures such as the arms and tentacles of cephalopods, the tongue of mammals and the trunk of the elephant that also rely on hydrostatic skeletal support but lack the fluid-filled cavities that characterize this skeletal type.
Although we normally consider arthropods to rely on a rigid exoskeleton, a hydrostatic skeleton provides skeletal support immediately following molting and also during the larval stage for many insects. Thus, the majority of animals on earth rely on hydrostatic skeletons. Animal skeletons serve a variety of functions in support and movement.
For example, the skeleton transmits the force generated by muscle contraction, providing support for maintenance of posture and for movement and locomotion. Also, because muscle as a tissue cannot actively elongate, skeletons provide for muscular antagonism, transmitting the force of contraction of a muscle or group of muscles to re-elongate their antagonists.
In addition, the skeleton often serves to amplify the displacement, the velocity or the force of muscle contraction. A wide range of animals and animal structures lack the rigid skeletal elements that characterize the skeletons of familiar animals such as the vertebrates and the arthropods. In this Commentary, I will summarize the arrangement of the muscle fibers and their role in support and movement in hydrostatic skeletons.
I will also describe the connective tissue fibers of hydrostatic skeletons that play a crucial role in controlling and limiting shape change. I will then describe the principles of support and movement and will illustrate these principles with an overview of the remarkable diversity of animals and animal structures that rely on hydrostatic skeletal support. Many of the animals and animal structures that rely on hydrostatic skeletons are approximately cylindrical in shape Wainwright, The musculature is typically arranged so that both the diameter and the length of the cylinder can be actively controlled Fig.
Three different muscle arrangements are observed that can control the diameter. All are arranged perpendicular to the longitudinal axis: 1 circular musculature, with a layer of fibers wrapping the cylinder circumferentially; 2 radial musculature, with fibers or fiber bundles originating near the central axis of the cylinder and extending towards the surface; and 3 transverse musculature, with fibers extending across the diameter of the cylinder as parallel sheets, typically in two orientations that are perpendicular to each other.
Control of the length is achieved by longitudinal muscle fibers, which are oriented parallel to the longitudinal axis and can be arranged as a continuous sheet or as isolated bundles. Helical muscle fibers are sometimes also present; their function is discussed below in the section on muscular hydrostats. The fundamental principles of support and movement in hydrostatic skeletons are straightforward. In addition to the musculature described above, conventional hydrostatic skeletons typically include a volume of enclosed fluid.
The fluid is usually a liquid essentially water and thus has a high bulk modulus, which simply means that it resists significant volume change. Contraction of circular, radial or transverse muscle fibers will decrease the diameter, thereby increasing the pressure, and because no significant change in volume can occur, this decrease in diameter must result in an increase in length.
Following elongation, shortening can be caused by contraction of the longitudinal muscle fibers, re-expanding the diameter and thus re-elongating the circular, radial or transverse muscle fibers. The longitudinal muscle fibers and the circular, radial or transverse muscle fibers thus function as antagonists, in a manner that is analogous to muscles on either side of a joint in an arthropod or vertebrate.
The function of the system thus depends on the pressurized internal fluid. Schematic diagram illustrating common muscle fiber orientations in hydrostatic skeletons. The cross-sectional area can be controlled by fibers oriented circumferentially, radially and transversely; fibers in these orientations are important for elongation and also support of bending movements. The length is controlled by longitudinal fibers, which shorten the organ or body and, through selective contraction, also create bending.
Helically arranged muscle fibers are found in muscular hydrostats and create torsion or twisting around the long axis. Both right- and left-handed helical muscle fiber layers are typically present and create torsion in either direction. The displacement and the velocity of contraction of the radial, circular or transverse musculature can thus be amplified. Furthermore, since the strain in the longitudinal muscles is proportional to the overall longitudinal strain during elongation, length changes such as those of a squid tentacle require longitudinal muscle fibers that are capable of a greater range of shortening and elongation than is typical for most vertebrate and arthropod muscle fibers.
Vertebrate tongues often rely on a form of hydrostatic support see below and in highly protrusible tongues, the longitudinal retractor muscles are either specialized for supercontraction, as is the case in the retractor muscles of the tongue of the chameleon Herrel et al. Supercontracting muscle is also found in the some insect larvae, which rely on hydrostatic support and undergo greater deformation than the adults Osborne, ; Hardie and Osborne, Relationship between diameter and length of a constant volume cylinder.
The positions on the graph of shapes A through D drawn to scale are indicated. A small decrease in diameter from shape B to D causes a large increase in length. From Kier and Smith Kier and Smith, The walls of many hydrostatic skeletons are reinforced with layers of connective tissue fibers that control and limit shape change.
Even though the connective tissue fibers are typically stiff in tension and are thus relatively inextensible, such an arrangement actually allows length change. Elongation and shortening is possible because the pitch of the helix changes during elongation the fiber angle, which is the angle relative to the long axis, decreases and shortening the fiber angle increases Clark and Cowey, ; Shadwick, Reinforcement with this type of connective tissue fiber array limits and controls shape change, allows smooth bending to occur, and prevents torsion or twisting around the longitudinal axis Fig.
As a comparison, consider the implications of wrapping the body wall with circular and longitudinal connective tissue fibers. Orthogonal arrays also resist bending forces and will fail in bending by kinking. They also offer little resistance to twisting Alexander, ; Alexander, ; Koehl et al. The role that the crossed-fiber helical connective tissue array plays in controlling shape change in wormlike animals has been analyzed with a simple geometrical model Clark and Cowey, ; Harris and Crofton, ; Seymour, ; Shadwick, The model considers a right circular cylinder wrapped with a single turn of an inextensible helical fiber and is used to solve for the enclosed volume of the cylinder as the fiber angle varies from 90 deg an infinitely thin disk with a circumference equal to the helical fiber length to 0 deg an infinitely thin cylinder with a length equal to the helical fiber length.
The volume approaches 0 as the fiber angle approaches 0 or 90 deg, with the maximum volume occurring at a fiber angle of 54 deg 44 min Fig. A worm with a volume equal to the maximum that could be enclosed by the fiber system at 54 deg 44 min would be unable to elongate or shorten significantly because any length change would entail a decrease in volume of an essentially incompressible fluid or strain in stiff connective tissue fibers.
In reality, most worms enclose a volume of fluid and tissue that is less than the maximum, and thus at a fiber angle of 54 deg 44 min, they are flattened in cross-sectional shape and lack turgidity.
Such a worm can change length, subject to the constraints of the helical fiber system. For instance, the worm can elongate, decreasing the fiber angle until it reaches the angle at which the volume contained by the helical fiber system equals the actual volume of the worm. No further elongation can occur beyond this angle because a decrease in the enclosed volume of fluid would be required.
By examining Fig. Diagram illustrating the differences between a pressurized cylinder reinforced with fibers in an orthogonal array AЧD and a cylinder reinforced with fibers in a crossed-fiber helical array EЧH. Orthogonal fiber reinforcement prevents length change B , provides stiffness in bending until failure occurs by kinking C , and allows torsion or twisting around the long axis D. Cylinders reinforced with a crossed-fiber helical array can change length F , bend in smooth curves G and resist torsion H.
After Wainwright Wainwright, A remarkable diversity of animals and animal structures rely on some form of hydrostatic skeletal support. In addition to variation in the arrangement of the musculature, considerable variation exists in the form of the fluid-filled cavities, ranging from extensive cavities to solid musculature or tissue with minimal extracellular fluid.
Below I review some examples of hydrostatic skeletons to provide an overview of their diversity of structure and function. Polyps, such as sea anemones, are one of the classic examples of animals that rely on a hydrostatic skeleton. The body of a sea anemone is in the form of a hollow column that is closed at the base and equipped at the top with an oral disk that includes a ring of tentacles surrounding the mouth and pharynx Fig.
By closing the mouth, the water in the internal cavity Ч the coelenteron Ч cannot escape and thus the internal volume remains essentially constant. The walls of an anemone include a layer of circular muscle fibers. Longitudinal muscle fibers are found on the vertical partitions called septa that project radially inward into the coelenteron, including robust longitudinal retractor muscles along with sheets of parietal longitudinal muscle fibers adjacent to the body wall.
Additional radial muscle fibers occur on the septa and a large circular sphincter muscle is present surrounding the base of the tentacles of the oral disk Batham and Pantin, With the mouth closed, contraction of the circular muscle layer decreases the diameter and thereby increases the height of the anemone.
Contraction of the longitudinal muscles shortens the anemone and re-extends the circular muscle fibers. Bending of the column can be achieved by simultaneous contraction of the longitudinal bundles on one side of the anemone and contractile activity of the circular muscle fibers to prevent shortening of the column due to compressional forces from the longitudinal musculature.
Thus, with this simple muscular arrangement a diverse array of bending movements and height change can be produced. Most anemones can also expel the coelenteric fluid while retracting the tentacles, the oral disk and the column towards the base in response to disturbances such as predators. The process involves first opening the pharynx and mouth by contraction of the radial muscle fibers.
Contraction of the longitudinal muscles then shortens the column and withdraws the oral disk and tentacles into the column. The sphincter finally contracts, reducing the diameter of the oral disk and closing off the top of the column. Reinflation occurs slowly as water is pumped into the column by a pair of ciliated grooves called siphonoglyphs that extend the length of the pharynx. Elastic energy storage in the fiber-reinforced gelatinous extracellular matrix called the mesoglea also aids restoration of shape Alexander, ; Gosline, Relationship between the volume contained by the crossed-fiber helical system and the fiber angle.
The actual volumes of several nemertean and turbellarian worms are indicated with fine horizontal lines; the heavy lines show the measured range of elongation and contraction. Note that a worm with a smaller relative volume e. Lineus longissimus is typically capable of a greater range of elongation and shortening than a worm with a greater relative volume e.
Some worms e. Polycelis do not reach the limits set by the crossed-fiber helical system because of other morphological constraints. From Clark and Cowey Clark and Cowey, Schematic cutaway view of a sea anemone showing the circular muscle fibers C , parietal longitudinal muscle fibers L , mouth M , pharynx P , retractor muscles R , sphincter muscle S , and tentacles T. After Ruppert et al. Ruppert et al. Diagram showing the stages in locomotory cycle of an earthworm.
Regions undergoing longitudinal muscle contraction are stationary indicated by large dots and are drawn twice as wide as those regions undergoing circular muscle contraction. The tracks of individual points on the body through time are indicated by the lines connecting each drawing, extending from left to right on the page.
From Gray and Lissman Gray and Lissman,