A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes referred to as hydrostatic for their possession of a hydrostatic organ instead of a hydrostatic skeleton. A hydrostatic organ and a hydrostatic skeleton may have the same capabilities, but they are not the same.
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Helically reinforced hydrostatic skeleton structure is typical for flexible structures as in soft-bodied animals.
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A muscular hydrostat, like a hydrostatic skeleton, relies on the fact that water is effectively incompressible at physiological pressures. In contrast to a hydrostatic skeleton, where muscle surrounds a fluid-filled cavity, a muscular hydrostat is composed mainly of muscle tissue. Since muscle tissue itself is mainly made of water and is also effectively incompressible, similar principles apply.
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Many animals with a wormlike cylindrical body have a hydrostatic skeleton with a flexible skin and a water-filled body cavity (coelom or pseudocoelom). They move by peristalsis, using opposed circular and longitudinal muscles, which act on the hydrostatic skeleton to change the body's shape. Hydrostatic skeletons are typically arranged in a cylinder. Hydrostatic skeletons can be controlled by several different muscle types.
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A simplified image showing earthworm movement via peristalsis The earthworm is a limbless annelid worm with a hydrostatic skeleton that moves by peristalsis. Its hydrostatic skeleton consists of a fluid-filled body cavity surrounded by an extensible body wall. The worm moves by radially constricting the anterior portion of its body, resulting in an increase in length via hydrostatic pressure. This constricted region propagates posteriorly along the worm's body.
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Fluid within the organism is evenly concentrated so the forces of the muscle are spread throughout the whole organism and shape changes can persist. These structural factors also persist in a hydrostatic organ. A non-helical hydrostatic skeleton structure is the functional basis of the mammalian penis.
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The other group of legged terrestrial invertebrates, the velvet worms, have soft stumpy legs supported by a hydrostatic skeleton. The prolegs that some caterpillars have in addition to their six more-standard arthropod legs have a similar form to those of velvet worms, and suggest a distant shared ancestry.
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Muscular hydrostats do not contain a fluid-filled cavity. These structures are constructed of muscle and connective fibers, densely packed into a 3-D structure. In many cases, the muscular hydrostat can be manipulated in all three dimensions. This allows for more precise movement compared to a typical hydrostatic skeleton.
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Organisms containing a hydrostatic skeleton have advantages and disadvantages. Their fluid shape allows them to move around easily while swimming and burrowing. They can fit through oddly shaped passages and hide themselves more effectively from predators. They are able to create a force when squeezing through rocks and create a “prying open” gesture.
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The tongue is a muscular hydrostat. A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton.
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Most molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.
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The head of a male wunderpus is wider than its mantle and for female wunderpus octopuses, their mantle is wider than their head. For females, this is due to the large ovary in their mantle. They have gill with 6-7 lamellae per demibranch present. The wunderpus octopus has a relatively small body and a flexible hydrostatic skeleton.
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Earthworms have rings of muscles that are filled with fluid, making their entire body hydrostatic. A sea anemone has a hydrostatic head, with arms radiating out around the mouth. This structure is helpful in feeding and locomotion. An example of a simple vertebrate containing a hydrostatic skeleton would be Enteropneusta, with the common name of acorn worm.
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This organism is classified as a Hemichordate, and they are marine worms that use their hydrostatic skeleton to tunnel and anchor themselves into the ground. This can be used for locomotion, but also can aid in the defense of the organism against outside forces as the worm can try to "hide" itself within the ocean floor.
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Capitella teleta lives in the shallow-water or intertidal marine environment. It is also found in salt marshes and is often found in high concentrations in disturbed soft sediments. It is a member of the infaunal benthic community. C. teleta burrows through the sediment by peristalsis, using its hydrostatic skeleton and contraction of longitudinal and circular muscles in the body wall.
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The corpus cavernosum contains connective fibers arranged both parallel and perpendicular to the longitudinal axis. These fibers remain folded when the penis is flaccid, but unfold as the penis fills with blood during an erection, which allows the penis to resist bending. The penises of turtles are structured similarly, although they evolved separately. Other vertebrates sometimes utilize a modified hydrostatic skeleton called a muscular hydrostat.
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Similar sets of muscles line the gut, and their actions move digesting food toward the worm's anus. Earthworms are hermaphrodites: each carries male and female sex organs. As invertebrates, they lack a true skeleton, but maintain their structure with fluid-filled coelom chambers that function as a hydrostatic skeleton. "Earthworm" is the common name for the largest members of Oligochaeta (which is a class or subclass depending on the author).
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A hydrostatic skeleton uses hydrostatic pressure generated from muscle contraction against a liquid filled cavity. The liquid filled cavity is commonly referred to as the hydrostatic body. The liquid within the hydrostatic body acts as an incompressible fluid and the body wall of the hydrostatic body provides a passive elastic antagonist to muscle contraction, which in turn generates a force, which in turn creates movement.R. B. Clark and J. B. Cowey.
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In medusae the only supporting structure is the mesoglea. Hydra and most sea anemones close their mouths when they are not feeding, and the water in the digestive cavity then acts as a hydrostatic skeleton, rather like a water-filled balloon. Other polyps such as Tubularia use columns of water-filled cells for support. Sea pens stiffen the mesoglea with calcium carbonate spicules and tough fibrous proteins, rather like sponges.
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Connective fibers, often collagenous, are arranged in a helical shape within the wall of the hydrostatic skeleton. The helical shape formed by these fibers allows for elongation and shortening of the skeleton, while still remaining rigid to prevent torsion. As the shape of the cylinder changes, the pitch of the helix will change. The angle relative to the long axis will decrease during elongation and increase during shortening.
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Factors controlling the change of shape of certain nemertean and turbellarian worms. J Exp Biol, 35(4):731–748, December 1958. This structure plays a role in invertebrate support and locomotor systems and is used for the tube feet in starfish and body of worms . A specialized version of the hydrostatic skeleton is a called a muscular hydrostat, which consists of a tightly packed array of three-dimensional muscle fibers surrounding a hydrostatic body.
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There is no coelom (body cavity) and the interior of the animal is filled with poorly differentiated connective tissue. In the macrodasyidans, Y-shaped cells, each containing a vacuole, surround the gut and may function as a hydrostatic skeleton. The mouth is at the anterior end and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen, lined by myoepithelial cells. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells.
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A coelom can absorb shock or provide a hydrostatic skeleton. It can also support an immune system in the form of coelomocytes that may either be attached to the wall of the coelom or may float about in it freely. The coelom allows muscles to grow independently of the body wall — this feature can be seen in the digestive tract of tardigrades (water bears) which is suspended within the body in the mesentery derived from a mesoderm-lined coelom.
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These colonies adopt a range of massive, branching, leaf-like and encrusting forms. Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like supports internally. Other anthozoans, such as sea anemones, are naked; these rely on a hydrostatic skeleton for support. Some of these species have a sticky epidermis to which sand grains and shell fragments adhere, and zoanthids incorporate these substances into their mesogloea.
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Zoological Journal of the Linnean Society, 83(4):307–324, 1985. Circular muscles decrease the diameter of a hydrostatic body, resulting in an increase in the length of the body, whereas longitudinal muscles shortens the length of a hydrostatic body, resulting in an increase in the diameter of the body. There are four categories of movements of a hydrostatic skeleton : elongation, shortening, bending and torsion. Elongation, which involves an increase in the length of a hydrostatic body requires either circular muscles, a transverse muscle arrangement, or radial muscle arrangement.
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In all species the lophophore is supported by cartilage and by a hydrostatic skeleton (in other words by the pressure of its internal fluid), and the fluid extends into the tentacles. Some articulate brachiopods also have a brachidium, a calcareous support for the lophophore attached to the inside of the brachial valve. The tentacles bear cilia (fine mobile hairs) on their edges and along the center. The beating of the outer cilia drives a water current from the tips of the tentacles to their bases, where it exits.
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Labelled cross section of a jellyfish The main feature of a true jellyfish is the umbrella- shaped bell. This is a hollow structure consisting of a mass of transparent jelly-like matter known as mesoglea, which forms the hydrostatic skeleton of the animal. 95% or more of the mesogloea consists of water, but it also contains collagen and other fibrous proteins, as well as wandering amoebocytes which can engulf debris and bacteria. The mesogloea is bordered by the epidermis on the outside and the gastrodermis on the inside.
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Since the anemone lacks a rigid skeleton, the contractile cells pull against the fluid in the gastrovascular cavity, forming a hydrostatic skeleton. The anemone stabilizes itself by flattening its pharynx which acts as a valve, keeping the gastrovascular cavity at a constant volume and making it rigid. When the longitudinal muscles relax, the pharynx opens and the cilia lining the siphonoglyphs beat, wafting water inwards and refilling the gastrovascular cavity. In general, the sea anemone inflates its body to extend its tentacles and feed, and deflates it when resting or disturbed.
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There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton, to control the water pressure in the cavity without expelling undigested food.
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Sipunculans are worms ranging from in length, with most species being under . The sipunculan body is divided into an unsegmented, bulbous trunk and a narrower, anterior section, called the "introvert". Sipunculans have a body wall somewhat similar to that of annelids (though unsegmented) in that it consists of an epidermis without cilia overlain by a cuticle, an outer layer of circular and an inner layer of longitudinal musculature. The body wall surrounds the coelom (body cavity) that is filled with fluid on which the body wall musculature acts as a hydrostatic skeleton to extend or contract the animal.
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Unlike the arthropods, velvet worms do not possess a rigid exoskeleton. Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many unrelated soft-bodied animals that are cylindrically shaped, for example sea anemones and various worms. Pressure of their incompressible internal bodily fluid on the body wall provides rigidity, and muscles are able to act against it. The body wall consists of a non- cellular outer skin, the cuticula; a single layer of epidermis cells forming an internal skin; and beneath this, usually three layers of muscle, which are embedded in connective tissues.
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The claws of the feet are used only on hard, rough terrain where a firm grip is needed; on soft substrates, such as moss, the velvet worm walks on the foot cushions at the base of the claws. Actual locomotion is achieved less by the exertion of the leg muscles than by local changes of body length. This can be controlled using the annular and longitudinal muscles. If the annular muscles are contracted, the body cross- section is reduced, and the corresponding segment lengthens; this is the usual mode of operation of the hydrostatic skeleton as also employed by other worms.
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Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components. Arthropods come from a lineage of animals that have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a hydrostatic skeleton, which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems.
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This genus also all have a hydrostatic skeleton, a muscular body filled with fluid similar to annelids, but also a chitinous exoskeleton, like the arthropods. Therefore, as the P. indigo continually grows, it must undergo ecydysis (moulting) every few weeks to enable this. The many legs walking in co-ordination together can over a distance of 200mm in about a minute, slow in comparison to species elsewhere The velvet worms have simple eyes, however it is primarily just for detecting light rather than detailed sight - this is mainly used to determine whether it is night or day, enabling it to come out at night to avoid desiccation P. indigo breathes through small pores in the side, called spiracles. These spiracles are usually able to be opened and closed in response to the environment both inside and outside the organism, but in the P. indigo this is not the case.
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