The hemichordates are among the many marine animals with hydrostatic skeletons and peristaltic locomotion. Hydrostatic skeletons are very common in invertebrates. A common example is the earthworm. Also, hydrostatic nature is common in marine life such as jelly fish, starfish, and sea anemones.
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However, it does not protect internal organs from external damage very effectively. Because the hydrostatic skeletons have limited ability for attachment of limbs, the organisms are relatively simple and do not have many abilities to grab or latch onto things. Organisms with complete hydrostatic skeletons need to be in an environment that allows them to re-fill themselves with their fluid that is necessary for survival. This is why hydrostatic skeletons are common in marine life.
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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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While in typical hydrostatic skeletons, movement is generated by applying force to a fluid-filled cavity, muscular hydrostats generate movement by muscle contractions. When one muscle contracts and decreases in area, other muscles within the structure must expand in response. Helical muscles may be present, which can create torsion, an ability that is restricted in hydrostatic skeletons. Muscular hydrostats are found in mammalian, reptilian, and amphibian tongues.
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They have a large amount of access to the necessary elements for survival. Terrestrial organisms that have hydrostatic skeletons generally have a lack of strength because they are not in a fluid environment. If one were to expand its body too much, it would collapse under its own weight.
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The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied body plans. Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms.
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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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The mammalian penis is a hydrostatic organ. The hydrostatic fluid, in this case blood, fills the penis during an erection. Unlike the hydrostatic skeletons of many invertebrates, which use the bending of the animal for locomotion, the penis must resist bending and shape changes during sexual intercourse. Instead of connective fibers arranged in a helical shape, the penis contains a layer called the corpus cavernosum.
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Not being a true phylogenetic group, soft-bodied organisms vary enormously in anatomy. Cnidarians and flatworms have a single opening to the gut and a diffuse nerve system. The roundworms, annelids, molluscs, the various lophoporate phyla and non-vertebrate chordates have a tubular gut open at both ends. While the majority of the soft-bodied animals typically don't have any kind of skeleton, some do, mainly in the form of stiff cuticles (roundworms, water bears) or hydrostatic skeletons (annelids).
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Hydrostatic organs are more common in advanced organisms, while hydrostatic skeletons are more common in primitive organisms. As its name suggests, containing hydro meaning "water", being hydrostatic means that the skeleton or organ is fluid-filled. As a skeletal structre, it possesses the ability to affect shape and movement, and involves two mechanical units: the muscle layers and the body wall. The muscular layers are longitudinal and circular, and part of the fluid-filled coelom within. Contractions of the circular muscles lengthen the organism’s body, while contractions of the longitudinal muscles shorten the organism’s body.
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