User:BrookBignell/sandbox

- Introduction

A Hydrostatic Skeleton is, in its simplest and least complex form is the skeleton of primitive and some modern day organisms. Hydrostatic skeleton can also refer to an organ in more advanced organisms. As its name suggests, containing Hydro meaning "water", a hydrostatic skeleton is a fluid-filled skeleton or organ. As a skeletal structure, it possesses the ability to affect shape and movement and involves two mechanical units: the muscle layers and the body wall (Longitudinal and circular) and 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. 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 comparison to a hydrostatic organ.

- Structure

Hydrostatic skeletons are typically arranged in a cylinder. Hydrostatic skeletons can be controlled by several different muscle types. Length can be adjusted by longitudinal muscle fibers, parallel to the longitudinal axis. They may be found in continuous sheets or isolated bundles. Diameter can be manipulated by three different muscle types. Circular musculature wraps around the circumference of the cylinder. Radial musculature extends from the center of the cylinder towards the surface. Transverse musculature arrange in parallel and perpendicular sheets crossing the diameter of the cylinder.

Within the cylinder usually lies a fluid, most often water. The fluid resists changes in volume. Contraction of circular, radial or transverse muscles increases the pressure within the cylinder, and results in an increase in length. Contraction of longitudinal muscles can shorten the cylinder.

Change in shape is limited by connective tissue fibers. 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.

Organismins

Hydrostatic Skeletons are very common in invertebrates. Some common examples are sea anemones and earthworms. A sea anemone has a hydrostatic head, with arms radiating out around the mouth. This structure is helpful in feeding and locomotion. Earthworms are rings of muscles that are filled with fluid, making their enter body hydrostatic.

An example of a simple vertebrate containing a hydrostatic skeleton would be Enteropneusta, with the common name of acorn worm. This organism is classified as a Hemichordate. They are marine worms that use their hydrostatic properties 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.

- Hydrostatic Skeletons in Vertebrates

The function of the mammalian penis relies on the use of hydrostatic skeleton. The hydrostatic fluid is blood, which fills the penis during an erection. Unlike the use of hydrostatic skeletons in many invertebrates, which use the bending of the animal for locomotion, the penis must resist bending and shape changes during sex. Instead of connective fibers arranged in a helical shape, the penis contains a layer called the corpus cavernosum. The corpus cavernosum contains connective fibers arranged both parallel and perpendicular to the longitudinal axis. These fibers remain folded whenthe penis is flaccid, but unfold as the penis fills with blood during an erection, allowing 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. Muscular hydrostats do not contain a fluid-filled cavity. These structures are constructed of muscle and connective fibers, densely packed into a 3D 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. 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. Mammalian tongues have the structure of a central core of muscle fibers surrounded by bundles of longitudinal muscles and alternating parallel sheets of transverse muscle fibers. Elephant trunks and tapir proboscises also utilize a muscular hydrostat. These structures are composed of longitudinal fibers surrounded by radial and helical fibers.

- Evelution

Add Citations

Comp- Vert Book - 428, 496, 18

http://jeb.biologists.org/content/215/8/1247

http://infinitespider.com/hydrostatic-skeleton/