Muscular hydrostat

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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. The principle behind the hydrostatic skeleton is that water is effectively incompressible at physiological pressures. Thus, a fiber-wound chamber full of water will act as a constant-volume system. What makes the muscular hydrostat unique is that it relies on the same principle, but there is no water-filled cavity. Instead, the bulk of the organ is made up of muscle, which also has constant volume and is effectively incompressible, its main material being water. Thus, instead of a cylinder wrapped with muscle and connective tissue that changes its shape, a muscular hydrostat is a cylinder made of muscle.

Name origin

Muscular hydrostat is a term coined by Dr. William M. Kier in 1982 to characterize the arms of octopuses and the arms and tentacles of squid. In a paper published in 1985, he showed that other organs, such as mammal and reptile tongues, and elephant trunks, fit in the category as well, although a difference worth noting is that while Octopus has no internal or exoskeleton, the Genioglossus muscle of the human tongue does originate from a bony prominence.

Common muscular hydrostats

Ordered by geologic age: oldest first, newest last.

• Whole bodies of many worms.
• Feet of mollusks, which became:
  • Arms and tentacles of cephalopods such as nautiloids, octopuses, squids.
• Tongues of animals.
• Trunks of elephants.

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