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Fluid pressure in the body of a spider is used to extend its legs. Hence, when a spider dies, the fluid pressure can no longer be sustained, and the legs contract and fold under the body.
A hydrostatic skeleton, also known as hydroskeleton, is a structure that comprises a fluid-filled cavity called coelom and the muscles that surround it. This fluid in the coelom (haemocoel), which is also called haemolymph, is present in open circulatory systems and is equivalent to a combination of blood and interstitial fluid. It is incompressible, and hence, maintains a constant volume against any pressure exerted on it. The contraction and relaxation of the muscles against the wall of the haemocoel bring about localization of the fluid pressure in a few segments of the body. This action causes movements in the animal body. In addition to this function, a hydrostatic skeleton also acts as a support structure for the body and can be used by the organism to modify its own shape.

This type of a skeletal system can be observed in the case of the animals belonging to phyla annelida (earthworm, leech), cnidaria (sea anemone, jellyfish, hydra), echinodermata (starfish, sea urchin, brittle stars, sea cucumbers), mollusca (snails, clams, octopus, nudibranch), nematoda (ascaris, hookworm), and platyhelminthes (tapeworm, liver fluke).

Hydrostatic Skeleton in Invertebrates

  • Hydra
  • Hydra
  • Jellyfish
  • Jellyfish
  • Sea Anemone
  • Sea Anemone
  • Flatworm
  • Flatworm
  • Liver Flukes
  • Liver Flukes
  • Earthworm
  • Earthworm
  • Leech
  • Leech
  • Brittle Star
  • Brittle Star
  • Sea Cucumber
  • Sea Cucumber
  • Sea Urchin
  • Sea Urchin
  • Star Fish
  • Star Fish
  • Cuttle Fish
  • Cuttle Fish
  • Snail
  • Snail
  • Nudibranch
  • Nudibranch
  • Octopus
  • Octopus
  • Oysters
  • Oysters
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Phylum Cnidaria

Cnidarians are very simple animals that exhibit a cylindrical body structure at the polyp stage, and an umbrella-like structure in the medusa stage.These animals have ciliated tissues called siphonoglyphs along both ends of the mouth all the way to the pharynx. The cilia function to pump water into its body cavity. Once the cavity is filled, the organism closes its mouth, while the cilia keep moving in order to create and maintain a positive pressure. These animals utilize this trapped sea water as the fluid required for the hydrostatic skeleton to function. The muscles are arranged in a circular manner along the wall of the body cavity.

When these muscles are contracted due to their circular arrangement, the cylindrical cavity is compressed and its diameter decreases. This shift in dimensions exerts pressure on the fluid which then causes the cavity to elongate. This elongation along the column affects the mesentery tissues that are situated longitudinally along the body wall, and cause them to stretch out. Once fully stretched, these muscles contract to return to their original position, which in turn causes the cavity column to shorten. Thus, the diameter of the cavity increases causing the mouth to open and release the water and thus flattens the animal. This process is carried out repeatedly to allow it to move and feed at the same time (since they are filter feeders). This action on the water is carried out by two opposing sets of muscles working against each other called antagonistic muscles. They are called so because the contraction of one set causes the relaxation of the other and vice versa.

Phylum Platyhelminthes

Flatworms have a more complex structure than cnidarians but they lack a true coelom. Despite this, they still possess a hydrostatic skeleton. These animals are of a flattened nature and possess muscle cells arranged in layers with a loose packing of cells derived from the mesoderm called mesenchyme.

Since these animals lack a body cavity, they also lack body fluid. Hence, the mesenchymal cells act as a non-compressible medium despite their non-flowing nature. This, in addition to the organized layers of muscle cells are utilized to help in the movement of the animal via contraction and relaxation of the muscle layers. However, due to it not being a true coelomate, the animal is able to conduct only gliding movements.

Phylum Nematoda

Nematodes are pseudoceolomates, i.e. the cavity is not completely lined with mesenchymal cells and the pseudocoelomic fluid bathes the internal organs enclosed in the cavity. This pseudocoel is covered by a body wall that bears longitudinally-arranged muscles divided into four fields due to the presence of the dorsal, ventral, and lateral cords.

However, the worm lacks the second set of antagonistic muscles for the proper functioning of the hydrostatic skeleton. This drawback is overcome by the nematode's ability to contract the four fields created by the cords, in groups. This grouped contraction causes localized shortening of the body cavity, and the pressure generated in the pseudocoelomic fluid results in the elongation of the other part of the body. This pattern of contraction and elongation produces sinusoidal waves along the animal's body. The undulations caused by the wave pattern allows the animal to move in a horizontal plane.

Phylum Annelida

Annelids exhibit a well-developed body and musculature along with a true coelom. They have a segmented body pattern with sets of setae projecting out from the body wall. Each segment possesses a segmental sphincter at its end which closes during the movement of the animal. This causes the quantity of fluid contained within a segment to remain constant. Each segment also shows presence of longitudinal and circular muscles (antagonistic muscles).

For the purpose of movement, the worm sequentially contracts and relaxes each segment of its body. As a result of the incompressible nature of the fluid, the contraction of the longitudinal muscles causes the circular muscles to stretch. This causes the segment to become short and fat, and the pressure causes projection of the setae. These setae allow the worm to get a foothold on the substance it is moving on. The sequential relaxation of the longitudinal muscles, coupled with the contraction of the circular muscles, allows the segments to become long and thin. The animal, then, utilizes its anchored position to withdraw the setae and move forward.

Phylum Echinodermata

Echinoderms exhibit the presence of tube feet all over their body, and a water vascular system (WVS) runs internally along the body wall, from the mouth all the way to the tip of these tube feet. The WVS is a series of specialized hydrostatic structures that transport sea water via ciliary action. Each of the several tube feet is a hollow, muscular structure, and is connected to an elastic, inflatable ampulla. The surface of the ampulla possesses a dense network of fibers, that expand when water is pumped into it. Uni-directional flow of water is ensured by the presence of one-way valves. When these fibers contract, the water is forced out into the tube foot, and the ampulla is deflated. This causes the tube feet to project out of their grooves.

Subsequently, these feet contract reversing the flow of water back into the ampulla, thus causing the retraction of the tube feet. If the feet are pressed against a rigid substrate, it causes the creation of a vacuum in that space allowing the animal to hold on due to the resultant suction. Contraction of the tube foot muscles forces water back into the ampulla and stretches the ampullar muscles. Many sea stars have muscle fibers attached to the bottom of the foot. When the bottom of the tube foot is pressed against a solid surface, contraction of these muscle fibers creates a vacuum allowing the foot to operate as a suction cup. These tube feet are also under the control of the nervous system of the animal and can be used to move the feet in any direction. These factors allow echinoderms to navigate and move.

Phylum Mollusca

In mollusks, the body cavity is host to its various organs, and the cavity itself, is also utilized by the circulatory system. This circulatory system utilizes a series of interconnected body spaces in the animal to transport haemolymph. These spaces are called sinuses, and are utilized for the functioning of the hydrostatic skeleton.

However, many zoologists suggest that a mollusk's use of its sinuses and haemolymph flow to generate movement portrays a hydraulic system rather than a hydrostatic skeleton. The underlying reason being that the animal transports its haemolymph from one space to another space instead of a single pressurized chamber of fluid. Contrary to the use of antagonistic muscles in a hydrostatic skeleton, a mollusk utilizes its ability to direct its haemolymph flow to achieve bodily projections and motility.

Pros and Cons

Mutable shape - It allows the organism to change its body shape intentionally, and this can be used by the animal either to fit into cramped spaces to defend itself or to swim with ease.

Hydrostatic strength - Such a skeleton allows the animal to act as a hydraulic lever. It can squeeze under structures and then expand itself, causing the space to expand.

Healing - These organisms heal quickly and with ease as their haemolymph is made out of water and the nutrients in it are also easily found around the animal's habitat.

Flexibility - The use of minimal number of muscles along with the manipulation of water pressure allows the animal to be highly flexible while moving.

Protection - The pressurized fluid in the body cavity acts as a shock-absorbent that protects the internal organs of the animal.

Adaptability - This skeletal system helps the animal adapt easily to moist or aquatic environment.

Structure - It is not a defined structure and cannot give rise to and support any form of limbs.

Desiccation - The working of this system is wholly dependent on an aquatic environment, and this system is also responsible for the animal to eat and survive. Hence, in a non-aquatic environment, the animal would die and dry up.

Limited strength - Since the skeleton is not a rigid structure, there is no scope for increase in the body mass of the animal.

Although this form of skeleton is unique in invertebrates, some vertebrates possess a similar analogous structure. Such a structure is called muscular hydrostat, and is made up of muscles. Despite its lack of a fluid-filled cavity, it works on the same principle since muscle tissues have high water content and are incompressible. Examples of muscular hydrostats include the human tongue, the trunk of an elephant, and the tentacles of an octopus.