Hydrostatic skeletons
Hydrostatic skeletons are structures found in various organisms that utilize fluid-filled cavities for support and movement, relying on muscle contractions to manipulate the pressure of the contained fluid. This type of skeletal support is considered one of the most primitive, as it does not require mineralization for its formation. Hydrostatic skeletons can be observed in a wide array of life forms, including plants, protists, and animals, and they function by creating pressure within a compartment to provide stiffness and facilitate locomotion.
In simpler organisms like protozoa, these skeletons allow for the formation of pseudopodia, which are temporary extensions that help in movement. In more complex invertebrates, such as annelids and cnidarians, hydrostatic skeletons enable various forms of movement, like swimming and burrowing, by controlling fluid dynamics within their segmented bodies. Some arthropods, such as spiders and certain bivalves, also exhibit hydrostatic features, particularly in their limbs and muscular extensions. Even vertebrates utilize hydrostatic principles in specific contexts, such as in the reproductive organs. Overall, hydrostatic skeletons demonstrate a versatile adaptation for support and movement across diverse biological groups.
Hydrostatic skeletons
Hydrostatic skeletons are fluid-filled cavities within a body that provides support and allows for movement by muscle contractions. The most primitive form of skeletal support evolved is mostly hydrostatic skeletons because no mineralization process is necessary for their formation. Hydrostatic skeletons have evolved in many organisms, including plants, protists, and animals. The basis of all hydrostatic skeletons has to do with the material properties of water. Water cannot be compressed under biological conditions and thus can act as a support and locomotory transient skeleton. Essentially, all hydrostatic skeletons act similarly. Water is contained in a compartment and is subjected to pressure. In this way, the compartment can become stiff and act as a skeleton. The method used to create the pressure differs among species.
Among the Protozoa, hydrostatic skeletons are found in the members of the phylum Sarcomas tigophora. Within this group are the amoeboid types that form pseudopodia, false feet that are transient structures formed by hydrostatic pressure within the cell. It is hypothesized that contractile proteins within the cell direct fluid toward the cell periphery in channels and, in so doing, cause the cell membrane to bulge outward as a pseudo pod. Thick pseudopodia, termed lobopodia, are found in shell-less amoebas. In this case, there appears to be a chemical difference in the fluid used in pseudopodia formation. Plasmasol has less viscosity and is pushed into the pseudopodia; when it is distributed laterally, it turns to plasmagel. In thin pseudopodia termed actinopodia, cytoplasm is forced along microtubules termed axonemes. These types of pseudopodia are normally found among foraminiferan and radiolarian amoebas.
Worm Hydrostatic Skeletons
In the animal phyla, hydrostatic skeletons are developed in the Cnidaria and function in relation to the gastrovascular cavity. In the polyp forms of colonial forms and especially within the Anthozoa, such as sea anemones, the myoepithelial cells surrounding the coelenteron make up a circular muscle band and thus can put pressure on the water in the coelenteron to extend the body or maintain the form of the polyp. This coelenteron extends into the tentacles of these forms, and thus, via contraction of the myoepithelium, can elongate the tentacles to capture food.
Various types of flatworms comprise the phylum Platyhelminthes. Despite the lack of a coelom, these worms are still considered to have a hydrostatic skeleton. Since the longitudinal and circular muscles lie external to fluid-filled parenchyma tissue, the pressure exerted by this muscle extends the body by elongating this tissue. In this way, the worms can undulate in swimming or have peristaltic motion while moving along a substrate.
The nematode worms, as well as other members of the Pseudocoelomate group, have developed a fluid-filled body cavity. This cavity in most group members has a space with an outer boundary of mesodermal muscle and an inner boundary composed of endodermal cells. In nematodes, the muscle layer is arranged in a longitudinal pattern beneath a cuticle that is composed of layers, including fibrous collagen, to maintain the shape of the worm under muscular pressure. Although less flexible than in true worms, contraction of the muscle layer can act against the pseudocoel fluid, thereby causing the body to undulate. Most nematodes need to act against a surface to have effective locomotion.
A more effective method of locomotion using a hydrostatic skeleton has been developed by annelid worms. These worms are segmented and have developed a true coelom. This means that the body cavity or coelom is bounded on all sides by mesodermally derived tissue: circular and longitudinally arranged muscles on the external boundary and membranes wrapping the gut tube. In addition, the coelom is divided in each segment into right and left halves, and each segment has a membrane that separates the coelom in one segment from that of another segment. Thus, the circular musculature can constrict one side of the body while the other side is relaxed and stretched. As a result, undulation is more effective in this group than in the pseudocoelomates. When such bending is coupled with setae or segmentally arranged parapodial extensions on the body, the undulations could be used for crawling and swimming, although the latter locomotory ability is poor in some groups.
Using the hydrostatic skeleton in burrowing necessitates another evolutionary strategy involving the coelom. Here the intersegmental partitions or septa are lost or perforated. This accomplishes the movement of coelomic fluid between segments during muscle contraction. Thus, circular muscles in posterior segments can drive fluid anteriorly, swelling and elongating the anterior portion of the animal. The posterior segments left behind can catch up with the anterior segments when the longitudinally arranged fibers contract. In this way, burrowing is affected. Similar contractile wave patterns are used by terrestrial oligochaetes, such as earthworms, to create peristaltic-type contractions that drive the animal forward. The contraction of circular and longitudinal muscles alternate to create thick and thin areas of the body. This corresponds with elongation and subsequent contraction of the body segments. In earthworms, the segments retain their intersegmental septa. The Hirudinida or leeches have done away with their intersegmental septa, and thus, the coelomic space is continuous. Constriction of the circular muscles extends the body forward, and the subsequent contraction of the longitudinal muscles will bring the rest of the body to meet it. The movement is accomplished by first attaching the posterior sucker, then elongating, attaching the anterior sucker, and then pulling the rest of the body forward toward the anterior sucker.
The Hydrostatic Skeletons of Arthropods, Bivalves, and Echinoderms
The development of an exoskeleton requires a change from a coelom-driven locomotion to one that uses muscles. The reduction of the coelom is a characteristic of the diverse arthropod taxon, but hydrostatic skeletons are still used in certain body areas. Flying insects, when attaining their adult state, emerge from their cocoons with folded wings. The insect must pump hemolymph into veins within the wing to expand them before they harden. These veins remain in the wings, and their walls and hydrostatic pressure may act to maintain the shape of the wing during flight. The ability of spiders to run fast, even though they do have eight legs, may lie in the hydraulic systems in their legs. Spiders have replaced extensor muscles with hydraulic spaces that, under pressure, automatically extend the legs. Thus, muscles are normally only used for flexion of the leg segments, and the legs can be moved faster than if muscles were used in both extension and flexion of the leg segments.
Although mollusks have a reduced coelom, hydrostatic skeletons are developed in certain members of this phylum. Bivalves normally burrow into the substrate and send up siphons through which water is brought in and out of the clam. In some species, water pressure is used to open and extend these siphons. In addition, the foot of the clam contains a blood sinus that, under pressure, fills with blood and expands in two ways. The foot can be extended into the substrate with subsequent swelling of the distal end. This action anchors the foot so that the clam can pull itself into the substrate. Among other mollusks, it is thought that the extension of tentacles to capture prey by squid and cuttlefishes is based upon the hydraulic action of muscles on these structures.
Most echinoderms have a well-developed water vascular system derived from the coelom. This system is composed of a sieve plate opening to the water. This plate is then connected via a stony tube to a ring canal. In asteroids or sea stars, this ring canal extends into the arms via radial canals. From the radial canal located in each, there extend bilaterally arranged lateral canals that enter a tube foot structure. The tube foot has two functional parts, an upper, bulblike ampulla and a lower tube foot. Contraction of the ampulla drives water into the podium, extending it and causing its tip to form a suctionlike disc that attaches to the substrate or prey. Relaxation of the ampulla withdraws water back into the ampulla, retracting the podial portion of the tube foot. In this way, many echinoderms move along the ocean bottom and manipulate prey.
Vertebrate Hydrostatic Skeletons
Like mollusks and arthropods, vertebrates have largely abandoned the coelom. Their endoskeletons have taken the place of hydrostatic skeletons. However, hydrostatic skeletons still occur, particularly in the reproductive system. The penis, or hemipenes, of mammals and reptiles contains spongy tissue that can engorge with blood. Venous return of the blood is largely prevented, extending the length and stiffness of the intromittent organ so that it may be inserted into the female’s reproductive tract.
Principal Terms
Circular Muscle: muscle fibers that run in a circular pattern around the body, perpendicular to the long axis of the body
Coelenteron: the fluid-filled gastrovascular cavity of Cnidarians
Coelom: the body cavity of higher invertebrates and vertebrates, where mesodermal tissues enclose a fluid-filled space
Longitudinal Muscle: muscle fibers that run along the longitudinal or anterior-posterior axis of the body
Pseudocoel: a fluid-filled body cavity that is bounded by mesodermal muscle on the outside and endodermal epithelium on the internal boundary
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