water-vascular system

The"water vascular system" is a hydraulic system used by echinoderms, such as starfish and sea urchins, for locomotion, food and waste transportation, and respiration^[1]. The system is composed of canals connecting numerous tube feet. Echinoderms move by alternately contracting muscles that force water into the tube feet, causing them to extend and push against the ground, then relaxing to allow the feet to retract.^[1][2].

Starfish

In starfish, water enters the system through a sieve-like structure on the upper surface of the animal, called the madreporite. This overlyies a small sac, or ampulla connected to a duct termed the stone canal, which is, as its name implies, commonly lined with calcareous material. The stone canal runs to a circular ring canal, from which radial canals run outwards along the ambulacral grooves. Each arm of a starfish has one such groove on its underside, while, in sea urchins, they run along the outside of the body.^[3]

a row of bulb-like ampullae. These are always staggered, so that an ampulla on the left follows one on the right, and so on down the length of the radial canal. The ampullae are connected to suckerlike podia, the entire structure is called a tube foot. In most cases, the small lateral canals connecting the ampullae to the radial canal are of equal length, so that the tube feet are arranged in two rows, one along each side of the groove. In some species, however, there are alternately long and short lateral canals, giving the appearance of two rows on each side of the groove, for four in total. [3]

Contraction of the ampullae causes the podia to stretch as water is brought into them. This whole process allows for movement, and is quite powerful but extremely slow^[4].

The central ring canal, in addition to connecting the radial canals to each other and to the stone canal, also has a number of other specialised structures on the inner surface. In between each radial canal, in many starfish species, there lies a muscular sac called a polian vesicle. The radial canal also has four or five pairs of complex pouches, called Tiedemann's bodies. These apparently produce coelomocytes, amoeboid cells somewhat similar to the blood cells of vertebrates.^[3]

Although the contents of the water vascular system are essentially sea water, apart from coelomocytes, the fluid also contains some protein and high levels of potassium salts.^[3]

Ophiuroids

Ophiuroids, the group including brittle stars and basket stars, have a somewhat different water vascular system from starfish, despite their superficially similar appearance. The madreporite is located on the underside of the animal, usually in one of the jaw plates. The stone canal runs upwards to the ring canal, typically located in a circular depression on the upper (i.e. internal) surface of the jaws. The ring canal has four polian vesicles.^[5]

Ophiuroids have no ambulacral groove, and the radial canals instead run through the solid bone-like ossicles of the arms. Unlike starfish, the tube feet are paired instead of staggered, and there are no ampullae. Instead, a simple valve at the upper end of the foot helps to control water pressure in the tube feet, along with contraction of the associated canals.^[5]

References

This animal-related article is a stub. You can help Wikipedia by expanding it. v • d • e