The water vascular system
is a hydraulic system used by echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, such as sea stars and sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
s, for locomotion, food and waste transportation, and respiration
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
. The system is composed of canals connecting numerous tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...
. 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.
The exact structure of the system varies somewhat between the six classes of echinoderm.
In sea stars (starfish), water enters the system through a sieve-like structure on the upper surface
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
of the animal, called the madreporite
The madreporite is a lightcolored calcerous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a small wart, on the aboral surface of the central disk of a...
This overlies a small sac, or ampulla
An ampulla was, in Ancient Rome, a "small nearly globular flask or bottle, with two handles" . The word is used of these in archaeology, and of later flasks, often handle-less and much flatter, for holy water or holy oil in the Middle Ages....
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 sea star has one such groove on its underside, while, in sea urchins, they run along the outside of the body.
Each side of the radial canals gives rise to 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.
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.
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 sea star 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 cell
A blood cell, also called a hematocyte, is a cell normally found in blood. In mammals, these fall into three general categories:* red blood cells — Erythrocytes* white blood cells — Leukocytes* platelets — Thrombocytes...
s of vertebrates.
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.
Ophiuroids, the group including brittle stars and basket star
Basket stars are a group of brittle stars. They are treated as a suborder Euryalina or order Euryalida. Many of them have characteristic many-branched arms. They generally live in deep sea habitats.The life span in the wild is up to 35 years. They weigh around 11 lbs, or 5 kg...
s, have a somewhat different water vascular system from sea stars, 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.
Ophiuroids have no ambulacral groove, and the radial canals instead run through the solid bone-like ossicles of the arms. Unlike sea stars, 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.
The madreporite of sea urchins is located within one of the plates surrounding the anus on the upper surface of the animal. The stone canal descends from the madreporite to the ring canal, which lies around the oesophagus, and includes a number of polian vesicles. Because sea urchins have no arms, the five radial canals simply run along the inside of the solid skeletal "test", arching upwards towards the anus.
The ampullae branching off from either side of the radial canals give rise to ten rows of tube feet, which penetrate through holes in the test to the outside. As in sea stars, the ampullae are arranged alternately, but in most (though not all) cases they split into two as they pass through the test before merging again on the outer side. The tube feet of sea urchins are often highly modified for different purposes. The radial canal ends in a small water-filled tentacle which protrudes through the uppermost plate of the ambulacral region.
Uniquely among echinoderms, crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
s have no madeporite. Instead, the oral surface is dotted within numerous minute ciliated funnels that run into the main body cavity. The ring canal has several small stone canals, located between the arms of the animal, but these open into the body cavity, and thus are only indirectly connected to the outside.
The five radial canals run into the arms and branch several times to supply all of the individual branches and pinnules lining the arms. As in other echinoderms, the radial canals give rise to lateral canals, but there are no ampullae, and clusters of three tube feet branch from the ends of each canal, except around the mouth, where they are found singly. In the absence of ampullae, water pressure is maintained by the ring canal, which is surrounded by contractile muscle fibres.
The water vascular system of sea cucumbers has no connection to the outside, and is thus filled with the internal coelomic fluid, rather than sea water. The madreporite is present, but lies within the body cavity, just below the pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
. The stone canal is relatively short.
The ring canal normally has one to four polian vesicles, but in the order Apodida
The Apodida is an order of littoral to deep-sea, largely infaunal holothurians. This order comprises three families, 32 genera and about 270 known species, called apodids.-Characteristics:...
, there may be as many as fifty. The radial canals run through notches in the calcareous plates surrounding the mouth and then run along the ambulacral areas along the length of the body. Lateral canals run to both the tube feet and the large oral tentacles, all of which possess ampullae. The Apodida, which have no tube feet, also have no radial canals, with the canals to the tentacles branching off directly from the ring canal.