Echinodermata: Comparative study of water-vascular system

Water-vascular system is a division of the coelom and most distinctive feature of echinoderms. lt is a system of canals and appendages of body wall. It is also termed as ambulacral system. Since the entire system is derived from coelom, the canals are lined with a ciliated epithelium and filled with watery fluid. Water-vascular system functions as a means of locomotion. It consists of madreporite or sieve plate, stone canal, water ring or ring canal, radial canals and podia or tube feet. In different classes of Echinodermata, water-vascular system is variously modified. Water-vascular system helps in locomotion. Animal moves by tube feet. Water after entering the madreporite passes into stone canal, ring canal, radial canal, podial canal and finally into ampullae of podia. As ampulla contracts, the water is forced into the tube foot which gets elongated. The sucker-like tip of tube foot touches the substratum and contracts again. Fluid goes back into ampulla and the body is drawn forwards bringing about the locomotion of sea star. [I] Asteroidea ln asteroids, water-vascular system based on the general echinoderm plan, with certain modifications. 1. Madreporite. Internal canals of the system connect with sea water outside through buttonshaped madreporite or sieve plate located on the aboral surface in interradial position. Sieve plate is calcareous having numerous ridges and furrows. Furrow have about 200 pores, each of which leads into a pore canal. Pore canals join to form a common canal to open into an ampulla beneath the madreporite. Although a great majority of asteroids have but one madreporite, multiplication of madreporites is not rare. 2. Stone canal. It is so named because of calcareous deposits located in its walls. A vertical Sshaped stone canal extends towards oral side. A longitudinal ridge divides the lumen of the stone canal into two passages. In Henricia, the internal ridge is very simple. ln Asterias and Asterina, ridge is bifurcated into two rolled vertical lamellae. ln Astropecten, ridge meets the opposite wall dividing the interior into tubes provided with a pair of scrolls each. The lumen of stone canal is ciliated. 3. Ring canal. Stone canal opens into a circular ring canal or water ring situated just internal to the peristomial ring of ossicles around the mouth margin. Often the walls of the ring canal are folded to divide its lumen into a more or less separate channels. 4. Radial canals. From the outer margin of ring canal are given out five radial canals. Each radial canal runs into each radial arm and terminates into the terminal tentacle at the tip of the arm. The radial canal runs on the oral side of ambulacral ossicles covering the ambulacral groove. 5. Lateral canals. From each side of the radial canal, lateral canals are given off alternately which pass between the ossicles on each side to enter the coelom. Each lateral canal has a valve and terminates into a bulb or ampulla connected to tube foot. 6. Polian vesicles. The ring canal, forms five polian vesicles on the outerside. They store water which is to be utilized when sea star comes outside the water. 7. Tiedemann’s bodies. The ring canal on its inner side also gives off interradially five pairs of small, irregularly shaped bodies. known as Tiedemann’s bodies. Some forms have only 9 such bodies. They produce coelomic corpuscles which are passed into ring canal as such giving the system, the name water-vascular system. Tube feet. A podium or tube foot, is a short hollow, elastic, thin walled closed cylinder located in the ambulacral groove. Tip of the podium is flattened forming a sucker for attachment. Inner or basal end of each podium the ambulacral ossicle, through a tiny ambulacral pore and expands to form a time bulb or bladder called ampulla, lying into the aboral side of the coelom. The walls of ampulla contain longitudinal and circular muscle fibres whereas the tube foot has longitudinal fibres only, there being no circular muscles in the tube foot. Podia are arranged in four rows along the length of ambulacral groove. As lateral canal on each side are alternately long and short, the podia look like in four rows instead of two. Species having only two rows of podia have lateral canals of equal length. [II] Ophiuroidea Madreporite lies on the oral surface. It bears a single pore and a pore canal. Some species have more than one madreporite. Stone canal ascends to the water ring, which is located in a groove on the aboral surface of the jaws. Stone canal gives off a madreporic ampulla just beneath the madreporic plate. Water ring or ring canal gives off four polian vesicles in each inter radius except one which contains stone canal. Sometimes polian vesicle are accompanied by long slender, tubular appendages, known as Simroth’s appendages. Ophiuroids lack Tiedemann’s bodies. Water ring gives rise to a radial canal in each radius which descend towards oral side, runs along the whole arm to terminate into terminal tentacle. Radial canals penetrate through the lower side of vertebral ossicles of the arms. In each ossicle, radial canal gives a V-shaped podial canal or lateral canal that enters into a pair of podia. Podia do not bear ampulla. There is a valve between podium and the lateral canal. Podia are reduced. Entire water-vascular system is lined with ciliated peritoneum.III] Echinoidea Water-vascular system of echinoids is like that of sea stars. One of the genital plates around the periproct bears pores and pore canal and also serve as madreporite. Stone canal descends orally to a water ring surrounding the oesophagus aboral to the lantern. Stone canal is accompanied by the axial gland. Five radial canals arise from the water ring and terminate into the terminal tentacle. Water ring also gives rise to five sponge like bodies called the Tiedemann’s bodies. From either side of radial canal alternately come out lateral canals to the bases of the ampullae. Suckers of podia in sea urchins are highly develop. IV] Holothuroidea They have water-vascular system like that of other echinoderms but madreporite is peculiar. Neither it is connected with the body surface nor is it attached in the coelom. It hangs just below the base of the pharynx and is connected to the water ring by a short stone canal. Pores and pore canals still persist in the madreporite but in place of sea water, coelomic fluid circulates in it. Water ring encircling the base of the pharynx gives rise to elongated polian vesicles which hang into the coelom. Their number maybe one in Cucumaria, 3 or 4 in Thyone, or 11 or 12 in certain Apoda. Five radial canals arise from water ring and pass upward to the inner side of the calcareous ring and then outward through a notch at the end of each radial plate. Before leaving the calcareous ring, each radial canal gives off smaller branches to the buccal tentacles. After leaving the ring, radial canals pass posteriorly within the body wall all along the length of ambulacral groove where they supply the podia. Ampullae are found on both podia and tentacles. In Apoda, which lack podia, water-vascular system is confined to oral water ring, polian vesicles and buccal tentacles.