If you have a very close look at a living starfish or sea urchin, you can see that they have hundreds of very mobile, translucent, tube-like structures with suckers on the end. In starfish (or sea stars as they are called in the States) these tube feet or podia are found on the underside in the ambulacral groove that runs the length of each arm. In sea urchins they are found protruding from all surfaces from between the spines. In both these types of Echinoderm the tube feet are used to capture food particles and transfer them towards the mouth – which is on the underside of the creatures. In addition, in starfish, they are used in locomotion – for walking across surfaces.
The tube feet are operated in an ingenious manner that is unique to the echinoderms. Echinoderms have what is called a water-vascular system providing the basis for podia movement. Water is taken into the animal, through a small special button-like structure known as a madreporite, to fill a system of canals, cavities, and valves lying within the body wall. The tube feet are extensions of this water vascular system. So the hollow tube feet are filled with fluid. There are longitudinal muscles in the podia walls; these are contracted and relaxed against the water pressure in order to extend, retract, and change the shape of the tube foot. The whole mechanism is like a hydraulic system. The flat-ended sucker tip of the tube foot also produces copious amounts of sticky secretion that helps it to adhere to surfaces.
[I took these photographs at the Hatfield Marine Science Center of Oregon State University, U.S.A.]
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