also called ambulacral feet (Latin ambulacra, "forest walks"). There is a water holder (ampulla), or muscular water bulb at the base of each tube foot (Fig. 58). These contract and force the water into the tube feet and extend them. The cuplike ends of the tubes cling to the ground by suction. The feet contain delicate muscles by which they contract and shorten. Thus the animal pulls itself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extended them being sent back into the ampulla. This system of water vessels (or water-vascular system) of the echinodermata is characteristic of them; i.e. is not found elsewhere in the animal kingdom. The grooves and the plates on each side of them occupy the ambulacral areas. The rows of spines on each side of the grooves are freely movable. (What advantage?) The spines on the aboral surface are not freely movable.
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Fig. 57.—Starfish, from below; tube feet extended.
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Fig. 58.—Section of one ray and central portion of starfish. f1, f2, f3, tube feet more or less extended; au, eye spot; k, gills; da, stomach; m, madreporite; st, stone canal; p, ampulla; ei, ovary.
