=Movements: the Water-tube System.=--The water, which is filtered through the perforated madreporite, is needed to supply a _system of canals_ (Fig. 56). The madreporite opens into a canal called the _stone canal_, the wall of which is hardened by the same kind of material as that found in the skin. The stone canal leads to the ring canal which surrounds the mouth (Fig. 56). The ring canal sends _radial canals_ into each ray to supply the double row of _tube feet_ found in the groove at the lower side of each ray (Fig. 57). Because of their arrangement in rows, the feet are 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. [Illustration: FIG. 57.--Starfish, from below; tube feet extended.] [Illustration: 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.] =Respiration.=--The _system of water vessels serves the additional purpose_ of bringing water containing oxygen into contact with various parts of the body, and the starfish was formerly thought to have no special respiratory organs. However there are holes in the aboral wall through which the folds of the delicate lining membrane protrude. These are now supposed to be _gills_ (_k_, Fig. 58). =The nervous system= is so close to the aboral surface that much of it is visible without dissection. Its chief parts are a _nerve ring_ around the mouth, which sends off a _branch_ along each ray. These branches may be seen by separating the rows of tube feet. They end in a pigmented cell at the end of each ray called the _eye-spot_.