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.
Fig. 57.—Starfish, from below; tube feet extended.
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]).
Fig. 59.—Starfish eating a sea snail.
b, stomach everted.
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.
The food of starfish consists of such animals as crabs, snails, and oysters. When the prey is too large to be taken into the mouth, the starfish turns its stomach inside out over the prey (Fig. [59]). After the shells separate, the stomach is applied to the soft digestible parts. After the animal is eaten, the stomach is retracted. This odd way of eating is very economical to its digestive powers, for only that part of the food which can be digested and absorbed is taken into the body. Only the lower part of the stomach is wide and extensible. The upper portion (next to the aboral surface) is not so wide. This portion receives the secretion from five pairs of digestive glands, a pair of which is situated in each ray. Jaws and teeth are absent. (Why?) The vent is sometimes wanting. Why?