PHYLUM ECHINODERMATA 



305 



Madreporite 



Stone canal 

 Ring canal 



Tiedemann's body 



Transverse cana 

 Ampulla 



Tube foot 

 Radial cana 



Figure 191. Starfish. Diagram of a part of the water vascular system; one of the radial canals 

 is cut off at the base and the other four near the base. 



The water-vascular system provides an 

 hydraulic pressure mechanism for locomo- 

 tion. The starfish walks by means of its tube 

 feet. Extension of the foot is brought about 

 by contraction of the bulblike ampulla, 

 forcing fluid into the cavity of the foot. 

 Contraction of the foot muscles causes the 

 fluid to run back into the ampulla. 



The action of the tube foot is usually 

 that of stepping forward. The foot, from a 

 contracted position, points forward; it then 

 elongates as the muscles of the ampulla con- 

 tract, thus forcing fluid into the tube foot; 

 and the bottom of the foot, the sucker, be- 

 comes pressed against the substrate. The 

 action of certain muscles in the foot pro- 

 vides a forward thrust of the animal's body; 

 thus the total effect of the forward thrusts 

 of the many tube feet produces the move- 

 ment of the starfish. After the forward 

 thrust, the sucker is detached from the sub- 

 strate and the foot contracts and points 

 forward again to start the process over. All 

 the tube feet act in a coordinated way by 

 extending in the same direction, but not at 

 the same time. The starfish advances slowly, 

 only about six inches in a minute. 



The tube feet are not only used for loco- 



motion, but for clinging to rocks and for 

 capturing and handling food. 



Digestive system 



The digestive tract (Fig. 193) is short and 

 greatly modified. The mouth opens into a 

 very short esophagus which leads into a 

 thin-walled sac, the stomach. The stomach 

 consists of two parts— a larger oral portion 

 and a small aboral or pyloric portion. From 

 the pyloric portion, a tube passes into each 

 ray, then divides into two branches, each 

 of which possesses a large number of lateral 

 pouches; these branches are called pyloric 

 or hepatic ceca. They are green in color. 

 Above the stomach is the slender intestine, 

 which opens to the outside through the 

 anus. Two branched pouches, brown in color, 

 arise from the intestine and are known as 

 intestinal ceca. 



The food of the starfish consists of fish, 

 oysters, mussels, barnacles, clams, snails, 

 worms, crustaceans, etc. Digestion is chiefly 

 extracellular. The stomach and pyloric ceca 

 secrete several digestive enzymes. Undi- 

 gested matter is ejected through the mouth. 

 The intestinal ceca secrete a brownish mate- 



