ECHINODERMATA 15 1 



arm, protected by the dermal plates, while the small closed end 

 extends between the plates, protruding out into the arm furrow 

 and ending in a disk-like expansion. The sea water is admitted 

 from the radial ambulacral vessel of the arm into the ampulla ; 

 by the contraction of this it is forced into the corresponding 

 tube-foot, which thus becomes greatly lengthened. When the 

 foot is thus extended and comes into contact with a stone or 

 other solid object the water is withdrawn from it, the middle of 

 its tip is drawn inward, forming under it a vacuum, and thus 

 the disk-like end of the foot is converted into a sucker which 

 clings to the rock. When water is again forced into the foot by 

 the ampulla, the hold of the sucker is relaxed. Thus the many 

 tube-feet acting independently reach forward in the general 

 direction of locomotion, attach themselves to some foreign object, 

 and through contraction pull forward the entire body with a 

 slow gliding motion, while the other feet already contracted loose 

 their hold and then reach forward. A fair speed is six inches a 

 minute. Although a starfish can move in any direction, it 

 usually proceeds with the arm lying immediately to the left of 

 the madreporic plate in advance. 



The skeleton or test consists of many calcareous plates em- 

 bedded in the sub-epidermis (mesoderm), which, except at the 

 mouth, completely surrounds the body. Since bands of muscular 

 fibers extend between all contiguous plates, the arms and disk 

 are movable. The protective spines are covered only by the 

 epidermis. 



The chief food of the starfish is moUusks (especially mussels, 

 clams and oysters), barnacles, worms, and small crustaceans. 

 In securing its food, such as a clam, it folds its arms about its 

 prey, attaching its hundreds of tube-feet to the valves of the shell, 

 and through the steady pull thus exerted tires the large adductor 

 muscles of the mollusk. Experiment has shown that a large 

 starfish can exert a steady pull of over two and a half pounds. 

 Between the valves thus opened the starfish rolls its very dis- 

 tensible stomach in the form of a thin sheet spread over the 



