ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 271 



Fig. 12-23. Regeneration in a starfish. It required six weeks for this starfish to accomplish the regeneration of the 



two arms seen here. 



body, and to capture food. If a single foot 

 is examined, the portion that protrudes ex- 

 ternally from the oral surface is found to be 

 an elongated tube. Internally, at the oppo- 

 site end, a bulbous structure, the ampulla, 

 joins a central tube, the radial canal, which 

 extends up the ray of the animal to join a 

 central circular tube, the ring canal. A 

 short tube, the stone canal, is joined to the 

 ring canal and runs up to the dorsal surface 

 of the disc, opening externally into the 

 madreporite plate (Fig. 12-21). 



Water enters through the madreporite 

 plate, passes into the stone canal, then 

 the ring canal, and to the ampullae. At the 

 margin of the ring canal are located the 

 Tiedemann bodies which produce the amoe- 

 bocytes found in the fluid of the ambulacral 

 system. When the ampulla contracts, the 

 fluid is forced into the tube foot, which is 

 thus elongated. If the sucker-like tip (Fig. 

 12-22) of the foot touches and attaches to 



an object, the muscular wall of the foot con- 

 tracts, forcing the fluid back into the am- 

 pulla, thus causing the foot to be shortened. 

 Since the foot adheres to the object it has 

 touched, the shortenino; of the foot draws 

 the body forward. In this manner the star- 

 fish is able to move. It also uses this mech- 

 anism to obtain food, but instead of the 

 alternating "push-pull" system of locomo- 

 tion, a steady contraction of the tube foot 

 is exerted to produce a constant pull. 



The endoskeleton, which is produced by 

 the mesoderm, is a calcareous framework 

 composed of many ossicles, most of which 

 are arranged in a definite pattern. Even 

 though the starfish has this strong endo- 

 skeleton, it is capable of autotomy. Thus it 

 can break off an arm and readily regenerate 

 the lost part, in a very short time (Fig. 

 12-23 ) . An arm may live briefly after it has 

 broken off the central disc, but it does not 

 usually regenerate a new animal unless a 



