658 EVOLUTION OF ANIMALS Part V 



tral system is associated with a network of nerves spread out below the surface 

 of the body. 



Feeding and Digestion. Most starfishes are carnivorous, feeding principally 

 upon clams, oysters, scallops, and mussels. If a starfish is placed in an aquarium 

 with one or two clams the sensitive tube feet at the ends of the arms soon wave 

 excitedly in their direction. Very soon, the starfish proceeds toward them and 

 attacks one of them. It climbs over the clam, its body tentwise above it 

 with its arms so placed that the tube feet finally pull on the opposite shells. 

 Many tube feet pull but not all of them at the same time so that there is a relay 

 of continuous pulling that fatigues the muscles of the clam which eventually 

 opens its shells. Immediately, the arms of the starfish contract pressing fluid 

 against the pouched part of the stomach which is everted through the mouth 

 and lowered between the shells. It envelops the clam's body and digestion 



.skin gi 



radial 

 canal 



nerve 



Fig. 32.7. Cross section of an arm of the starfish. It shows the separateness 

 of the ossicles; the free passage ways between the roomy body cavity (coelom) 

 and the skin gills, between the radial canal and the whole extent of the tube 

 feet; and the openings of the gonads, the ovaries or testes whichever the sex 

 may be. (After Brown: Selected Invertebrate Types. New York, John Wiley and 

 Sons, 1950.) 



