THE PHYLUM ECHINODERMATA 



Structures and Functions Related to Metabolism and Responsiveness. 



The food of the starfish consists principally of mollusks, such as clams, 

 oysters, scallops, and snails. In feeding, the animal assumes a characteristic 

 "humped" position over the prey; the more proximal tube feet are attached 

 by their suckers to the outer surface of the shell, and the more distal ones 

 commonly appear to secure a hold upon the substrate. In some manner still 

 imperfectly understood, the starfish ^oon causes the valves of the mollusk to 

 gape open. An older theory, without any demonstrable basis, held that the 

 starfish secretes some noxious substance which finds its wav between the valves, 

 paraKzing the adductor muscles of the bivalve. Experimental work has shown 

 that b\' powerful, sustained contraction of its muscles (probablv those connecting 

 the ambulacral ossicles), the starfish can exert enough force through its 

 firmly attached tube feet to pull the valves apart, at least a few millimeters. 

 It is clear that such a small gape can be produced, without damaging the 

 adductor muscles of the clam, by the application of a degree of force which 

 the starfish is capable of bringing to bear on the valves. When even a small 

 opening has been produced, the stomach of the starfish is everted through the 

 mouth and inserted into the mantle cavity of the bivalve. It has been 

 established by experiment that the folds of the everted stomach can pass 

 through a surprisingly small orifice. Having penetrated the outer defenses of 

 the clam, the starfish releases into the mantle cavity digestive enzymes so 

 powerful that they gradually bring about the complete disintegration of the 

 soft parts of the mollusk. The products of this extracellular digestion are con- 

 ducted by powerful flagellary currents into the digestive cavitv of the starfish. 

 After completing its meal of the body of the mollusk, the starfish with- 

 draws its stomach, closes its mouth, and crawls away, leaving behind only 

 the empty shell of its prey. By this peculiar method of feeding, Astenas Jorbesi 

 and A. vulgaris destroy large numbers of shellfish. One investigator reported 

 that during a 6-day period, a single starfish devoured 56 clams, some as long 

 as an arm of the starfish itself. Starfishes also feed on dead or injured animals 

 other than shellfish and even attack other starfishes and sea urchins. In the 

 absence of food, a starfish can survive starvation for several months. 



The central portion of the digestive system occupies the short oral-aboral 

 axis of the starfish (Fig. 16.3). The mouth opens into the cardiac stomach, the 

 folded walls of which take up much of the cavity of the disk. Above the 

 cardiac stomach, and communicating broadly with it, lies the small, flattened 

 pyloric stomach, which leads upward into the short intestine. The intestine 

 opens to the exterior at the anus, which is somewhat eccentricallv placed on 

 the aboral surface of the disk. The intestine bears a pair of branched, 

 tubular organs, the rectal caeca, which lie between the roof of the pyloric 

 stomach and the aboral body wall. These may be considered as homologous 

 with the "water lungs" of holothurians (pp. 504-505), but their functions 

 are unknown. 



In addition to these central parts, the digestive tract also includes branched 

 diverticula which extend into the cavities of the arms. These are the five 



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