196 METAZOAN PHYLA 



226. Feeding and Metabolism. — As the starfish moves about over a 

 surface it secures animals which are unable to run away, such as oysters, 

 mussels, barnacles, clams, snails, and tube-dwelling worms, or those 

 which are surprised and captured before they can escape, such as crus- 

 taceans and even small fish. When an object is captured which can be 

 passed through the mouth into the stomach, this is done and digestion 

 takes place within the body, after which the indigestible part is thrown 

 out through the mouth. If, however, a starfish finds itself over such an 

 animal as an oyster or mussel, which is firmly attached and protected 

 by a shell, it has recourse to a novel mode of circumventing its prey. 

 Tube feet are attached to the two valves of the shell and then a steady 

 pull is exerted which tends to draw the valves apart. This pull may be 

 resisted by the mollusk'for some time, but sooner or later the muscles of 

 the victim relax. The stomach of the starfish is then everted through 

 the mouth and immediately inserted into the crack between the two 

 valves. Though the mollusk may for a time again attempt to close 

 this crack, the starfish ultimately succeeds in inserting enough of its 

 stomach so that it can be wrapped about the body of the mollusk, which 

 is then digested within its own shell. When digestion is complete the 

 starfish draws the stomach back into its body by means of retractor 

 muscles and moves on to find other prey. It is stated by MacBride 

 that if the mollusk is not firmly attached, the starfish will pick it up 

 between its rays; on one occasion a starfish which was confined was 

 observed to walk about all day carrying with it a mussel which it was 

 unable to open. 



When the food is digested it is absorbed through the walls of the 

 alimentary canal into the coelom and is thus distributed to all parts of 

 the body. Elimination probably takes place in part through the rectal 

 caeca but also in another and very curious manner. Ameboid cells, 

 known as amehocytes, lying in the coelomic fluid, pick up particles of 

 waste and make their way out through the walls of the dermohranchiae — 

 literally, skin gills — thus getting outside the body with the waste, not 

 again to return. These dermohranchiae (Fig. 103 A) are pouches of 

 peritoneum filled with coelomic fluid and may be projected outward 

 through thin places in the wall of the body, pushing the epidermis in 

 front of them. Their function is mainly respiratory. When the animal 

 is exposed by the ebbing of the tide they are retracted within the body 

 wall and remain so until the animal is again covered by the return of the 

 tide. The effect of these dermohranchiae, when fully extended, is more 

 or less completely to cover the surface of the body with a soft tissue 

 through which one who touches the animal can feel the firmer wall of 

 the body. The amebocytes are produced in structures attached to the 

 ring canal of the water-vascular system, known as polian vesicles (Fig. 

 104) and Tiedemann's bodies. 



