196 



COLLEGE ZOOLOGY 



its muscles and allow the valves to gape. The edge of the stomach 

 is then inserted between the valves and applied directly to the 

 soft parts of the prey, which is thus completely digested. When 

 the starfish moves away, nothing but the cleaned shell is left 

 behind. If the bivalve is small, it may be completely taken into 

 the stomach, and the empty shell later rejected through the 

 mouth." (MacBride.) Schiemenz has shown " (1) that 

 whilst a bivalve may be able to resist a sudden pull of 4000 

 grammes it will yield to a pull of 900 grammes long continued; 

 (2) that a starfish can exert a pull of 1350 grammes; (3) that 

 a starfish is unable to open a bivalve unless it be allowed to 



raise itself into a hump 







Fig. 136. — View of starfish {Echinaster) 

 devouring a mussel. i, madreporite. 

 (From the Cambridge Natural History.) 



(Fig. 136) so that the pull 

 of the central tube-feet is 

 at right angles to the prey. 

 A starfish confined between 

 two glass plates walked 

 about all day carrying with 

 it a bivalve which it was 

 unable to open." (Mac- 

 Bride.) 



The lining of the stomach 

 secretes mucus; that of the 

 pyloric sac and casca secretes 

 ferments; these change proteids into diffusible peptones, starch 

 into maltose, and fats into fatty acids and glycerine. Thus is- 

 digestion accomplished. Undigested matter is ejected through 

 the mouth, and very little, if any, matter passes out of the 

 anus. The rectal caeca secrete a brownish material of unknown 

 function, probably excretory. 



Circulation. — The fluid in the ccelom is kept in motion by 

 cilia and carries the absorbed food to all parts of the body. 



Excretion. — This is accomplished by the amoebocytes (neph- 

 rocytes) in the coelomic fluid (p. 193), probably aided by the 

 rectal caeca. 



