194 



PRACTICAL ZOOLOGY 



The water-vascular system is peculiar to' echinoderms. Sea 

 water is forced into this system of canals by cilia. The most 

 interesting structures of the water-vascular system are the tube 

 feet by means of which the starfish moves from place to place 

 and holds its food. 



The food of the starfish consists of fish, oysters, mussels, 

 barnacles, clams, snails, worms, Crustacea, etc. When a mussel 

 .... , is to be eaten, the animal 



1 '■''■'' seizes it with the tube feet 



" and places it directly 

 under its mouth, folding 

 its arms down over it in 

 umbrella fashion (Fig. 114). 

 The muscles which run 

 around the arms and disk 

 in the body wall contract 

 and partially turn the 

 stomach inside out. The 

 everted edge of the stomach 

 is wrapped round the prey. 

 Soon the bivalve is forced to relax its muscles and allow the 

 valves to open. 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. 

 Oyster beds are seriously affected by starfishes. One star- 

 fish which was placed in a dish containing clams devoured over 

 fifty of them in six days. Formerly starfishes were taken, cut 

 in two, and thrown back; this only increased the number, since 

 each piece regenerated an entire animal. They are now often 

 captured in a moplike tangle, to the threads of which they 

 cling. They are then killed in hot water or thrown out on the 

 shore above high-water mark and left to die in the sun. 



Fig. 114. — Diagram of starfish eating 

 a mussel. (From Cambridge Natural 

 History.) 



