HOW STAR-FISH WALK. 87 
we have a regular water-supply taken in at the porous 
plate and carried along all the five rays. But we 
want next a separate cistern for each tube-foot, for 
we have seen that they move separately, and so can- 
not all be filled with water at the same time. These 
separate cisterns we find in a number of elastic bags 
or vesicles (v v, Fig. 3 7) placed along the water-canals, 
and opening into them on the one hand, and on the 
other into the tube-feet. Now when water is taken 
in at the grating h above* the canals are filled, and 
they fill the vesicles, and each vesicle is able to con- 
tract and force its water down into its own foot-tube, 
thus stretching it out. Then the foot-tube while 
stretched at full length can, by drawing in its walls a 
little, force some water back, and so draw up the 
centre of the round cushion at the end of its tube, 
making a sucker just as a schoolboy does with wet 
leather on a pavement ; then the foot holds fast. 
Lastly, by drawing up the muscles which run down 
the tube, the body is drawn on, the sucker released, 
and the foot pulled back to begin again. 
This is how the star-fish walks, and when we re- 
member how many hundreds of feet he has, how 
firmly each one holds, and how slightly it moves, we 
cease to wonder that he glides so smoothly and clings 
so firmly to the rock. He is a greedy creature, w r hose 
whole care is his stomach, and he will eat any animal 
food he can find, from small crabs, shell-fish, and 
other sea-creatures, to mere garbage and decaying 
matter, so that he is very useful as a scavenger of the 
sea. He in his turn is eaten by the cod, the haddock, 
and other fish, but he is better protected from smaller 
* This grating is called the Madreporiform tubercle. 
