144 
ANIMAL ACTIVITIES. 
with a larger tube running lengthwise of the ray, and 
this in turn connects with a tube surrounding the 
mouth. With this oral tube the radiating tubes all 
unite and from this there extends the stone canal 
(another tube) reaching to the madreporic body on the 
aboral surface. The ambulacral feet have muscular 
walls. The madreporic body is pierced with minute 
holes through which water can enter the water-system. 
Fig. i i 7.—A Brittle Starfish. 
When the starfish wishes to advance, some of the am¬ 
bulacral feet are elongated in the direction of progres¬ 
sion by squeezing the ampullae and forcing the water 
into the feet. When the feet are fully extended the 
sucker at the end of each foot fastens itself to the rock, 
or other support near, and the longitudinal muscles of 
the tube contract, shortening the ambulacral foot and 
pulling the starfish to the rock. 
