PHYLUM ECHINODERMATA 



231 



foot. The ampulla is bulblike ajid is located above the roof of the 

 ambulacral groove in the coelom. It is connected through its 

 ambulacral pore with the contractile tube foot which hangs down 

 into the ambulacral groove. The distal or free end of the foot 

 has a slightly inverted, suckerlike shape. The proximal pair of 



Radial canal J^ 



tiadreporite 

 Stone canal 



Tiedemanri's body 



Tube foot 



Fig. 126. — Diagram of water-vascular system of the starfish. 



Fig. 127. — Starfish "walking" on glass. Notice the extended tube feet. (Courtesy 



of General Biology Supply House.) 



ampullae in each arm of some starfish lack the tube feet and are 

 sometimes erroneously called Polian vesicles. Alternate tube feet 

 are farther from the radial canal than the others on each side. The 

 ampullae and tube feet function effectively in locomotion, the am- 

 pullae contracting to force water into their respective tube feet 



