ASTERIAS. 71 



are nine small vesicles, Tiedemann bodies. They are smaller 

 than the ampullae and project in toward the mouth. The posi- 

 tion where the tenth Tiedemann body might be expected, is 

 taken by the stone canal. 



3. Leaving the circular canal are five radial water tubes, 

 one for each arm. These tubes lie along the oral surfaces of 

 the ambulacral plates, and are accordingly not visible on the 

 inside of the animal. The position of the tube can best be 

 understood by making a transverse section of an arm. It will 

 then be seen either in injected or uninjected specimens, lying 

 immediately below the ambulacral plates. In injected speci- 

 mens it may be followed by dissecting from the oral side, from 

 the circular canal to the extremity of the arm, where it ends 

 in a small tentacle. 



4. Along the sides of the ambulacral ridges, within the body- 

 cavity, are rows of little bag-like ampullae. Determine their re- 

 lation to the ambulacral pores. If the specimen is fresh, press a 

 few ampull and see if the corresponding tube feet are affected. 

 Can you determine their function? In a dissection it is hard to 



* 



find the connecting tubes that join the radial tubes to the tube 

 feet, but they can sometimes be seen in sections of arms of 

 injected specimens. They can readily be .seen in microscopic 

 preparations. 



The water-vascular system is very distinctive for the Echi- 

 nodermata, and you should understand perfectly: 



(a) How the tube feet are extended. 



(6) What causes them to adhere. 



(c) The connection between tube feet, ampullae, connecting 

 canals, radial water tubes, circular canal, stone canal, and mad- 

 reporic plate. 



(d) How it is possible to extend one foot without extend- 

 ing others. 



Make a drawing showing the arrangement of the water-vascu- 

 lar system. 



Nervous System. This is not easily studied by dissection. 

 It consists of a nerve ring which encircles the mouth and lies 

 just ventral to the circular water canal, and five radial nerves 



