ASTERIAS 217 



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 saclike ampullae. Determine 

 their relation to the ambulacral pores. If the specimen is 

 fresh, press a few ampullae 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 

 Echinodermata, 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, connect- 

 ing canals, radial water tubes, circular canal, stone canal, and 

 madreporic plate. 



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

 ing others. 



Make a drawing showing the arrangement of the water- 

 vascular 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 

 that extend down the arms just beneath the radial water 

 tubes, to end at the tips of the arms in pigment spots, the 

 eye spots. The whole central nervous system is superficial 

 and forms a portion of the outer covering of the body. The 

 radial nerves can be seen by separating the rows of ambu- 

 lacral feet, but it is much more satisfactory to study them in 

 prepared sections. 



Muscular System. — Examine the walls of the starfish for 

 its muscular system. If time permits, it will be desirable to 



