74 ECHINODERMATA. 



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. 



(b) 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 draining showing the arrangement of the ivater-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 canals 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 ambulacral 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 

 macerate a portion of an arm to see the skeleton to which these 

 muscles are attached. 



Study prepared sections of the arm of a small starfish and 

 determine the relation of organs. 



1. The hepatic cseca. How are they supported? What 

 is their structure ? 



2. The radial canal, connecting tubes, tube feet, and am- 

 pullae. 



