138 THE STARFISH 



appearance in the two sexes. The ova and sperm are discharged into 

 the water, where fertilization occurs. Show the reproductive organs 

 in two of the arms in which the pyloric caeca are not shown on your 

 previous drawing. 



Exercise 6. — The Ambulacral System. 



(f) If the directions have been followed carefully, the specimen 

 will now show the main parts of the ambulacral system, or water- 

 vascular system with only a small amount of additional dissection. 

 Specimens having the system injected with a colored mass are prefer- 

 able, but almost all parts of the system can be recognized in an 

 uninfected specimen. On the floor of the ccelom in each arm will be 

 seen the ampullae (c/. Fig. 63). Determine their arrangement in the 

 specimen under observation and also by reference to the dry specimen. 

 Cut the arm of the dry starfish transversely with a sharp scalpel and 

 observe a conspicuous tube, the radial canal of the water- vascular 

 system. Press one of the ampullae and determine the effect on the 

 tube foot with which it is connected. Examine several sections cut 

 from the injected specimen and look for connections between the radial 

 canal and the ampullae and tube feet. After determining the anatomi- 

 cal relations of these parts, including the valves in the lateral canals 

 by which the ampullae and tube feet are connected with the radial 

 canal, you should be able to explain the mechanism by which the tube 

 feet are expanded and contracted. Traced toward the disk the radial 

 canals will be seen attached to a ring canal which encircles the mouth 

 and which is marked by nine protuberances, the Tiedemann's vesicles. 

 Note their location with reference to the arms, and determine what 

 region lacks a Tiedemann's vesicle. What relation to the ring canal 

 has the stone canal which leads downward from the madreporite? 



(g) Draw the ambulacral system (XI) as seen from the aboral 

 view. Outline the disk and one arm, showing the other arms cut off 

 near their bases. Show only a few of the ampullae and their connec- 

 tions with the tube feet and radial canal. 



Exercise 7. — The Nervous System. 



(h) It is difficult to observe all parts of the nervous system, but 

 certain features may be observed that show its radial nature and rela- 

 tions to other structures. Examine the aboral surface of a dry specimen 

 that has the ambulacral grooves well opened. Remove the spines 

 guarding the groove and the peristome. In the median line of each 

 groove is a dark line, the dried remains of a radial nerve. Traced 

 toward the disk, these nerves are seen to arise from an oral nerve ring 



