216 ECHINODERMATA 



Reproductive System. — Turn the hepatic caeca to one side 

 and notice the ovaries or testes. The sexes are separate, but 

 the organs have the same general appearance in both sexes. 

 They vary in size according to the season of the year, some- 

 times being so small that they are not easily found, and again 

 being nearly or quite as large as the hepatic caeca. With a 

 pair of forceps lift up one of these organs and see where it is 

 attached. It is at this point that the reproductive cells reach 

 the exterior. How many gonads are there? 



Draw the gonads into another arm of your figure. 



Water-vascular System. 1 — 1. Carefully remove the side of 

 the stomach next to the bivium, being very careful not to dis- 

 turb the stone canal, which runs from the madreporic plate 

 to the margin of the membrane around the mouth. By the 

 side of the stone canal is a thin band of tissue formerly sup- 

 posed to be a heart. It is generally referred to as the axial 

 organ of the hemal system. See Chadwick's monograph on 

 Asterias for a discussion of the theories concerning the nature 

 of the hemal system. 



2. The circular canal, which is joined by the stone canal 

 at the outer margin of the peristomial membrane, follows the 

 margin of the membrane and so encircles the mouth. Origin- 

 ating from it at points very near the ampullae of the first 

 tube feet are nine small vesicles, Tiedemann bodies. They 

 are smaller than the ampullae and project in toward the 

 mouth. The position 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 uninfected specimens, lying 



1 This may be injected in fresh specimens, either with gelatin or fine 

 starch mass, by picking up one of the radial canals with a hypodermic 

 syringe and injecting toward the disk. 



