58 ECHINODERMATA. 



taclied to the ridges formed by the ambulacral plates in each arm. 

 How is it possible for the stomach to be everted? What reason 

 is there for two divisions? 



3. In each arm is a pair of long, glandular organs, the hepatic 

 cccca. The ducts of each pair unite and join the pyloric divi- 

 sion of the stomach by a common duct. These are digestive 

 glands. What reason is there for having ten enormous digestive 

 glands? Does this have anything to do with the method of feed- 

 ing? 



Make a drawing of the digestive system of the disk and one 

 arm. 



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 3'ear, sometimes 

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

 nearly or quite as large as the hepatic ca^ca. 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. Carefully remove the side of the 

 stomach next to the bivium, being very careful not to disturb 

 the stone eanal, 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 supposed to be a 

 heart. It is now generally believed to be connected with the 

 reproductive system, and is frequently referred to as the axial 

 organ. It has nothing to do with the system now under con- 

 sideration. 



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

 the outer margin of the peristomial membrane, follows the mar- 

 gin of the membrane and so encircles the mouth. Originating 



^ 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. 



