ASTERIAS. 73 



Water-vascular System.^— 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 supposed to be 

 a heart. It is generally referred to as the axial organ of the 

 Jmmal sijstem. See Chadwick's monograph on Asterias for a 

 discussion of the theories concerning the nature of the haemal 



system. 



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 

 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 posi- 

 tion 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 uninjected specimens, lying 

 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 bag-like ampullar. Determine their re- 

 lation 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 



^ 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. 



