DISSECTION Of STARFISH. 281 



Sketch these plates in the ray of the drawing left incom- 

 plete. 



Turn this same ray over, remove the ambulacra, and see 

 the ambulacral plates from the oral surface. They meet, 

 forming an ambulacral groove, the edges of which are 

 formed by smaller plates (interambulacrals) bearing mov- 

 able spines. 



Cut off the arm as yet left intact about half an inch from 

 the disc, and draw the section, including in the sketch the 

 ambulacral plates forming the roof of the ambulacral 

 groove; outside of these the interambulacrals, and then the 

 plates of the aboral surface. Add to these parts the bran- 

 chiae, ambulacra, ampullae, hepatic caeca, and mesenteries in 

 their proper position. 



In the groove of that part of the arm which remains at- 

 tached to the disc notice a tube, the radial canal. Insert 

 into this the canula of a hypodermic syringe or other in- 

 jecting apparatus (see Appendix), and force in some colored 

 fluid (solution of carmine or Prussian blue). What happens 

 to the ampullae and ambulacra ? Part the ambulacra and 

 follow the colored radial canal to the region of the mouth, 

 and see how this is surrounded by a ring-canal. Are stone- 

 canal, racemose vesicles, or Polian vesicles filled with the 

 fluid ? Insert the radial and ring canals, ampullae, and 

 ambulacra in the drawing of the stone-canal, etc. 



Beneath the radial canal is a thickening of the skin, the 

 radial nerve which connects with circumoral ring-nervt: 

 just below the ring-canal. 



