Echinodermata. 1 8 1 



8. The eye-spot is borne on a distinct, but minute, plate. 

 Compare young and old specimens to see that whatever the size, 

 this single ocular plate with its eye-spot is always at the end of 

 the ray. Count the ambulacral plates in a short and in a long 

 ray. Where do the new plates develop ? 



DISSECTION OF THE STARFISH (IN WATER). 



1. The ray opposite the madreporic body is the anterior ray. 

 Cut into its aboral wall near the outer end, and from this point 

 cut along the upper part of each side of the ray, an inch or two 

 toward the disk; raise the flap thus freed, and, avoiding internal 

 organs, continue the cut on each side to the disk. 



2. Attached to the aboral wall find a pair of elongated, branched 

 bodies, the digestive glands, or ceca. Note how each cecum is 

 held in place by the thin mesentery. 



3. Along the middle line of the aboral wall, inside, is a 

 yellowish streak, the extensor muscle of the ray ; with forceps 

 prove its general structure. 



4. Along each side of the ridge in the floor of the ray, observe 

 rows of thin-walled sacs, sometimes distended, but more often 

 collapsed in alcoholic specimens. These are the ampullae, or 

 ambulacral vesicles. Watch the ampullae while pressing on the 

 tube feet, and vice versa. If a specimen injected with coloring 

 matter be at hand, it should now be examined. 



5. Near the base of the ray find, on each side, an elongated 

 body resembling a bunch of grapes, and of a lighter color than the 

 ceca; these are the ovaries and spermaries, and are very much 

 alike in appearance in the two sexes, and only distinguishable by 

 color (the spermaries being lighter colored), or by microscopic 

 examination in the living specimens. Find the point of attach- 

 ment of one of them. The openings in the interradial angle are 

 not very evident. 



6. Cut along the sides of the two rays lying on the right and 

 left of the anterior ray, connect the cuts at the interradial angles, 

 and turn back the cover of the three rays and disk. Within the 



