STARFISH. 113 



side and find the ten Polian vesicles (much like the am- 

 pullae) and, inside of these, the small sacculated racemose 

 vesicles. How many are there of these? What do you 

 find in the place of the one needed to make symmetry? 

 Beside the stone-canal is a thin-walled sac, the so-called 

 heart. Sketch the organs in this paragraph and keep the 

 drawing for further additions. 



Remove the ampullae, membranes, etc., from the floor of 

 one of the rays and see the ambulacral plates which meet 

 in the median line. Notice the openings in this ambula- 

 cral area by means of which the ampulla? connect with the 

 ambulacra. Are these ambulacral pores in or between the 

 plates? How many rows of them do you find in an arm? 

 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 (inter ambulacrals) 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 

 branchiae, ambulacra, ampullae, hepatic caeca, and mesen- 

 teries in their proper position. 



In the groove of that part of the arm which remains 

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

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

 jecting apparatus, and force in some colored fluid (solu- 



* The points in this and the next paragraph are best made out 

 in a dried arm. 



