104 THE BIOLOGY OF THE ANIMAL 



In the grooves are 

 12. The ambulacral feet. — How are they arranged 

 in the groove? What is their shape? Size? 

 Do you find any variations in shape or size? 

 Color ? Structure ? Are they found anywhere 

 else than in the grooves ? Pull off a foot and 

 insert a fine bristle into the torn end. Notice 

 that the foot is tubular. Examine one of the 

 feet under the low power of the microscope, 

 and note at the extreme end the thickened 

 ring with the creased central membrane form- 

 ing a sucking disk ; also the circular ridges in 

 the walls of the tube, the groups of circular 

 muscles. Can you determine whether any or 

 all of the feet are connected? Sketch a longi- 

 tudinal section of an ambulacral foot magni- 

 fied. Make an enlarged drawing of the sucking 

 disk to show its structure. 



Lying just above a membrane stretched 

 across the roof of the groove find 



13. The radial -water-tube. — How far does it ex- 



tend? Pass a fine guarded bristle into its 

 cavity and trace the course of the tube. Ex- 

 amine injected specimens to see how the radial 

 water-tubes are connected with one another. 

 Along with the water-tube run a " blood-ves- 

 sel " and a nerve so intimately connected with 

 the tube as to be almost indistinguishable ex- 

 cept in trans-sections of the ray prepared for 

 microscopic examination. 

 Make a diagram of a cross section of a ray. At the 

 ends of the rays look for 



14. The eyes. — Position ? Shape? Size? Color? 



What is the arrangement of the spines around 

 the eye? 



