STRONGYLOCENTROTUS. 77 



possible they should be observed in their native places and the 

 conditions noted. 



1. What apparently serves as food for the animal? Can you 

 determine how this is obtained ? • 



2. Do you find attempts at concealment? 



3. Are the animals able to climb? 



Put a living sea-urchin in a dish of sea-water and study its 

 movements. 



1. When placed on its back, how does it turn over? 



2. What is the normal method of progression? 



3. How are the spines arranged when the animal is creeping 

 on the bottom? 



4. What difference do you note between the spines on the 

 lower and upper surfaces? 



5. How long are the tube feetf Are they used with the 

 spines in moving or do both sets of organs act independently ? 



6. Grasp a spine with your forceps and see if neighboring 

 spin^es respond. Do they form defensive armor? 



7. In what directions may a spine be moved? Remove a 

 spine from a preserved specimen and determine how it was 

 attached and how the muscles that moved it were attached to 

 the spine and to the test. 



Make a diagram showing the arrangement. 



8. Do the spines have any definite arrangement? 



9. By means of the tube feet, notice that there are five ambu- 

 lacral areas, between which are five inter-amhulacral areas. 



10. Notice an area on the aboral surface which is free from 

 spines. This is the periproct. 



11. Notice the membrane around the mouth, the peristome. 



12. Look for pedicellarice on the peristome. In what other 

 places are pedicellariae found? Do they differ from those of 

 the starfish? 



Draw one. 



13. Notice the tentacles (modified tube feet) on the peri- 

 stoirie. 



