220 ECHINODERMATA 



on the surface of the arm, the entire arm being encased with 

 four or more rows of shields. They can be studied best in 

 sections. 



Grave: Ophiura brevispina I. Mem. Biol. Lab. Johns Hopkins Univ., 



4, 1900. Mem. Nat. Acad., 8, 1899. 

 : Ophiura brevispina II. An Embryological Contribution and Study 



of the Effect of Yolk Substance upon the Developmental Process. 



Jour. Morph., 27, 1916. 



ECHINOIDEA 



ARBACIA OR STRONGYLOCENTROTUSi (Sea Urchin) 



In some localities sea urchins can be found in tide pools 

 or near low-tide mark, where they may be very abundant. 

 In other localities they can be obtained only by dredging. 

 When 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 creep- 

 ing on the bottom? 



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

 lower and upper surfaces? 



5. How long are the tube feet? 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 

 spines 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. 



1 These directions will serve for any of our common sea urchins. 



