SEA-URCHIN. 



This will apply to either of the common urchins, Strongy- 

 locentrotus or Arbacia, of the northern Atlantic coast. Each 

 student should be provided with an alcoholic specimen and 

 with a dried, cleaned, and bleached aboral region of the shell 

 (test) of another specimen. For this specimens used in pre- 

 vious years may be employed. They are easiest cleaned by 

 rubbing off the spines and then bleaching in Eau de Javelle 

 or Labarraque's solution (potassium or sodium hypochlorite), 

 to be had of druggists. This requires several weeks. A more 

 rapid method is to boil the test in very weak potash or soda 

 lye, but if the lye be too strong or the boiling too long con- 

 tinued the whole will fall to pieces. 



EXTERNAL. 



What is the general shape? Are the spines movable? 

 Can you find ambulacra between the spines? In how 

 many areas are they arranged? At one pole of the urchin 

 find the oral area closed by a thin membrane (peristome) 

 and in its centre, teeth (how many?). Do the ambulacra 

 radiate from this mouth? If so, where should you look for 

 the eye-spot (compare starfish)? 



In a cleaned specimen of the test make out the ambu- 

 lacra! areas radiating from the region of the mouth. They 

 may be recognized by the presence of the ambulacral pores. 

 Do these pores pass through or between the plates? How 

 does this condition compare with that found in the star- 



115 



