SEA-URCHIN: LABORATORY WORK. 



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 * make out the ambulacra! 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 starfish ? 

 Between each two sets of ambulacral plates are found the 

 larger interambulacrals. Which plates, ambulacral or in- 

 terambulacral, bear rounded prominences for the articula- 

 tion of the spines ? Making a comparison with a starfish, 

 where would you draw the line between two rays of the sea- 

 urchin ? Illustrate by a sketch. 



Follow a ray from the oral area to the pole opposite the 

 mouth. Notice in the centre of this pole a circular anal 



* For this purpose the parts of the shell (test) of specimens used 

 in previous years may be employed. They are easiest cleaned by 

 rubbing off the spines and then bleaching in Eau de Javelle or Labar- 

 raque's solution (potassium or sodium hypochlorite), to be had of 

 druggists. 



