Echinodermata. 187 



8. Radiating from the apex of each genital plate is the zig- 

 zag interradial suture. How many kinds of plates are found 

 within the area included by two adjacent interradial sutures ? 

 The perforated plates are the ambulacral plates, and the unper- 

 forated, the interambulacral plates. Compare these two sets of 

 plates with the corresponding parts of a starfish. 



9. The ambulacral plates form the ambulacral areas. Trace 

 each of the ambulacral areas to its aboral end, and find at its 

 apex a small plate wedged in between two adjacent genital plates. 

 These smaller ones are the ocular plates. Note the small open- 

 ing from which projects an unpaired tentacle, the end of the 

 radial water tube. 



10. Carefully compare the hard parts of the starfish and sea 

 urchin. Wherein are they alike, and wherein do they differ ? 

 What changes in growth would be necessary to convert one of 

 these forms into the other ? What part of a starfish is homolo- 

 gous with the anal area of a sea urchin ? 



1 1 . Make careful drawings of the oral surface, of the aboral 

 surface, and of the side of the test. 



ALCOHOLIC SPECIMEN. 



For the sake of review and comparison, it is well to have the 

 cleaned test before you during this study. 



1. Observe the soft membrane, the peristome, on the oral 

 surface and the teeth projecting from the mouth. 



2. At the aboral pole look for the anus and genital plates. 



3. Examine one of the largest spines ; move it about to see 

 its range of motion. Remove it and make out how it is articu- 

 lated to the test. The fleshy tube ensheathing the base is 

 muscular tissue, by the shortening of which the spine is moved. 

 Clean the spine and make a drawing of it. 



4. Note any variations in the size and shape of the spines in 

 various regions. 



5. Study carefully the arrangement of the spines, using the 

 cleaned test for comparison. 



