162 PBACTICAL ZOOLOGY. 



zigzag inter-radial suture. How many kinds of 

 plates are found within the area included by two 

 adjacent inter-radial sutures ? The perforated plates 

 are the ambulacral plates, and the unperforated, the 

 inter-ambulacral 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 opening 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 homologous with 

 the anal area of a sea-urchin ? 



11. 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 one in 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 



