222 ECHINODERMATA 



lacral series of plates. These plates and the genital plates 

 together form what is known as the apical system. 



5. In the ambulacral series of plates, note the arrangement 

 of the openings (ambulacral pores) through which the tube 

 feet protrude. 



6. Do all of the plates bear balls to which spines were 

 articulated? Are the balls of equal size? Do they have a 

 definite arrangement? 



Can you homologize the positions of the ambulacral, inter- 

 ambulacral, ocular, and genital plates in the sea urchin and 

 starfish? What portion of the starfish is represented by the 

 periproct of the sea urchin? 



Make a drawing of the test, showing the ambulacral, inter- 

 ambulacral, and apical systems of plates. 



7. Around the peristome, on the inside of the test, note 

 the five auricles forming arches or bridges over the bases of 

 the ambulacral areas. Their purpose will be seen later. 



Cut around the equatorial region of an alcoholic specimen, 

 taking care to cut through the test only. Break the aboral 

 portion away bit by bit with forceps until near the genital 

 plates, freeing the fragments from the internal organs with- 

 out disturbing their positions. 



Reproductive System. — How were the gonads (their ap- 

 pearance is the same in both sexes) attached to the test? 

 How many are there? Opposite what areas of the test are 

 they placed? Where do they open to the exterior? Without 

 mutilating, find the narrow strip of tissue that connects the 

 gonads to each other near their aboral ends. This is the 

 genital rachis. Connected with the genital rachis and lying 

 alongside the stone canal, which leads from the madreporite, 

 is the genital stolon. 



Digestive System. — Remove the gonads from the three 

 areas farthest from the madreporic plate, lift the remaining 

 aboral portion of the test slightly, examine the alimentary 

 canal, and note: 



1. The large and conspicuous jaws, frequently called the 

 lantern. They will be studied later. 



