70 ECHINODERMATA. 



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

 ulated? 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. 



With a scalpel or strong scissors, cut around the equatorial 

 region of an alcoholic specimen, taking care to cut through the 

 test only. Break or cut the aboral portion away bit by bit until 

 near the genital plates, freeing the fragments from the internal 

 organs without disturbing their positions. 



^ Reproductive System. How were the gonads (their appear- 

 ance 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, and examine the alimentary canal. 



1. The large and conspicuous jaws, frequently called the 

 lantern. They will be studied later. 



2. The esophagus, passing between the jaws, and bending 

 over to one side to join the intestine. 



3. The intestine. Notice its size and its shape. Do its 

 loops have any relation to the positions of the gonads? 



4. The intestinal siphon, lying along the intestine and 

 attached to it at both ends. 



