ARBACIA. 63 



Skeleton.^ — Examine the aboral surface of a cleaned ''test" 

 and note: 



1. The periproct has four triangular plates which cover the 

 anal opening. (Scattered plates in Strongylocentrotus.) 



2. Around these anal plates are five large ones, that form 

 the apices of the inter-ambulacral series of plates. These are 

 the genital plates, and each is perforated by a small opening, 

 the genital pore. 



3. That one of the genital plates is larger than the others 

 and is full of very minute pores. This is the madreporite, which 

 is homologous with the madreporite of the starfish. Determine 

 its structure with a len«. 



4. Between the genital plates are five smaller ocular plates, 

 also perforated, which form the apices of the ambulacral 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, 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 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 b}^ the 

 periproct of the sea-urchin? 



Make a drauring 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. 



^ If a preserved specimen of Strongylocentrotus be placed in a solution 

 of nitric acid (about 15 percent) from five to ten minutes, the plates of the 

 test can be more easily seen, especially after drying. This is apparently 

 due to the coloring-matter in the animal itself. Arbacia is not helped by 

 the treatment. 



