ARBACIA OR STRONGYLOCENTROTUS 221 



8. Do the spines have any definite arrangement? 



9. By means of the lube feet, notice that there are five 

 ambulacral areas, between which are five inter ambulacra! 



areas. 



10. Notice an area on the aboral surface which is free 

 from spines. This is the periproct. 



11. Notice the membrane around the mouth, the peris- 

 tome. 



12. Look for pedicellariae on the peristome. In what 

 other places are pedicellariae found? Do they differ from 

 those of the starfish? 



Draw one. 



13. Notice the tentacles (modified tube feet) on the peris- 

 tome. 



14. The dermal branchiae are shrublike appendages at the 

 outer edge of the peristome. They are situated opposite each 

 interambulacral area. 



Skeleton. 1 — Examine the aboral surface of a cleaned 

 "test." 



1. The periproct has scattered plates which cover the 

 anal opening. (Four triangular ones in Arbacia.) 



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

 the apices of the interambulacral series of plates. These are 

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

 the genital pore. 



3. 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. Deter- 

 mine its structure with a lens. 



4. Between the genital plates 1 are five smaller ocular 

 plates, also perforated, which form the apices of the ambu- 



1 If a preserved specimen of Strongylocent'rotus be placed in a solu- 

 tion of nitric acid (about 15 per cent) 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. 



