STRONGYLOCENTROTUS. 75 



8. Do the spines have any definite arrangement? 



9. By means of the tube feet, notice that there are five ambu- 

 lacral areas, between which are five inter-ambulacral 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 peristome. 



12. Look for pedicellarice 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 peristome. 



14. The dermal branchice are shrub-like appendages at the 

 outer edge of the peristome. They are situated opposite each 

 inter-ambulacral area. 



Skeleton. 1 Examine the aboral surface of a cleaned "test" 

 and note: 



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 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 lens. 



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. 



1 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. 



