n 4 ELEMENTARY ZOOLOGY 



TECHNICAL NOTE. Remove the spines from the underlying 

 shell or test (fig. 21) and wash the test until perfectly clean, or place 

 in a solution of lye for a short time and then wash. 



Note, the characteristic radial symmetry of the shell or 

 test. Note on the aboral aspect, diverging from the 

 medial anal aperture, five double rows of pores. What 

 are these for ? Each of the five divisions set with pores 



FIG. 20. A sea-urchin, Strongylocentrotus franciscanns. (From specimen 

 from Bay of Monterey, Calif. ) 



is called an ambulacral area, while the intervening seg- 

 ments which support the long spines are called the 

 inter ambulacral areas. Note on the aboral surface, sur- 

 rounding the median-placed anal aperture, a series of 

 small plates. Those which are located in the interambu- ] 

 lacral areas are the genital plates. Through these plates 

 the ducts from the reproductive organs open by small 

 pores. Note a very much enlarged plate with a striated j 

 appearance. This is the madreporite, which, as in the j 

 starfish, is the external opening of the stone canal and! 

 water-vascular system. Note the small ocular plate at | 

 the tip of each ambulacral area. The ocular plates con-] 



