How does the external surface of the sea-urchin differ from that of the starfish? Can you find the very long tube-feet? Where is the mouth-opening? With what is it surrounded? Each tooth is enclosed in a calcareous framework. The whole structure is known as "Aristotle's lantern."

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.

Fig. 20.—A sea-urchin, Strongylocentrotus franciscanus. (From specimen from Bay of Monterey, Calif.)

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 is called an ambulacral area, while the intervening segments which support the long spines are called the interambulacral areas. Note on the aboral surface, surrounding the median-placed anal aperture, a series of small plates. Those which are located in the interambulacral 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 appearance. This is the madreporite, which, as in the 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 contain small pigment-cells and communicate with the nervous system.

Fig. 21.—"Test" of sea-urchin, Strongylocentrotus franciscanus, with spines removed. (From specimen.)