SEA-URCHIN. 117 



Most prominent at first will be the yellowish reproduc- 

 tive organs occupying a position above everything else. 

 Are its lobes connected? Can you trace the ducts of this 

 organ? Sketch the reproductive system and then remove 

 it. This will expose the alimentary canal (brown in color) 

 supported by a mesentery. Trace its course, making draw- 

 ings as you proceed. How many turns does it make? At 

 its oral end the alimentary canal connects with a compli- 

 cated apparatus Aristotle's lantern composed of numer- 

 ous harder portions and muscles to move them. Have the 

 teeth any relations to this apparatus? Look on the inside 

 of the test for the ampullce, and between them for the 

 radial canal. 



Usually in preserved urchins the stone-canal becomes so 

 tender as to be easily destroyed. It goes downward from 

 the madreporite to the inner end of Aristotle's lantern, 

 where it connects with a ring-canal, and from this arise 

 the radial canals, in much the same way as in starfishes, the 

 whole forming a water-vascular system. As in the star- 

 fish, the nervous system follows this water-vascular system. 



