DISSECTION OF SEA-URCHIN. 283 



area, made up of small anal plates. How many plates 

 make up the boundary of this circle ? Examine them 

 under the lens and decide which one compares in structure 

 with the madreporite of the starfish. Is it radial or inter- 

 radial in position ? How many of these plates bear small 

 pores ? Sketch this region, showing the anal area and the 

 tips of the rays, and label the parts, deciding which of the 

 perforated plates must be genital and which must be 

 ocular plates by comparing with their relative position, 

 radial or interradial, in the starfish. With what is the 

 madreporite associated ? What parts must belong to the 

 aboral surface of the starfish ? 



INTERNAL STRUCTURE. 



Open an alcoholic urchin by breaking into the equator 

 of the test, and then continue the opening around by 

 breaking, bit by bit, with the forceps around the shell, 

 taking care that the fleshy parts beneath be not injured. 

 Then carefully lift the aboral pole and, separating every- 

 thing from it, leave all the soft portions in the oral half. 



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 ampullae, and between them for the 

 radial canal. 



