THE SEA-UKCHIN. 161 



b. Examine a microscopic section of the decalcified 

 body wall to see that there was soft living matter, 

 both on the outside and on the inside of the cal- 

 careous plates. 



c. Grind down and mount a thin section of a plate, 

 as in the case of the starfish, and see that not 

 only is the plate wholly enclosed in the body wall, 

 but that it forms a network whose meshes were 

 penetrated by the soft living substance of that 

 body-wall. It should now be clear that the plates 

 were formed by the deposition of calcareous mat- 

 ter within the living tissues of the body wall. 

 The joints, or sutures, between the plates are 

 formed by the absence of the deposit of calcare- 

 ous matter. 



4. Eeturning to the entire test, study the arrangement 

 of the plates, their variations in shape, size, etc. 



Into how many similar areas may the surface of the 

 test be divided? To make out these points, and the 

 shapes of the plates, pull apart a piece of a dried test 

 that was left over from previous dissection. 



5. At the aboral pole, observe a small distinctly marked- 

 off area, including numerous small plates. This is 

 the anal area, and the plates are the anal plates. 

 Unlike the other plates, these, in the living sea-urchin, 

 are movable. They surround the anus. 



6. Surrounding the anal area are the five large genital 

 plates, each having a genital opening near its outer 

 angle. 



7. With a lens examine the largest of the genital plates ; 

 its perforated portion serves as a madreporic body. 



8. Eadiating from the apex of each genital plate, is the 



