1 86 Practical Zoology. 



opposite pole, the aboral pole, is a circular area composed of 

 several small plates, near the center of which is the anal opening. 



2. Note that the test is composed of distinct pieces or plates. 

 Put one of the plates into a little dilute acid and note what 

 occurs. 



3 . To make out the real nature of the skeleton, proceed thus : 



a. Handle an entire decalcified specimen, i.e. one from 

 which the calcareous matter has been removed by chromic or 

 other acid. Observe that the body walls and spines are still 

 present. 



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 calcareous 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 

 inclosed 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 matter within the living tissues of the 

 body wall. The joints, or sutures, between the plates are formed 

 by the absence of the deposit of calcareous matter. 



4. Returning to the entire test, study the arrangement of the 

 plates, their variations in shape, size, etc. Into how many simi- 

 lar 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. 



