62 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



ambulacra! plates, and unites with the row on the other 

 side of the ray, to form a terminal tuft of spines upon the 

 upper surface of the ray close to the tip. 



m. Remove the ab-oral wall from the central disc, and 

 having cleaned away the soft parts, in order to expose the 

 inner surface of its floor, notice : 



1. The pentagonal mouth-opening. 



2. The five anibulacral areas converging at the mouth 

 to form the sides of the pentagon. 



3. Notice that the alternating arrangement of the am- 

 bulacral pores gradually disappears at the proximal end of . 

 the ray, so that the last three pores are arranged almost in 

 a straight line. 



4. The last pair of ambulacra! ossicles are much shorter 

 and thicker than the others, and their proximal edges form 

 the slightly convex sides of the mouth-pentagon. The 

 ambulacral pores of this last pair of ossicles pass directly 

 through the stony matter of the plates. 



5. Notice the five inter-radial partitions which separate 

 the ambulacral areas of adjacent rays, and are formed by 

 the union of the inter-ambulacral ossicles of one side of 

 one ray to those of the opposite side of the adjacent ray. 

 Each of the inter-radial partitions abuts upon one of the 

 rounded angles of the mouth-pentagon. 



6. On each side of the partition there is a perforation 

 somewhat larger than the ambulacral pores ; the internal 

 end of the reproductive orifice. Pass a bristle into this 

 tube, and try to find the external opening on the outer sur- 

 face of the specimen. 



n. Make a sketch of the inner surface of the floor of 

 the disc. 



o. On the lower external surface of the disc, notice that 

 the rows of inter-ambulacral spines approach each other 



