STRUCTURE OF THE STARFISH. 61 



plate will be at the same end, and the two will form a 

 tube. The tube between this second plate and the third 

 will, on the contrary, lie at the central end, 



3. On the outside of the ambulacral area there is an 

 area marked by a double row of very minute pores ; the 

 area of the inter-ambulacral ossicles. These are equal in 

 number and thickness to the ambulacral ossicles, to the 

 outer ends of which they are united. 



4. Outside the inter-ambulacral plates there is a row of 

 much larger plates, each of which articulates with three 

 or four inter-ambulacral plates. They are indefinitely 

 cross-shaped, and are united by the long arm of the cross 

 to the inter-ambulacral plates, by the cross-bar to adjacent 

 plates of the same row, and by the top of the cross to the 

 inegular plates of the al>oral surface. More careful ex- 

 amination shows each to be made up of three distinct 

 ossicles. Between the arms of adjacent plates are large, 

 nearly circular foramina, closed by membrane. 



A\ Make an enlarged sketch of a small poi-tion of the 

 floor of a ray, showing all these points. 



I. Clean and examine the lower or external surface of 

 the same specimen, removing the spines from part of it in 

 order to expose the plates, and notice : — 



1. The double row of ambulacral ossicles and their 

 pores. 



2. A row of inter-ambulacral ossicles on each side of 

 the ambulacral area. 



3. The double row of slender, movable spines, which 

 these ossicles carry. 



4. The row of three or four series of thick spines on 

 the cross-shaped plates. 



5. Trace this latter row of spines to the tip of the ray, 

 and notice that it passes around the ambulacral and inter- 



