220 RADIATES: ECHINODERMS. 



faces. So much can these suckers be extended that a 

 Sea-Urchin has been seen to put them forth from the 

 top, and, bending them downwards, cling to the bot- 

 tom of the basin in which the animal was lying ! Fig- 

 ure 463 shows a common kind of Sea-Urchin as it ap- 

 pears when alive. When the animal dies, the skin, 

 which covers the shell and holds the spines in their 



Fig. 464. Top View of Sea-Urchin. Spines removed. 



places, dries up, and the spines fall off, and then the 

 shell, with all its beautiful structure and markings, is 

 plainly seen. In the one represented in Figure 464 

 we find ten double rows of plates which run along the 

 curved surface from the bottom to the top of the shell. 

 In five of these double rows the plates are large, with- 

 out holes, and are covered with large tubercles. Al- 

 ternating with the double rows of large plates are five 

 double, rows of smaller ones, bearing few and small 

 tubercles, and each plate is perforated with the holes 

 for the suckers. The plates which bear the holes are 

 called the ambulacra! plates, from a Latin word which 

 means a walk, or alley ; and the large plates without 

 holes are called the interambulacral plates. At the 

 termination of each of the five belts or zones of ambu- 

 lacral plates there is a little triangular plate with a 



