SEA URCHINS. 



235 



certain plates there are rows of holes through which 

 pass fleshy organs called suckers, or ambulacral feet, 

 with the end slightly expanded. By means of these 

 suckers, which can be extended much beyond the 

 spines, these animals can cling firmly to other bodies, 

 and thus move about over the rocks, even up and 

 down their smooth sides, as well as on level surfaces. 

 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 bottom of 

 the basin in which the animal was lying. Figure 463 

 shows a common kind of Sea Urchin as it appears 

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

 covers the shell and holds the spines in their places, 



Fig. 464. — Top view of Se i Urchin Spines removed. 



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 



