SEA-URCHINS. 219 



pansion and contraction, and containing particles of 

 limestone. There are many kinds, varying from an 

 inch to a foot in length. They live in the sea and are 

 exceedingly interesting, and very beautiful when the 

 long and delicate fringes around the mouth are ex- 

 panded. When taken from the water they shrink and 

 lose their beauty of form and color. They must be 

 seen in the ocean, or in the aquarium, in order to get 

 a good idea of them. Figure 462 shows one kind 

 which is very common at Grand Menan, and Eastport, 

 and other places in the North Atlantic. The Chinese 

 call these animals Trepang, and use them for food. 



SEA-URCHINS, OR ECHINOIDS. 



True Sea-Urchins are hemispherical, or flattened, 

 and have a hard shell composed of plates which are 

 regular in form and firmly bound together. Upon 



Fig. 463. Sea-Urchin. 



these plates are tubercles, and on these tubercles hard 

 spines. In certain plates there are rows of holes 

 through which pass fleshy organs called suckers, or 

 ambulacra, 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 sur- 



