8TABFI8H, SEA-TmOHIN8 AND SEA- CUCUMBERS. 49 



starfishes and sea-urchins exterminated over a given region, 

 the species would survive, as the young might be hundreds 

 of miles away, and in safety at the surface of the ocean. 

 Sea-urchins. — Another example of Echinoderms is the 



Fio. 50.— The common Sea-urohm, Echinus, d, framework of mouth and teeth 

 seen In front; c, the same seen sideways; a, &, side and external view of a 

 single tooth (pyramid). (All natural size.) 



Echinus, which is more like a starfish than one would at 

 the first glance think. A good idea of the general structure 

 of sea-urchins may be obtained by an examination of the 

 common Echinus (Eig. 50). 



It is common among rocks, ranging from low-water mark 



FiQ. &1.—Echmarachnius, common Sand-cake. (Natural size.) 



to fifty or more fathoms. It eats sea-weeds, and is also a 

 scavenger, feeding on dead fish, etc. 



The shell consists of five double rows of limestone pieces 

 called anibulacral plates, which are perforated for the exit 

 of the suckers or feet, which are like those of star-fish. 

 There are also fiv^ double rows of inUramiulacral plates, 



