306 COMMON OBJECTS OF THE SHORE. 



so much accuracy that the eye can but with 

 difficulty trace the lines of union. Tell a human 

 artisan, however versed in geometrical proportions, 

 to cut a thousand plates, polygonal in form, and 

 fit them to each other, leaving not the slightest 

 space between their margins anywhere, so that 

 the whole shall form a hollow sphere of certain 

 given proportions, how would he succeed? Doubt- 

 less he would pronounce the problem quite im- 

 practicable. But in the shell before us, this is 

 just what nature has achieved most perfectly. 

 First we should observe five double rows of oblong 

 plates, pentagonal in form, which on their outer 

 surfaces present the spine-supporting tubercles. 

 On either side are found innumerable pieces, of 

 smaller size, but equally exact in shape, through 

 which are bored the perforations for the ambulacral 

 feet; and these are again separated from another 

 row of perforated plates, by other intermediate 

 pieces, having spines affixed externally. These 

 plates are mostly pentagons in form, with sides of 

 various length ; but all combined, fit each other so 

 closely that their combination serves to form one 

 solid compact shell." 



The species of sea-urchin which is most frequent 

 on our shores, is that called the Common Sea-egg 

 (Echinus sphara}. Forbes gives as one of its 

 familiar names that of Sca'ad man's head. Though 

 usually of globose shape, it varies in form, and is 

 sometimes so much flattened that even experienced 

 zoologists have mistaken it for another species. 

 These stony balls are to be found of all sizes up 

 to a globe of fourteen inches in diameter. This 

 sea-egg has usually a reddish or purplish tinge, 

 with white spines sometimes tipped with purple. 



