OCEAN LIFE. 



33 



mathematical precision, and conjured with so much accuracy, 

 that the eye can but with difficulty trace the line of union. 

 Tell a human artisan, however versed in geometrical proportions, 

 to cut out a thousand plates, polygonal in form, and fit them to 

 each other, leaving not the slightest space between their margins 

 any where, so that the whole shall form a hollow sphere of certain 

 given proportions, how would he succeed ? Doubtless he would 

 pronounce the problem quite impracticable. But in the shell 

 before us, that is what nature has achieved most perfectly. 

 First, we observe five double rows of oblong plates, pentagonal 

 in form, which on their outer sides present the spine-supporting 



tubercles. On either side are found innumerable pieces of a 

 smaller size, but equally exact in shape, through which are bored 

 the perforations for the ambulacral feet, and these again are 

 separated from another row of perforated plates by other inter- 

 mediate pieces having spines affixed externally. These plates 

 are mostly pentagons in form, with sides of various lengths, but 

 all combined, fit each other so closely, that their combination 

 seems to form one solid, compact shell. 



" We shall not stop to count the number of the plates com- 

 prised in every series, or to calculate how many perforations are 

 provided for the feet. Let the reader try to number them when 



