66 SPINES OF ECHINI 



fitted together with greater precision than the 

 various parts which make up a coat of mail. 

 The shell adapts itself in the matter of 

 growth to the creature it contains. It is a 

 marvellously strong structure when we con- 

 sider how very light it is. 



Its beauty may be seen by removing all the 

 spines. Sailors, who, generally speaking, have 

 plenty of time on their hands, are adepts at 

 polishing these natural boxes. The work re- 

 quires great gentleness of handling, but when 

 well done amply rewards the polisher. 

 Previous to the polishing process, but with 

 the spines removed, the surface of the plates 

 will be seen to be covered over with tubercles. 

 Each tubercle consists of a rounded boss, on 

 the centre of which is a small pimple or 

 mamelon. 



'These tubercles are of three different sizes, 

 primary, secondary, and miliary. The primary 

 are the largest, and bear the largest spines. 

 Scattered irregularly over the plates are the 

 secondary tubercles, which carry the secondary 

 spines, and between these are numbers of 



