% ECHINUS. 205 



support of the spines,* which have grooves or 

 sockets at their base, allowing of their accurate 

 application to the spherical surface of the tu- 

 bercles. They thus constitute ball-and-socket 

 joints, allowing of free motion in all directions. 

 Each joint is connected with the plate on which 

 it turns, by means of the integument, which 

 acts the part of a capsular ligament ; and sets 

 of radiating muscular fibres are provided for 

 effecting the movements of the spines. By 

 employing these spines as levers, the Echinus 

 advances with great facility along plane sur- 

 faces at the bottom of the sea. This animal 

 is also aided in its progressive motion by the 

 employment of suckers, which are placed at 

 the end of the slender tubes, protruding from 

 the pores of the ambulacra, and analogous to 

 those of the Asterias. 



The Spatangus, a genus belonging to this order, 

 buries itself in the sand by the action of its 

 spines, which on its under surface are short, 

 thick, and expanded at the ends, like the handle 

 of a spoon, with the convexity downwards ; and 

 which have a limited rotatory motion. Those 

 which grow from the sides are more slender, 



* It has been ascertained by Mr. Haidinger, that the struc- 

 ture of these spines is crystalline, and that their cleavage pre- 

 sents the exact rhomboidal angles characteristic of carbonate 

 of lime. See his Translation of Mohs's Mineralogy, vol. ii. 

 p. 91. 



