18(3 THE MFXHANICAL FUNCTIONS. 



On the outer spherical surface of the external 

 crust, there are formed a great number of calca- 

 reous tubercles, arranged with beautiful regularity 

 and symmetry in double lines, passing, like meri- 

 dian circles, from the upper to the lower pole of 

 the sphere. Each appears, when magnified, to be 

 a smooth and solid ball, projecting from the surface 

 of one of the polygonal plates of the crust. These 

 balls serve for the support of the spines,* which 

 have grooves or sockets at their base, allowing of 

 their accurate application to the spherical surface 

 of the tubercles. They thus constitute ball-and- 

 socket joints, allowing of free motion in all direc- 

 tions. 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 surfaces 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 Spatmigus, 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 



* It has been ascertained by Mr. Haidinger, that the structure of 

 these spines is crystaHine, and that their cleavage presents the exact 

 rhomboidal angles characteristic of carbonate of lime. See his 

 translation of Mohs's Mineralogy, vol, ii. p. 91. 



