ECHINUS. 151 



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 sphe- 

 rical surface of the tubercles. 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 effect- 

 ing the movements of the spines. By employing these spines 

 as levers, the Echinus advances with great facility alono- 

 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 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, w^ith the convexity downwards; and 

 which have a limited rotatory motion. Those which grow 

 from the sides are more slender, and taper towards the ex- 

 tremities, and when not in use they f\dl flat upon the body 

 with their points directed backwards. Besides these, there 

 are a few longer bristles, arranged in a crescent on the back, 

 and converging till their points meet, but capable of being 

 erected to a perpendicular position. The animal, when 

 placed on sand, commences its operations by revolving the 

 lower spines, thus soon creating a hollow quicksand, into 

 which it sinks by its own weight so far as to enable the low- 

 est of the lateral spines to co-operate with them, by scatter- 

 ing and throwing up the loosened particles; while these, at 

 the same time, contribute, by their re-action, still fiirther to 

 depress the body. As the animal sinks, a greater number 

 of spines are brought into action, and its progress becomes 



• It has been ascertained by Mr. Haiding-er, that the structure of these 

 spines is crystalline, and that their cleavag^e presents the exact rhomboidal 

 angles characteristic of carbonate of lime. See his Translation of Mohs's 

 Mineralog-y, vol. ii. p. 91. 



