206 THE MECHANICAL FUNCTIONS. 



and taper towards the extremities, and when 

 not in use they fall 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 lowest of 

 the lateral spines to cooperate with them, by 

 scattering and throwing up the loosened par- 

 ticles ; while these, at the same time, contribute, 

 by their reaction, still farther to depress the 

 body. As the animal sinks, a greater number 

 of spines are brought into action, and its pro- 

 gress becomes more rapid ; while the sand, 

 that had been pushed aside, flows back, and 

 covers the body, when it has sunk below 

 the level of the surface. In this situation the 

 long dorsal bristles come into play, preventing 

 the sand from closing completely, and preserv- 

 ing a small round hole for the admission of 

 water to the mouth and respiratory organs.* 



Whenever, in following the series of organic 

 structures, new forms are met with, we always 



* The account here given is taken from Mr. Osier's papers in 

 the Philosophical Transactions for 1826, p. 347. 



