ECHINUS. 187 



limited rotatory motion. Tliose which grow from 

 the sides are more slender, 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, ar- 

 ranged 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 re- 

 volving 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 particles ; 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 

 progress becomes more rapid; while the sand, which 

 had been pushed aside, flows back, and covers the 

 body, when it has sunk below the level of the sur- 

 face. In this situation the long dorsal bristles come 

 into play, preventing the sand from closing com- 

 pletely, and preserving 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 find 

 them accompanied by corresponding modifications 

 in the processes of developement. The organization 

 of the animals belonging to the lowest division of 

 the series is not sufficiently perfect to afford the 

 means, which are supplied in the higher animals, 



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

 Philosophical Transactions for 1826, p. 347. 



