220 THE AQUARIAN NATURALIST. 



the plates which have completed their growth and 

 bear fully formed spines, down to the smallest com- 

 mencement of plates as yet unfurnished with spines. 



Passing from pole to pole of the globular shell, the 

 reader will next observe ten rows of plates, disposed 

 in pairs, avenues as they are fancifully called ; these 

 are at once recognizable by innumerable minute per- 

 forations, the ambulacral orifices, disposed with great 

 regularity. The ambulacral orifices in the living 

 Echinus give passage to numerous tentacular-looking 

 organs, each of which is terminated by a minute 

 sucker, and endowed with a truly wonderful power of 

 contraction and extension ; insomuch that when un- 

 employed they are retracted, something like the horns 

 of a snail, until they quite disappear ; but when ac- 

 tively employed, they are protruded until their sucker- 

 like extremities reach beyond the points of the longest 

 spines, and thus are enabled to lay hold on foreign 

 objects. 



The number of these suckers is very great. In a 

 moderate sized Sea-urchin Professor Forbes reckoned 

 sixty- two rows of pores in each of the ten avenues. 

 Now, as there are three pairs of pores in each row, 

 their number multiplied by six, and again by ten, 

 would give the great number of 3720 pores ; but, as 

 each sucker occupies a pair of pores, the number of 

 suckers would be half that amount, or 1860. Imagine 

 therefore the complicated mechanism wielded by one 

 of these animals even for the simple purpose of taking 

 a walk. 



In order to understand the manner in which the 

 Sea-urchin employs these remarkable organs, let us 



