SEA-UKCHINS AND SEA-CUCUMDERS. 



335 



SUCKER-PLATE OF tJKCnlN. 



The round aperture in the centre plays an important 

 part in the function of the organ. The foot adheres on 

 the same principle as tliat by ^hich children take up 

 larije flat stones Avith a i)iece of 

 Avetted leather, to the middle of 

 which a string is attached. The 

 hoy drops his sucker on the 

 stone, and treads firmly on it, to 

 brines it into close contact with 

 the surfjice ; then he pulls at the 

 string perpendicularly, by which 

 the central part of the leather is 

 lifted a little way from the stone, 

 leaving a vacuum there ; since the contact of the edges 

 with the stone is so perfect that no air can find entrance 

 between them. Now the pressure of the atmosphere 

 upon the leather is so great that a considerable weight, 

 perhaps half-a-dozen pounds, may be lifted by the stiing 

 before the union yields. 



Well, the very counterpart of this amusing opera- 

 tion is repeated by the clever " Urchin" whose per- 

 formances we are considering. The tube is his string ; 

 the dilated end with the plate in it, his leather ; his 

 muscular power acts like the other urchin's tread, to 

 press the bottom of the sucker against the surface of 

 the rock. Then he pulls the string ; in other words, 

 he drags inwards the centre of the muscular bottom of 

 the sucker, which is, as it were, sucked up into the cen- 

 tral orifice of the plate. Tluis a vacuum is formed 

 beneath the middle of the sucker, on which the weight 

 of the incumbent water and atmosphere united presses 

 with a force far more than sufiicient to resist the 

 weight of his body, when he drags upon it, and, as it 

 were, warps himself vp to the adhering point. 



