292 EVENINGS AT THE MICROSCOPE. 



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 con- 

 siderable weight, perhaps half-a- 

 dozen pounds, may be lifted by 

 the string before the union yields. 

 Well, the very counterpart of 

 this amusing operation is repeated 

 by the clever " Urchin" whose 

 performances we are considering. 



SUCKER-PLATE OF URCHIN. 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 central orifice of the plate. Thus 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 sufficient to resist the 

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

 warps himself up to the adhering point. 



Here is in my cabinet a specimen of a Sea-Urchin of a 

 less regular form : it is the Heart-Urchin (Amphidotus 

 cordatus). Essentially, its structure agrees with that of 

 the more globular forms, but it is heart-shaped, and the 

 two orifices, instead Qf being at opposite poles, are sepa- 

 rated only by about one-third of the circumference. It 

 shows also singular impressed marks on its shell, as if 

 made by a seal on a plastic substance. 



But what I chiefly wish to direct your attention to are 

 the spines. These differ much from the kindred organs 

 in Ecldnus, being far more numerous, very slender, curved, 

 thickening towards the tip, and lying down upon the shell 

 in the manner of hair, whence the species is sometimes 



