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 VRCHIX. ^^ ^^ ^ j^ ^^ . ^ tffofefi 



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 (Amjihidotus 

 cordatus). Essentially, its structure agrees with that of 

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

 two orifices, instead of 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 Echinus, 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 



