SEA-URCHINS AND SEA-CUCUMBERS 331 



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. 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 inward 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 re- 

 sist 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 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 toward the tip, and lying down upon the shell 

 in the manner of hair, whence the species is sometimes 

 called the Hairy Sea- egg. The array of spines has a glit- 

 tering silky appearance in this dried state. 



We will now put a few of them under a low power 



