830 EVENINGS AT THE MICROSCOPE 



caustic potash, we see that it carries a beautiful glassy 

 plate of extreme thinness, which lies free in the swollen 

 cavity of the termination of the tube. This plate is cir- 

 eular in form, apparently notched at the margin, and cut 

 with four or five (for the number varies) incisions, which 

 reach almost to the centre. The substance is formed of 

 the common clear brittle calcareous matter of the skele- 

 ton, hollowed into numberless cavities, according to the 

 general plan. The centre is perforated with a larger 

 round orifice. The appearance of marginal notching is 

 deceptive; and indicates a structure analogous to what we 

 see in the spine. The notched line indicates the extent 

 of the spongy structure; but beyond this the plate extends 

 into a perfectly circular smooth edge, but is constituted of 

 a layer of calcareous substance so thin that there is no 

 room for the ordinary cavities within it. 



The round aperture in the centre plays an important 

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



the same principle as that by which 

 children take up large flat stones 

 with a piece of wetted leather, to 

 the middle of which a string is at- 

 tached. The boy drops his sucker 

 on the stone, and treads firmly on 

 it, to bring it into close contact with 



SUCKER-PLATE OF URCHIN. the surf ace ; 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 



