SEA-URCHINS AND SEA-CUCUMBERS. 291 



so please, its swollen tip is brought close up to the pores. 

 By mechanism so beautiful and simple is the prolongation 

 or abbreviation of these organs effected. 



"We noticed, however, that the extremities of the tubes 

 had an adhesive power, which faculty it is that constitutes 

 them feet. They are prehensile ; and thus they afford, as 

 we observed in the living Urchin, the means by which it 

 takes hold of even a smooth and vertical surface, as the 

 side of a glass tank, and drags up its body thereby. 



Putting, now, the extremity of this cut-off tube under 

 graduated pressure, having first applied to it a drop of 

 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- 

 cular 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 

 skeleton, 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 attached. The boy drops his sucker on 

 the stone, and treads firmly on it, to bring it into close 

 contact with the surface; then he pulls at the string per- 

 pendicularly, by which the central part of the leather is 

 lifted a little way from the stone, leaving a vacuum there ; 



u 2 



