334 EVE2fINGS AT THE MICROSCOPE. 



cause of the injected water. Suppose, now, in tnra, 

 the fibres of the tube contract, while those of the blad- 

 der relax ; the fluid is driven back, the bladder dilates, 

 and the tube shortens, until, if the animal so please, its 

 swollen ti}) is brought close up to the pores. By me- 

 chanism so beautiful and simple is the prolongation or 

 abbreviation of these organs eflected. 



AYe noticed, however, that the extremities of the 

 tubes had an adhesive power, which faculty it is that 

 constitutes them feet. Tliey 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 cames a beauti- 

 ful glassy plate of extreme thinness, which lies free in 

 the swollen cavity of the termination of the tube. Tliis 

 plate is circular 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 sub- 

 stance is formed of the common clear brittle calcareous 

 matter of the skeleton, hollowed into numberless ca^'- 

 ities, according to the general plan. The centre is per- 

 forated with a larger round orifice. The appearance of 

 marginal notching is deceptive ; and indicates a struc- 

 ture analogous to what we see in the spine. Tlie 

 notched line indicates the extent of the spongy struc- 

 ture ; 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. 



