SEA-URCHINS AND SEA-CUCUMBERS o29 



is perforated with, a pair of pores; so that the cavity of 

 every tube communicates with the interior of the shelly 

 box by two orifices. 



Now on the interior side of these two pores that is, 

 within the cavity of the shell there is placed a little 

 membranous, or rather muscular, bladder, filled with a 

 fluid which is not materially different from sea-water. 

 There is a free communication between the bladder with- 

 in and the tube without the shell, by means of the pair 

 of pores through, which the fluid passes. By means of the 

 muscular fibres, which are under the control of the Ur- 

 chin's will, any portion of this double vessel can be con- 

 tracted to a certain extent. Suppose it is the interior 

 bladder; the effect of the contraction of its walls is to 

 diminish its capacity, and the contained fluid is forced 

 through the pores into the tube without. The longitudi- 

 nal fibres of this part being at the same moment relaxed, 

 the tube is lengthened because of the injected water. Sup- 

 pose, now, in turn, the fibres of the tube contract, while 

 those of the bladder relax; the fluid is driven back, the 

 bladder dilates, and the tube shortens, until, if the animal 

 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 cat- off tube under 

 graduated pressure, having first applied to it a drop of 



