290 EVENINGS AT THE MICROSCOPE. 



rminute holes or pores, which are arranged in ten longi- 

 tudinal lines, associated so as to make five pairs of lines. 

 Now with a lens examine more minutely a portion of any 

 one of these lines, and you discern that it is composed 

 of a multitude of pores, which have a peculiar order of 

 arrangement among themselves; that is to say, they form 

 minor rows which cross, obliquely or diagonally, the 

 course of the longitudinal line. These rows are them- 

 selves double, the pores running in pairs, not however 

 with mathematical symmetry. In this species, there are 

 three pairs of pores in each row, and so there are in the 

 one which I have here alive, but in other of our native 

 species the rows consist of five pairs. 



These pores are intimately connected with the tubular 

 feet, each of which springs from a portion of the shell 

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

 of every tube communicates with th3 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 within 

 ^ind 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 

 Urchin's will, any portion of this double vessel can be 

 contracted 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 longitudinal 

 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 



