SEA-UKCHmS AND SEA-CUCTJMBEES. 333 



are arranged in ten longitudinal or meridional lines, as- 

 sociated so as to make five pairs of lines. Now, with a 

 lens, scrutinize 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 ar- 

 rangement among themselves ; that is to say, they form 

 minor rows which cross, obliquely or diagonally, the 

 course of the meridional 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 tubu- 

 lar feet, each of which springs fi'om a portion of the 

 shell that 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. 



Ifow 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 and the tube without the shell, by means 

 of the pair of pores through which the fluid passes. 

 By means of the muscular flbres, 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 con- 

 tained 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 be- 



