SEA-URCHINS AND SEA-OUCUMBEBS. 277 



So I brought it home, and now produce it as the text of 

 our conversazione. 



Every part is a wonder ; but we must examine each in 

 order. Take the spines first. 



As we examine these organs on the animal crawling at 

 ease over the bottom of a saucer of sea-water, using this 

 triple lens, we see that each is a taper pillar, rounded at 

 the summit, and swelling at the base, where it seems to 

 be inserted into a fleshy pedestal, on which it freely 

 moves, bending downward in all directions, and describing 

 a circle with its point, of which the base is the centre. 

 Each spine is for the greater portion of its length of a 

 delicate pea-green hue, but the terminal part is of a fine 

 lilac or pale purple. The whole surface appears to be 

 fluted, like an Ionic column, but this is an illusion, as you 

 will see presently. 



I now detach one of the spines, cutting it off with fine- 

 pointed scissors as near the base as I can reach. I put it 

 with as little delay as possible into the live-box, and exa- 

 mine it with a high power, say 600 diameters. Look at 

 it. You see the ciliary currents very distinctly; and if 

 you move the stage so as to bring the basal portion into 

 view, you may discern even the cilia themselves, very 

 numerous and short, quivering with a rapid movement. 

 The currents are not longitudinal, but transverse, and 

 somewhat peculiar. The floating atoms which come within 

 their vortex are drawn in at right angles to the axis of the 

 spine, and are presently hurled away in the same plane ; 

 forming a circle, whose plane is perpendicular to the direc- 

 tion of the spine. The surface upon which these cilia are 

 set is a transparent gelatinous skin, of extreme tenuity, 

 stretched tightly over the solid portion, which it completely 

 covers, and studded with minute oval orange-coloured 

 grains. 



The substance of which the spines are composed is best 

 seen by crushing a few of these organs into fragments. 



