SEA-STARS AND SEA-URCHINS. IOI 



no less than 4,800. The calculation is as follows : ten bands, 

 each consisting of eighty rows of six holes (10x80=800x6 = 

 4,800). The specimens that the crabbers take out of their 

 crab-pots, and smash against the rocks, are commonly much 

 larger than this. It always grieves me to see such wonderful 

 structures destroyed in that fashion. 



The five thousand pores are in pairs, each pair giving rise to 

 one pedicel or sucker-foot, like those we described in the Five- 

 fingers. The ten bands of pores are also arranged in pairs, 

 the bands forming a pair being separated by about five rows of 

 spines, and each pair of bands being separated from the next 

 by twenty (4 x 5) rows of spines. These intervening spines are 

 borne on two series of long, yet still five-sided, plates ; the 

 number of spines to each, in a growing specimen, varying ; but 

 from counting many of these I should suppose a fully-grown 

 plate, from the middle of a series, would support twenty spines ; 

 at the top and bottom of the series, however, there are only two 

 or three spines to each plate. These spines are not mere rigid 

 outgrowths like the prickles on a chestnut bur ; they are beau- 

 tifully finished pieces of mechanism, with considerable latitude 

 for movement in any direction. Although only about five- 

 eighths of an inch in length, each is a beautiful column in ala- 

 baster, tapering slightly to the top, and decorated from near 

 the base with a series of thirty (6 x 5) parallel rounded ridges. 

 The bottom of this spine is hollowed out and polished perfectly, 

 to enable it to move freely on the polished knob upon which it 

 fits. These knobs are the bosses left on the shell when the 

 spines have been cleaned off; the spines being held to them 

 and moved by a circular band of muscular tissue. 



If we look at the underside of the Urchin we shall find the 

 mouth occupying the centre, with five polished white teeth pro- 

 truding. Although these are not much to look at from outside, 

 they form a large and complicated structure within, which 

 goes by the name of the " Lantern of Aristotle," because the 

 famous Stagyrite appropriately compared its shape to a lantern. 



