Objects for the Microscope. 211 



the blue selenite the structure is better seen by polarized 

 light. 



A short account of the animal to which this spine 

 belongs may be interesting to those who cannot read its 

 perfect history in the work of Forbes on the Radiata. It 

 belongs to the same division as the Star-fish, the Holo- 

 thurise, the Medusae, or Jelly-fish, the Entozoa, Polypes, 

 and Infusoria, in all of which the external or internal parts 

 radiate like a star, and which are therefore called the 

 radiatre. In all these, but especially in the Star-fish and 

 Sea-urchin, the parts are divided and formed by the number 

 five in a most remarkable manner, and few things would 

 afford a pleasanter study than one of these sea-urchins, 

 easily procured in every fish-market in London, or at the 

 sea-side. The animal is easily killed in cold fresh water, 

 and then the spines may be examined, with their curious 

 ball and socket joint, so firmly fixed, yet so easily bending 

 on every side at the will of the Echinus, who uses them, 

 not only for defence, but for burrowing in the sand. 

 Between the spines are multitudes of minute organs, the 

 uses of vvhich are as yet unknown, called pedecilariae ; they 

 are of three kinds, pearly white with dotted and toothed 

 beaks, and move about when the Echinus is alive, opening 

 and shutting their trifid beaks as if each had an independent 

 life. 



They are beautiful objects mounted in balsam, and viewed 

 with a low power. When the spines have been examined 

 they are easily removed by dipping the shell into boiling 

 water and brushing them off; then fresh beauty appears in 

 the tesselated wall of that wonderful house built up by the 

 Almighty for the Sea-urchin according to a certain plan, 

 and with such contrivance for its comfort as it is worth while 

 to examine quietly. First we notice double rows of very 

 minute holes, dividing the shell into five divisions ; through 

 each hole a small sucker protruded by which it walked, or 

 attached itself to rocks or stones; 1,860 of these suckers 

 occupying each two of these pores. The plates between 

 each double row of pores are studded with the balls which 

 fitted into the socket of each spine ; these fine plates are 



