SECTION OP ECHINUS SPINE. 



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 

 Holothuriae, the Medusae, or Jelly-fish, the Intozoa, 

 Polypes, and Infusoria, in all of which the external or 

 internal parts radiate like a star, and which are therefore 

 called the radiatse. 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 which are as yet unknown, called 

 pedecilarise ; they are of three kinds, pearly white with 

 dotted and toothed beaks, and move about when the 

 Echinus is alive, opening and shutting their tritid 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 ; 

 1860 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 ; 



