336 EVENINGS AT THE MICROSCOPE 



true you may cut open the animal and find nothing at all 

 more solid than the somewhat tough and leathery skin; 

 but a calcareous skeleton is there notwithstanding, though 

 in truth only a rudimentary one. If we were to cut off a 

 considerable fragment of the skin, and spread it out to 

 dry upon- a plate of glass, and then cover it with Canada 

 balsam, we should find assisted by the translucency which 

 is communicated to the tissues by the balsam that the 

 skin is filled with scattered atoms of the calcareous struct- 

 ure, perfectly agreeing with that with which the solid 

 framework of the Urchin is built up, but minute and 

 isolated in the flesh instead of being united into one or 

 more masses of definite organic form. 



But the atoms I speak of are still more perfectly seen 

 by dissolving the piece of skin in boiling potash, and 

 washing the sediment twice or thrice in pure water; this 

 may then be spread upon a glass slide, and covered with a 

 plate of thin glass, when it forms an interesting and per- 

 manent object for study. I have here a slide which is the 

 result of such treatment; to the naked eye it appears 

 sprinkled with the finest dust, but under magnifying 

 power it is seen to consist of numberless calcareous 

 bodies, of great beauty, and very free from extraneous 

 matter. 



The elegance of the forms is remarkable, and also their 

 uniformity; for though there do occur here and there 

 among them plates of no regular shape, perforated with 

 large or small roundish orifices, yet the overwhelming ma- 

 jority are of one form, subject to slight modifications, 

 in shape and size. 



Neglecting, then, the irregular pieces, we perceive that 

 the normal form is an oval of open work, built up by the 



