SEA-URCHINS AND SEA-CUCUMBERS. 341 



tlie Urchin is built up, but minute and isolated in the 

 flesh instead of being united into one or more masses 

 of delinite 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 permanent 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 over- 

 whelming majority are of one form, subject to slight 

 modifications, in shape and size. 



N^eglecting, then, the irregular pieces, we perceive 

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

 by the repetition of a single element. That element is 

 a piece of clear glassy material, highly refractive, of 

 the shape of a dumb-bell — two globes united by a 

 thick, short column. The oval is constructed thus : — 

 suppose two dumb-bells to be placed in contact, side 

 by side, and soldered together, there would be of course 

 an oval aperture between their columns. Then two 

 others dumb-bells are united to these in a similar man- 

 ner, but one on each side, so that the globes of each 

 shall rest in the valley between the former globes now 

 united. Tliese then are soldered fast in like manner . 



