SEA-URCHINS AXD SEA-CUCUMBERS. 297 



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 mag- 

 nifying power it is seen to consist of numberless cal- 

 careous 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. 



Neglecting, 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 other dumb-bells are united to these 

 in a similar manner, but one on each side, so that the 

 globes of each shall rest in the valley between the former 

 globes now united. These then are soldered fast in like 

 manner ; and the result is that there are three oval aper- 

 tures. The next step is that on the top of the four united 

 globes two other dumb-bells stand erect, and lean over 

 towards each other till their upper globes come into con- 

 tact, their lower ones remaining remote; these are soldered 

 to the mass and to each other at the points of contact, 

 .leaving a fourth aperture. The same is repeated at the 



