328 EVENINGS AT THE MICROSCOPE. 



the middle, where the blade-bases meet each other. 

 They fit accurately, and each has a deep oblong cavity 

 in its bottom, which does not, as I conceive, commmii- 

 cate with the interior. 



By selecting one of these heads, which has been 

 divested of its fleshy parts by immersion in caustic 

 potash, and then well cleansed by soaking in clean 

 water, and placing it under a low power of the micro- 

 scope — 100 diameters, for example — with a dark ground, 

 and the light of the lamp cast strongly upon it by 

 means of the Lieberkuhn, or the side-condenser, we 

 shall have an object of most exquisite beauty. The 

 material has all the transparency and sparkling bril- 

 liance of flint-glass, while the elegantly-shaped pins, 

 the perfect symmetry of the prismatic bases, the arch 

 which is lightly thrown across their cavity, the minute 

 teeth of the tips locking accurately into each other, 

 and the oval cavities in the whole structure set in reg- 

 ular rows, and reflecting the light from thousands of 

 points, constitute a spectacle which cannot fail to elicit 

 your admiration. 



P. globifera is formed on the same model as P. 

 l/riphylla^vA. is more globose, and each piece appears 

 to have a deep cleft at the point, which does not ex- 

 tend to the interior side, where a thick ridge runs 

 down from the point to the base. At the summit of 

 this ridge, in each of the three divisions, there is set 

 a strong acute spine, directly horizontally inwards, so 

 that the three cross each other when the blades close, 

 which they do energetically — a formidable apparatus 

 of prehension ! The stem is much more slender than 

 in P. t/riphylla, and the height of the head of one of 

 average size is only y gd of an inch. It is peculiar also 



