122 THE DIATOMACE.E. [Jime, 



loped, sometimes the latter, — presenting the greater diversity of 

 form^ structure, and markings, and affording the more essential 

 characters for the genera and groups. 



The external sm'face of this flinty structui'e is, almost without 

 exception, smooth and even : internally, however, glandular thick- 

 enings of the most varied kind are fomid projecting. The par- 

 ticles of silica which compose these, lie less compactly than those 

 on the perfectly smooth planes, — presenting umbilical nodes of 

 definite form (globular, oblong, cuneate, or funnel-shaped) . Some 

 of them appear as little dots, arranged regularly, and extending 

 in the direction of the length or breadth of the Diatom : others, 

 again, present opaque lines, traversing the same directions, and, 

 according to their width, designated ridges, ribs, stripes, or 

 lines, — these also being often bordered with dots. 



Thus the Diatomacece are so characterized by theii" form and 

 structm-e, that at first glance they may be distinguished from 

 every other organism. Proceeding inwards from this siliceous 

 exterior, we come next upon a delicate membrane — the cell pro- 

 per — by which the flinty skeleton is produced. This membrane 

 encloses the contents of the cell, which consists of a slimy sub- 

 stance, and of a peculiar brown or golden-yellow colouring- 

 matter, quite distinct from the chlorophyll of plants. Chloro- 

 phyll is soluble in alcohol, and in contact with alkalies dissolves, 

 forming a yelloArish-green substance; whereas the colouring- 

 matter of the Diatomacece is insoluble in alcohol (though bleached 

 by it after some time), and remains unacted upon by alkaUes. 

 While the Diatom is yet young, this coloiuing-m after is equally 

 distributed ; but in a short time it undergoes various changes, in 

 common with the interior of the cell generally. It assumes the 

 most diversified forms, — these mostly symmetrical, and often 

 extremely elegant. Granular bodies are at the same time pro- 

 duced, which circulate with a spii'al motion in the interior of the 

 cell, — a phenomenon resembhng that which takes place in the 

 ceUs of some AlgcB [Spirogyra) . 



The Diatom thus formed, is found to be enveloped in a kind of 

 mantle, corresponding to the cuticle of the higher plants. Being 

 of a gelatinous consistence, it is now apparently absent, and 

 again very evident. Sometimes it forms the medium of attach- 

 ment between numbers of Diatoms, which appear as groups or 

 masses, in seeming disorder, or in regular roAvs. Occasionally 



