Class 5. BACILLARIE^. 



THIS class of Algse includes a large number of minute plants 

 known as the Diatoms. They are perhaps better known under 

 the name of the Diatomacese, but the earlier name ' Bacillarieae ' 

 has been in use for many years among systematists, particularly 

 in continental Europe, and the name ' Diatomacese ' is here retained 

 for one family only. The class is a very large one, with well- 

 marked characters, and includes about 10,000 species. As would 

 be expected among such a large number of species there is great 

 variability of form, but at the same time the Diatoms always 

 possess those salient features which mark them off from all other 

 Algse. 



They are universally distributed in both fresh and salt water, 

 and as the wonderful sculpture of their cell-walls renders them 

 objects of great beauty, they have long been made the subject of 

 special study by numerous students of natural history. 



Diatoms are unicellular plants, mostly of minute size, the cell- 

 walls of which are composed of an organic matrix impregnated 

 with silica. The silica can be removed by the action of hydro- 

 fluoric acid, leaving the organic matrix behind ; or the organic 

 matrix, which is allied in composition to cellulose, can be. removed 

 by calcination leaving behind the siliceous constituent. 



Each individual Diatom is termed a frustule, and the cell-wall 

 consists of two more or less equal valves, joined together by two 

 connecting -bands which overlap. Each half of the Diat* i is thus 

 composed of two pieces, a valve and a connecting-band, and the 

 connecting-band of the older half fits over that of the younger 

 half like the lid of a cardboard-box. The connecting-bands, 

 although closely fitted to their respective valves, are distinct from 

 them, and the two bands together form what is termed the f/irdle. 

 The latter does not usually consist of two closed hoops, but as 



