THALLOPHYTA. 625 



plants in the coal measures. It is thought that Bacillus ATnylobacter has been 

 identified in a silicified state. 



Of living Bacteria a very large number have been distinguished. 



Class II.— DINOFLAGELLATA, Peridinere. 

 Alliance IV. 



This compact group of unicellular organisms is, as has been said above, a 

 branch of the Flagellate Protozoa. They have therefore no very near affinities with 

 other plant groups, though the nutrition of many is thoroughly plant-like. They 

 would come nearest to the motile (Flagellate) forms of Protococcoideee (see p. 628). 



The great characteristic of the group is the possession of two flagella, one directed 

 longitudinally and attached to the anterior end of the body, the other transversely 

 directed and often situated in a circular transverse groove. There is sometimes also 

 a longitudinal furrow. It is this transverse flagellum which is specially concerned 

 in movement. 



There is often a cell membrane of cellulose, and the cell possesses green or 

 brownish chromatophores containing chlorophyll and a single large nucleus. 



Reproduction is effected by binary fission, usually during a resting stage of the 

 cell. 



Most of the forms are marine, and some are the cause of sea-phosphorescence. 



The group is divided into two sections — the Adinida without, and the Dinifera 

 with a transverse furrow. 



Ceratium and Peridinium are two well-known genera. 



Class III.— BACILLAEI ALES. 



Alliance V. 



Family: DiatomaceoB, Diatoms. 



These are a large group of unicellular plants which grow both in fresh and salt 



water and upon moist soil. As a rule they occur together in large numbers. The 



protoplasm is coloured brown by a brown pigment, diatomm, which masks the 



chlorophyll which is also present. The colouring matter is restricted to special 



chromatophores, which may be few or numerous. The cell- wall is incrusted with 



silica, and is a very characteristic feature of the Diatom. The wall consists of two 



halves or valves (frustules) which fit into one another like the lid on to a pill-box. 



These valves are smooth or variously sculptured, dotted, ribbed, &c., and enjoy a 



wide popularity as microscopic objects on account of the beauty and delicacy of 



their tracery. Some idea of the variety and form of Diatom-cells may be obtained 



from the accompanying figure 369. In the colonial forms the cells are attached 



to the substratum directly (fig. 369 1) or by means of branching filaments (fig. 369 "). 



Others are attached to one another in zigzag chains or continuous ribbons (figs. 



369 15 and 869 ^% Others, again, are embedded in mucilage. Many of the forms 

 Vol. II. 90 



