86 



Bacillariede 



perforated and approximately parallel to the valve-faces. The number of 

 longitudinal septa depends upon the number of intercalary bands. In 

 Mastogloia each cell has two longitudinal septa, in Tabellaria there are 

 from two to twelve or more, and in Tetracydus and Rkabdonema there 

 may be as many as twenty. These septa are generally plane, although they 

 may be undulated, as in Qrammatophora. Karsten has distinguished those 

 intercalary bands which possess longitudinal septa as intercalary valves 

 (fig. 58 C and D). The perforations are large but variable. Sometimes 

 one large window-like foramen is situated in the median part of the septum, 



Fig. 59. A and B, Tetracydus lacustris Balfs ; B is a longitudinal section of cell showing one 

 of the partial septa. C, longitudinal section of cell of Licmophora Lyngbyei (Kiitz.) Grun, 

 showing partial septum. D and E, Tabellaria fenestrata (Lyngb.) Kiitz. ; E is a longitudinal 

 section of cell showing the extent of the septa. F, Climacosphenia moniligera Ehrenb., 

 section of upper portion of cell to show the perforated septum, s, septum. C and F are 

 after 0. Miiller. 



and at other times the septum is only partial, the foramen occupying most 

 of the median part and the whole of one end (fig. 59 A E). In Clima- 

 cosphenia each septum has a series of numerous perforations (fig. 59 F). 

 In Denticula the partial longitudinal septa are fused with numerous im- 

 perfect transverse septa which pass inwards from the valves. 



Thus the interior of the diatom-cell may become chambered in various 

 ways, and as will be noted subsequently these chambers very largely 

 accommodate lobes of the chromatophores. 



SYMMETRY OF THE DIATOM-CELL. The diatom-cell may be perfectly 

 symmetrical or it may be completely asymmetrical, and the various degrees 



