298 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



be observed to invest the frustule d, from wbieh tbe frustule e, 

 being the terminal one, has more completely freed itself. 



187. It is by the, persistence of the connecting membrane, not- 

 withstanding the dissolution of the adhesion between the sur- 

 faces of the valves, that the frustules are so held together after 

 self-division as to form zigzag chains in the next group; of 

 which the genus Diatoma (Fig. 94), that gives the name ("cut- 

 ting through") to the whole order, is a typical example. Its 



Fio. 96. 



Fig. 94. 



Fig. 95. 



Fig. 94. Diaioma vulgare : — a, side view of frustule; 6, frustule undergoing self-diYisioii. 



Fig. 95. GrammcUophffra serpentina: — a, front and side views of single frustule; &,&, front and end 

 views of divided frustule ; c, a frustule about to undergo self-division ; d, a frustule completely 

 divided. 



Fig. 96, Isthmia nervosa, 



valves, when turned sideways, are seen to be strongly marked 

 by transverse strise, which extend into the front view. The 

 proportion between the length and the breadth of each valve is 

 found to vary so considerably, that, if the extreme forms only were 

 compared, there would seem adequate ground for regarding them 

 as belonging to dift'erent species. This genus inhabits fresh 

 water, preferring gently running streams, in which it is some- 

 times very abundant. The genus G-rammatophora (Pig. 95) is 

 nearly allied to the preceding ; but its transverse strise are ex- 

 tremely faint and difficult of detection, so that its valves serve as 

 "test-objects" (§ 102, III) ; and the frustules, when seen in front 

 view, are marked by peculiar bands, usually sinuous, which are 

 termed vittce. The curious Biddulphia, whose self-division has 

 already been described (§ 176), belongs to this group ; its frus- 

 tules have a very peculiar external form (Fig. 81), and they are 

 believed to be divided internally by partitions which correspond 



