274 THE MICROSCOPE AND ITS REVELATIONS. 



usually designated as frustules) are sometimes found adherent side by- 

 side (as at Z>) so as to form filaments, but are more commonly met with 

 in a state of partial separation, remaining connected at their angles only 

 (usually the alternate angles of the contiguous frustules) so as to form a. 

 zigzag chain. A similar cohesion at the angles is seen in the allied genus 

 Grammatophora (Fig. 174), in Isthmia (Fig. 181), and in many other 

 Diatoms ; in Biddulphia (Fig. 167) there even seems to be a special 

 organ of attachment at these points. In some Diatoms, however, the 

 frustules produced by successive acts of binary subdivision habitually 

 remain coherent one to another, and thus are produced filaments or 

 clusters of various shapes. Thus it is obvious that when each frustule is 

 a short cylinder, an aggregation of such cylinders, end to end, must form 

 a rounded filament, as in Melosira (Figs. 177, 178); and whatever may 

 be the form of the sides of the frustules, if they be parallel one to the- 

 other, a straight filament will be produced, as in Achnanthes (Fig. 185). 

 But if, instead of being parallel, the sides be somewhat inclined towards, 

 each other, a curved band will be the result; this may not continue 

 entire, but may so divide itself as to form fan -shaped expansions, as 

 those of Lichmophora flabellata (Fig. 172); or the cohesion may be- 

 sufficient to occasion the band to wind itself (as it were) round a central 

 axis, and thus to form, not merely a complete circle, but a spiral of 

 several turns, as in Meridian circular e (Fig. 170). Many Diatoms, 

 again, possess a stipes, or stalk-like appendage, by which aggregations of 

 frustules are attached to other plants, or to stones, pieces of wood, etc. ; 

 and this may be a simple foot-like appendage, as in Achnanthes longipes 

 (Fig. 185), or it may be a composite plant-like structure, as in Lichmo- 

 phora (Fig. 172), Gomphonema (Fig. 186), and Mastogloia (Fig. 189). 

 Little is known respecting the nature of this stipes; it is, however, quite 

 flexible, and may be conceived to be an extension of the cellulose coat 

 unconsolidated by silex, analogous to the prolongations which have been 

 seen in the Desmidiacece ( 261), and to the filaments which sometimes 

 connect the cells of the Palmellacem ( 243). Some Diatoms, again, 

 have a mucous or gelatinous investment, which may even be so substan- 

 tial that their frustules lie as it were in a bed of it, as in Mastogloia 

 (Figs. 189 B, 190), or may form a sort of tubular sheath to them, as in 

 Schizonema (Fig. 188). In a large proportion of the group, however, 

 the frustules are always met with entirely free; neither remaining in the 

 least degree coherent one to another after the process of binary subdi- 

 vision has once been completed, nor being in any way connected either 

 by a stipes or by a gelatinous investment. This is the case, for example, 

 with Triceratium (Fig. 164), Pleurosigma (Fig. 165), Actinocyclus (Fig. 

 191, #, #), Actinoptychus (Fig. 180), Arachnoidiscus (Plate XL), Cam- 

 pylodiscus (Fig. 176), Suriretta (Fig. 175), Coscinodiscus (Fig. 191, a, a 

 a), Heliopelta (Plate I., Fig. 3), and many others. The solitary discoidal 

 forms, however, when obtained in their living state, are commonly found 

 cohering to the surface of Seaweeds. 



274. We have now to examine more minutely into the curious struc- 

 ture of the silicified casing which incloses every Diatom-cell or ' frus- 

 tule,' and the presence of which imparts a peculiar interest to the group; 

 not merely on account of the elaborately-marked pattern which it often 

 exhibits (Fig. 277), but also through the perpetuation of the minutest 

 details of that pattern in the specimens obtained from fossilized deposits 

 (Fig. 299). This casing consists of two valves or plates, usually of the 

 most perfect symmetry, closely applied to each other, like the two valves 



