FRAGILLARIA ACHNANTHES. 



297 



FIG. 93. 



" are literally covered in the first warm days of spring with a 

 ferruginous colored mucous matter, about a quarter of an inch 

 thick, which on examination by the microscope, proves to be 

 filled with millions and millions of these exquisitely beautiful 

 siliceous bodies. Every submerged stone, twig and spear of 

 grass is enveloped by them ; and the waving plume-like appear- 

 ance of a filamentous body covered in this way is often very ele- 

 gant." The genus Bacillaria, so named from the staff-like form 

 of its frustules, is now limited to the species B. paradoxa (Fig. 

 92, B), whose remarkable movements have been already de- 

 scribed ( 179). Owing to this displacement of the frustules, its 

 filaments seldom present themselves with straight parallel sides, 

 but nearly always in forms more or less oblique, such as those 

 represented above. This curious object is an inhabitant of salt 

 or of brackish water. Many of the species formerly ranked under 

 this genus are now referred to the genus Diatoma ( 187) ; to 

 which also the genus Fragillaria is nearly allied, the difference 

 between them lying chiefly in the mode of adhe- 

 sion of the frustules. These in Fragillaria form 

 long straight filaments with parallel sides ; the 

 filaments, however, as the name of the genus im- 

 plies, very readily break up into their component 

 frustules, often separating at the slightest touch. 

 Its various species are very common in pools 

 and ditches. Among the numerous genera be- 

 longing to this group, we may stop to notice 

 Achnanthes; some of the species of which, par- 

 ticularly A. longipes (Fig. 93), are furnished 

 with a single nearly straight stipes, attached to 

 one end of the lower margin of the first frustule 

 of the filament. There is a curious difference 

 in the markings of the valves of the upper and 

 lower frustules ; for while both are traversed by 

 strife, which are resolvable under a sufficient 

 power into rows of dots, as well as by a longitu- 

 dinal line, which sometimes has a nodule at 

 each end (as in Navicula), the lower valve (), 

 has also a transverse line, forming a stauros or 

 cross, which is wanting in the upper valve (e). 

 A persistence of the connecting membrane may 

 sometimes be observed in this genus, so as to 

 form an additional connection between the 

 cells; thus, in Fig. 93, it not only holds to- 

 gether the two new frustules resulting from the 

 subdivision of the lowest cell, a, which are not 

 yet completely separated the one from the other, 

 but it may be observed to invest the two frus- 

 tules b and <?, which have not merely separated, but are them- 

 selves beginning to undergo binary subdivision ; and it may also 



Achnanthes lanffipes : 

 a, 6, c, d, e t successive 

 frustules in different 



