MICROSCOPIC FORMS OF VEGETABLE LIFE. 



28T 



sideways (a), are seen to be strongly marked by transverse striae, 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 ex- 

 treme forms only were compared, there would seem adequate ground for 

 regarding them as belonging to different species. The genus inhabits 

 fresh water, preferring gently-running streams, in which it is sometimes 

 very abundant. The genus Fragillaria is nearly allied to Diatoma, the 

 difference between them consisting chiefly in the mode of adhesion of 

 the f rustules, which in Fragillaria form long straight filaments with paral- 

 lel sides: the filaments, however, as the name of the genus implies, very 

 readily break-up into their component frustules, often separating at the 

 slightest touch. Its various species are very common in pools and 

 ditches. This family is connected with the next by the genus Nitzscliia, 

 which is a somewhat aberrant form distinguished by the presence of a 

 prominent keel on each valve, dividing it into two portions which are 

 usually unequal, while the entire valve is sometimes curved, in N. sig- 

 moidea, which is sometimes used as a Test-object, but is not suitable for 

 that purpose on account of the extreme variability of its striation. 

 Nearly allied to this is the genus Bacillaria, so named from the elongated 

 staff-like form of its frustules; its valves have a longitudinal punctated 

 keel, and their transverse stria3 

 are interrupted in the median 

 line. The principal species of 

 this genus is the B. paradoxa, 

 whose remarkable movement 

 has been already described ( 

 282). Owing to this displace- 

 ment of the frustules, its fila- 

 ments seldom present them- 

 selves with straight parallel 

 sides, but nearly always in 

 forms more or less oblique, 

 such as those represented in 

 Fig. 171. 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. The 



Genera Nitzschia and Bacillaria are now associated by Mr. Ealfs/ with 

 some other genera which agree with them in the bacillar or staff -like form 

 of the frustules and in the presence of a longitudinal keel, in the Sub-fam- 

 ily NitzschiecB, which ranks as a section of the Surirellece. Another Sub- 

 family, SynedrecB, consists of the genus Synedra and its allies, in which 

 the bacillar form is retained (Fig. 192, I), but the keel is wanting, and 

 the valves are but little broader than the front of the frustule. 



285. In the SurirellecB proper, the frustules are no longer bacillar, 

 and the breadth of the valves is usually (though not always) greater than 

 the front view. The distinctive character of the genus Surirella, in ad- 

 dition to the presence of the supposed 'canaliculi' ( 275), is derived 

 from the longitudinal line down the centre of each valve (A), and the 



1 See Pritchard's " Infusoria," 4th Ed. p. 940. The genus Nitzschia was in the 

 first instance placed by Mr. Ralfs in the family Fragillariece, and the genus 

 Bacillaria in the family Surirellece. 



Surirella constricta : A, side view ; B, front view ; c, 

 binary subdivision. 



