92 



Bacillariex 



termed the axial area and varies much in width. Less often, smooth areas 

 occur parallel to the axial area but nearer the margin of the valve ; these 

 are known as lateral areas. 



The raphe (or median line) may be straight, undulating, or sigmoid. 

 In its most highly developed state it is a fissure of a more or less complicated 

 character by which the protoplast is'placed in contact with the surrounding 

 medium. In the Naviculaceae the cleft of the raphe is not in a vertical 

 plane, but is always bent, and not infrequently V-shaped in cross-section. 

 (Consult fig. 65 5). Moreover, the cleft is not wholly open, being in many 



L 



Fig. 65. Navicula ( Pinnularia) viridis Kiitz. 2, diagram with the raphe of one valve almost 

 superimposed upon the other ; ekn, terminal nodule ; ckn, central nodule. 2, longitudinal 

 section through the central nodule showing the two canals (vk) joining the outer (a) and 

 inner (i) clefts of each half of the raphe ; schl, the canal which joins both halves of the inner 

 fissure. 3, terminal nodule showing ends of the two fissures of the raphe ; isp, inner fissure 

 terminating in the funnel-shaped body (tk) ; asp, outer fissure ending in the polar cleft (psp). 

 4, superimposed terminal nodules of epivalve and hypovalve. 5, transverse section across 

 the cell, showing the inner (isp) and outer (asp) fissures of the raphe (r), the nature of the 

 costae in the valves, and the disposition of the two chromatophores. (From Oltmanns ; 

 14, after 0. Miiller; 5, after Lauterborn.) 



forms closed in its middle region ; that is, along the bend (or in cross-section 

 about the point of the 'V'). Thus, in reality there are two cleft-like 

 fissures, one on the inner side and one on the outer side of each valve. 



The raphe is interrupted by the central nodule which is perforated 

 by two canals, each of which joins together the outer and inner fissures of 

 one half of the valve. There is also a -canal running along the inner side of 



