Bacillariece 



263 



Fig. 124. Transverse section of the 

 frustule of a Navicula. (From Van 

 Heurck, after W. Prinz.) cb and cb', 

 the two connecting-bands forming the 

 girdle; r, raphe; en, central nodule; 

 c, costa of valve. (Very highly magni- 

 fied.) 



Some Diatoms possess a pseudo-raphe, which is a simple line, 

 central or sub-central, and without a central nodule. Often the 

 raphe is conspicuous in both valves, but sometimes it is only 

 present in one valve (Achnan- 

 tties, Cocconeis, Rhoicosphenia). 

 In many Diatoms the raphe is 

 obscure and marginal or sub- 

 marginal. In some species, such 

 as Navicula nobil is Ehrenb., there 

 is a smaller accessory raphe which 

 lies alongside the primary one, 

 and is united to it close to the 

 central and terminal nodules. 

 In Vanheurckia there is a sili- 

 ceous rib on each side of the 

 raphe and nodules, and in Am- 

 phipleura the central nodule is 

 greatly elongated. 



Some Diatoms (Fragilari- 

 oideas, Mastogloia) possess longi- 

 tudinal septa which are always more or less considerably perforated. 

 These septa are parallel to the valve-faces and are usually situated 

 between the girdle and the valves. 



Each individual Diatom contains a more or less centrally dis- 

 posed nucleus, generally with a nucleolus. A very weak solution 

 of methylene blue will bring out the nucleus in living Diatoms, 

 staining it clearly before the rest of the protoplasm. The proto- 

 plasmic mass surrounding the nucleus is connected with the 

 primordial utricle either by two broad bands of protoplasm or by 

 a number of radiating or anastomosing threads. One or several 

 large vacnoles occupy the central portions of the cell. 



The chromatopkores of Diatoms are somewhat variable. One 

 or many may be present in each cell ; they may be small and 

 discoidal, large and plate-like, or extensive anastomosing masses 

 occupying a large part of the lining protoplasm. In many species 

 they are extremely irregular in form, being band-like, lobed, or 

 even presenting the appearance of perforated plates. They are of 

 a golden-yellow or brown colour, or very rarely green, as in some 

 forms of Navicula viridis Kvitz. and N. cuspidata Kiitz. They 

 contain chlorophyll, but this is masked by the presence of a brown 



