84 



BacillariesB 



largest of which are exactly opposite each other and are called the valves. 

 In most cases the characteristic sculpture of the wall is confined to the 

 valves, each of which forms one entire face of the diatom. No matter 

 what the external shape of the valve, its edges are always bent at right 

 angles to the general plane of the valve-face, and it thus possesses a flange 

 like that 011 the lid of a box. Closely joined to the flange of each valve 

 is a connecting band, so that the wall of the diatom consists of two halves, 

 each half being constituted by a valve and its corresponding connecting 

 band. Owing to the method of cell-division one half is older than the other, 

 and as the older valve is very slightly larger than the younger valve the 

 connecting band of the older half fits over that of the younger like the lid of 

 a cardboard box. The valve of the older half is known as the epivalve ; that 

 of the younger half as the hypovalve. (Consult figs. 57 and 65 5.) 



The two connecting bands, which in many cases are not very firmly 

 united with their respective valves, together form the girdle. The 

 connecting bands are not closed hoops, but as shown by Palmer & 



Keeley ('00) and others each is a two- 

 ended strip with the ends overlapping ; 

 and although in the majority of di- 

 atoms this is not apparent by mere 

 inspection it can readily be seen on 

 boiling rather fiercely large diatoms, 

 such as Surirella nobilis,Navicula major, 

 etc., in fuming nitric acid, as under 

 such circumstances the connecting 

 bands become loose and their free ends 

 can be easily observed. 



The aspect in which the girdle of a 

 diatom is exposed to view is known 

 as the girdle view, and that in which 

 the face of the valve is exposed to view 

 as the valve view. (Fig. 57.) 



In the average diatom just discussed 

 the cell-wall consists of four segments, 

 but sometimes the girdle is more com- 

 plicated in structure owing to the 

 interpolation of intercalary bands be- 

 tween the valves and connecting bands 

 (fig. 58A,B,E and F). One, two, or 

 more of these intercalary bands may 

 be present, the cell being correspond- 

 ingly widened when seen in girdle view. The greatest number of such 



-ep. 



- -r 



j-c.n 





B 



Fig. 57. Navicula ( Pinnularia) riridin 

 Kiitz. A, girdle view; B, valve view. 

 cp., epivalve; hi/., hypovalve; c., costas; 

 c.n., central nodule; t.n., terminal no- 

 dule; r., raphe. x 800 (after Pfitzer). 



