256 



XX. STRUCTURE OF THE THALLOPHYTA. 



narrow furrows, running from the edges towards, but without 

 reaching, the middle (compare the figure). They are now usu- 

 ally considered to be chambers in the valve,' open towards the 

 interior. The central, smooth space, free from the furrows, 

 shows at its middle and each end, a strongly-refractive thicken- 

 ing, which we distinguish as a nodule. The two end nodules 

 are joined to the median nodule by a line, which bends out 

 symmetrically close on either side of the median nodule, and 

 ends in a slight enlargement. The end 

 nodules are surrounded by the ends of 

 the line in the form of a crescent, to 

 effect which the line bends out at both 

 ends laterally in the same direction as 

 at the median nodule. In its course be- 

 tween the nodules, the line broadens a 

 little. We assume that it is a cleft lead- 

 ing into the interior of the cell ; it is the 

 raphe, The furrows do not pass across 

 the girdle-side (B) ; we see them only 

 at the edges of the figure. By focussing 

 for the optical section, and careful ex- 

 amination of the ends of the cell, we can 

 demonstrate the remarkable fact that a 

 median strip of the wall is double. From 

 exhaustive investigation, it is determined 

 that there is here an overlapping, box- 

 wise, of separated parts of the wall. At the 

 edges of the two elliptic wall- segments, 

 which we saw in the view of the valve- 

 side, portions of membrane adjoin, which 

 end with free margins. The wall of this 

 cell, therefore, consists of two halves, of 

 which the one is inserted inside the other. The structure of this wall 

 corresponds throughout to an elliptic box with a cover placed upon 

 it. The side walls of the cover are just as high (deep) as those of 

 the box, but they are not completely slipped the one into the other. 

 If, in our cell, we pass from the optical section to the surface view, 

 we can follow the thin edges of the two halves of the cell 

 as delicate lines. The flat, furrowed surfaces of the cell-wall 

 we distinguished as valves, their smooth free-ending side- walls 

 as girdles ; whence the use of the terms in question to indicate 



FIG. 97. Pinnularia mri- 

 dis. A , view of the valve-side. 

 B, view of the girdle-side (x 

 540). 



