Extei ? lal symmet ) // 



87 



of symmetry or asymmetry have been brought about by unequal growth of 

 either the valves or the girdle. This question of symmetry was regarded 

 as of primary importance from a classificatory point of view by both Heiberg 

 and Pfitzer, but unless taken in conjunction with other characteristics it is of 

 little real importance. 



O. Miiller ('95) has given an excellent synopsis of the different kinds 

 of external symmetry observed in various types of diatoms. Some are 

 zygomorphic in one plane only, some in three planes at right-angles, and 

 others exhibit a radial symmetry (fig. 60). In the majority of diatoms 



Fig. 60. Diagrams to illustrate the symmetry of the diatom-cell. A, Navicula inridis; B, 

 Gomphonema elegans; C, Amphora ovalis ; D, Eupodiscus Argus ; E, Isthmia enervis. A C 

 are peunate diatoms, D and E are centric diatoms, v, valve view ; 0, girdle view ; s, section 

 of cell ; pp, plane of section. (All the figures somewhat modified from 0. Miiller.) 



the sculptures on the valves are also arranged in relation to the external 

 symmetry, a fact of such importance from a classificatory standpoint that 

 it allows the division of diatoms into the two well-marked primary groups of 

 the Centricse and Pennata?. 



STRUCTURE OF THE CELL-WALL. The cell-wall of diatoms consists of 

 an organic ground substance which is more or less strongly silicified. The 

 girdle is usually relatively thin, but the valves vary much in strength and 

 thickness. In the great majority of species they are more or less sym- 

 metrically sculptured, such markings generally having the appearance of 



