266 Bacillariece 



and suggested that the absence of silica along this line could be 

 accounted for by the obstruction of the moving cilia. Biitschli (1892) 

 also imagined that the presence of a cilium or a fine flagellum 

 would explain the phenomenon, but no methods of staining have 

 ever demonstrated the existence of such structures. 



The movements of some of the larger species of Navicula 

 (Pinnularia) have been explained by Biitschli (1892) 1 and by 

 Lauterborn (1894) 2 to be due to the production of a delicate 

 filament which is protruded from the raphe at a point opposite 

 the central nodule. The frustules of Navicula major Kiitz., 

 N. nobilis Ehrenb., and N. viridis Ktitz. are surrounded by a 

 distinct mucilaginous envelope, and the protruded filament is 

 quite colourless and transparent, resisting all attempts to stain 

 it with aniline dyes. It lies alongside the raphe, but not in 

 contact with it, and it elongates by a series of jerks. Btitschli 

 puts this forward as the explanation of the jerky movement of 

 Diatoms, the frustule being pushed backward by the elongation of 

 the filament, the distal end of which is fixed to the substratum. 



In 1893 0. Miiller 3 again emphasized his previous explanation 

 of the movements of Diatoms, affirming that they were dependent 

 on the forces connected with the protoplasmic currents on the 

 surface of the valve, and he denied that the movements could be 

 the result of the filaments described by Biitschli and Lauterborn. 

 He likewise stated that these filaments were composed of proto- 

 plasm, and not of mucilage. Lauterborn (1894) 4 contested that 

 the production of motility by the streaming of protoplasmic 

 currents, as suggested by Miiller, would be an isolated phenomenon 

 in either the vegetable or animal kingdom, whereas movements 

 are known to occur in the Desmidiaceae and Oscillatoriaceae as a 

 result of the excretion of mucilage, and, according to Schewiakoff, 

 in the creeping Gregarinidse also. Miiller (1894) 5 replied again to 

 the criticism of his hypothesis, and stated that the analogy which 

 had been drawn between the movements of Diatoms and of 

 Desmids was a false one. 



There is no doubt that in many of the smaller motile species 

 there is a complete absence of gelatinous filaments such as those 



1 Biitschli in Abhandl. naturh.-med. Ver. Heidelberg, 1892, Bd iv, Heft 5. 



2 Lauterborn in Berichte Deutsch. Bot. Gesellsch. Bd xii, 1894, p. 73. 



3 O. Miiller in Berichte Deutsch. Bot. Gesellsch. Bd xi, 1893. 



4 Lauterborn, torn. cit. xii, 1894. 



5 0. Miiller, torn. cit. xii, 1894. 



