100 



Bacillariede 



is much evidence to show that movements do take place when the active 

 diatom is only in contact with the surrounding water. The power of loco- 

 motion is particularly manifested in those diatoms of a naviculoid form, and 

 various explanations have at different times been put forward to account for 

 it. The following historical survey, although brief and rather imperfect, will 

 give some idea of the suggestions which have been made concerning this 

 interesting biological problem. 



Fig. 73. 



A, Asterionella formosa Hass. B, Tabellaria fenestrata (Lyngb.) Kiitz. 

 var. asterionelloides Grun. 



Ehrenberg (1838) imagined the movement to be due to the protrusion of 

 cilia or of a pseudopodium through the raphe of the valve, whereas Nageli 

 (1849) attributed it to the passage of osmotic currents through the cell-wall. 

 Max Schultze (1865), who observed the movements of minute foreign particles 

 down the length of the raphe, attributed the locomotion to the contractility 

 of a small portion of the protoplasm which was protruded through the raphe. 

 Hallier (1880) considered it to be due to a contractile layer of protoplasm, 



