Bacillariece 205 



other times jerky, and is generally along a fixed substance with 

 which the Diatom is in contact. It is not at all comparable to 

 the free-swimming movements of many other Algte, arid, as a rule, 

 only takes place when one valve-face of the Diatom is in contact 

 with the fixed object. 



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) 1 , 

 who observed the movements of minute foreign particles down 

 the length of the raphe, attributed the locomotion to the contrac- 

 tility 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, and Onderdonk (1885) also 

 regarded it as due to an external movement of protoplasm, but 

 Mereschkowsky (1880) 2 concluded that the evidence was in favour 

 of Nageli's theory of osmotic currents through the cell-wall. 

 0. Miiller (1889) 3 demonstrated the presence of a large number of 

 minute pores and anastomosing fissures in the valves of certain of 

 the large species of Navicula (Pinnutaria), and showed that the 

 central and terminal nodules are traversed by straight and curved 

 canaliculi which run towards the raphe and are eventually merged 

 with it. Owing to intracellular pressure the protoplasm emerges 

 from the pores of the central or terminal nodules and passes 

 down the whole length of the raphe, returning into the cell-cavity 

 through the pores of the terminal or central nodules as the case 

 may be. There are thus two sets of currents on each valve of the 

 Diatom. The cause of the movement of the frustules was ascribed 

 by Mtiller to the reaction of the motive forces of this living stream 

 of protoplasm upon the surrounding water. Schiberszky (1891 ) 4 , 

 from observations on Synedra, agreed with Pfitzer that the move- 

 ment was due to a coating of protoplasm which escapes from the 

 raphe, and which is in a condition of vibratile motion. He believed 

 that the currents along the raphe were usually interrupted jerking 

 or pulsating movements. 



Cox (1890) 5 revived the idea of a line of cilia along the raphe, 



1 Max Schultze in Archiv fiir Mikr. Anat. Bd i, 1865, p. 376. 



2 Mereschkowsky iu Bot. Zeitung, 1880, p. 529. 



3 O. Miiller in Berichte Deutsch. Bot. Gesellsch. Bd vii, 1889. 



4 Schilberszky in Hedwigia, xxx, 1891. 



5 J. D. Cox in The Microscope, July, 1890. 



