TIIK NATURK OF DIATOM MOTION. II9 



another will be seen making evident attempts to revolve 

 itself through an angle of 90° in a direction at right angles to 

 the longer axis. These attempts are intermittent, evidently of 

 an energetic nature, and in the end generally successful. The 

 cryptic mechanism is plainly situated along the raphe. Once 

 the diatom has balanced itself upon its valve, with this raphe 

 in contact with the slide, it proceeds to crawl, but not until 

 this has been aeeomplishcd /'^ The manifest struggle undergone 

 in order to attain this position shows both the necessary rela- 

 tion between the raphe and a solid surface, and the unprece- 

 dented nature, in the diatom's experience, of a surface of 

 such glassy smoothness as is presented by a microscopical 

 slide. In the native pool, as a matter of course, the endless 

 inequalities of the bottom always present themselves invit- 

 ingly. Under the microscope, also, any chance filament, or 

 large grain of sand, or collection of solid debris, is utilized 

 when within reach of the raphe. 



Several familiar motile diatoms, such as Sitriniia, most 

 species of lYitzschia, etc., have no occasion for any such pre- 

 liminary struggle, for the very excellent reason that it is prac- 

 tically impossible for them to rest upon a solid surface without 

 some part of their raphes being in contact therewith. In the 

 case of Campylodiscus, also, there is no conceivable position 

 of stable equilibrium in which they would be helpless. On 

 the other hand, certain species of Navicida avoid the neces- 

 sity of such struggle by going about in groups of four joined 

 socially girdle to girdle in the bond of a common coleoderm ; 

 and in the world-famed Baeilla}-ia paradoxa we have the per- 

 fection of coordinated activity, with a correspondingly perfect 

 ability in dealing with conditions untoward for locomotion.! 



*In the paper already cited, the writer was inclined, on the strength of 

 observations pre\'iously on record, to agree with the view that the Navicnlai 

 swim. Further study at first hand has shown this notion to be untenable. 



t The relation between shape and structure of the frustule and nor- 

 mal habitat and habit is a most enticing branch of diatom ])hilosophv 

 still laryelv unexnloited. 



