68 



THE AMEKICAN MONTHLY 



[April, 



opposite sides of the frustule. These 

 masses moved along the sides, some- 

 times the whole length of the diatom, 

 sometimes only to the middle where 

 they would rest a while, and then 

 either complete the motion or go 

 back. They did not always move 

 simultaneously, nor with the same 

 soeed^ but, ■'.vitb a general agreement 

 of motion. This action was contin- 

 ued half an hour, the diatom not get- 

 ting free. 



Turning to another part of the 

 slide, I found another free moving 

 specimen with a similar gelatinous 

 mass in contact with it. The diatom 

 was moving freely and towing the 

 matter along with it, attached to its 

 hinder end. Soon the mass began to 

 move forward on the shell, the mo- 

 tion of the diatom ceased and was 

 presently reversed, the order of se- 

 quence being distinctly as stated. In 

 several instances the motion of the 

 gelatinous mass from the rear end 

 of the diatom forward, plainly pre- 

 ceded the change in the direction of 

 the frustule, as if the change of ciliary 

 motion (assuming that to be the mo- 

 tive power, for the sake of illustration,) 

 did not instantly stop the headway 

 of the diatom, but required an ap- 

 preciable moment of time to over- 

 come the momentum. My observa- 

 tion of this shell continued for a full 

 hour, the changes of direction being 

 frequent, and all the accidental modi- 

 fications and phases of the phenom- 

 ena were strikingly confirmatory of 

 the existence of some force applied 

 along the line of the raphe, acting 

 sometimes in one direction and some- 

 times in another, in such a way as 

 would be fully explained by suppos- 

 ing ciliary action along that line, but 

 which do not seem to be so easily ac- 

 counted for by osmotic action, cer- 

 tainly not by osmotic action at the 

 ends of the frustule. 



On one or two occasions the acting 

 force did not appear to be reversed 

 at the same instant at the two ends 

 of the diatom. Twice the foreign 

 matter moved against the current of 

 general motion, slowly, it is true, but 



really in such a way as to indicate 

 that the force acting upon it was not 

 in the same line of direction as was 

 that exerted on the other half of the 

 frustule. But when the motion control- 

 ling the gelatinous mass became 

 vigorous, it either became dominant 

 or was indicative of harmonious 

 action at both ends of the shell, so 

 that the motion of the diatom through 

 the water became very pronounced 

 and strong. 



I looked for similar phenomena 

 among the other kinds of diatoms in 

 the gathering, but saw nothing of the 

 sort except in the instances described. 

 The Naviculae were very lively, but I 

 saw no examples of action upon 

 foreign matter that came in their way. 

 Neither could I detect any current, 

 even along the Nitzschias; the motion 

 of the gelatinous substance occurring 

 only when it came in contact with the 

 shell and apparently sticking to it. 



My study of the diatom-shell has 

 led me to accept the opinion that the 

 raphe is a real fissure in the shell, but 

 in many species it is not a simple and 

 vertical linear-opening of the shell. 

 It is more like the joint formed by 

 the overlapping of the edges of 

 curved tiling on a roof : a thickened 

 line of silica borders one lateral half 

 of the shell, while the other half dips 

 under it with a thin film. It is true 

 that an osmotic force may be con- 

 ceived as working along the raphe, as 

 well as that a line of cilia should do 

 so ; but the difficulty is to account 

 for such action upon an extraneous 

 mass as that which I have described, 

 or to make osmosis from such a place 

 upon the shell move the diatom 

 in the direction of its length. The 

 assumed presence or absence of a 

 gelatinous film enveloping the diatom 

 does not materially vary the condi- 

 tions of the problem in either case. 

 If we assume that the osmotic action 

 is at the extremities of the shell, the 

 observed phenomena, as to the action 

 upon the gelatinous mass when in the 

 middle of the frustule, are unaccount- 

 ed for. 



As to the manner in which the lap- 



