, INTRODUCTION. XXlll 



The movements of the Diatomacese appear rapid and viva- 

 cious under the microscope ; but it must be remembered that the 

 high powers usually employed in the observation of these mi- 

 nute organisms magnify theii' motions as well as their bulk. I 

 have noted the movements of several species with the aid of an 

 eye-piece micrometer and a seconds watch, and found that one 

 of the most rapid, viz. Bacillaria paradoxa, moved over -20 u^^ 

 of an inch in a second ; Pinnidaria radiosa, one of the slowest, 

 ov^i' 3 io o th of an inch in the same time ; and that the same 

 period was occupied by Pinnularia ohlonga in traversing Yooo^h 

 of an inch, Nitzsclda linearis 25^00 ^^ ^f '^^ inch, and JPleurosigma 

 strigosum -g^ooth of an inch. Or, expressing the spaces and 

 times by other units, we find that the most active required some- 

 what more than three minutes to accomplish movements whose 

 sum w^ould make one inch, and the slowest nearly an hour to 

 perform the same feat. 



These movements are usually noticed only in the free species 

 of the class, as in those belonging to the genera Navicula, Nitz- 

 schia, and others, and especially in the more minute or younger 

 individuals. Motion is not, however, confined to these, but may 

 at times be detected in other forms, and even the frustules of 

 attached species, as those of Gomphonema, when forcibly sepa- 

 rated from their stipes, occasionally exhibit an evident tendency 

 to change their position. The movements in the latter are, how- 

 ever, exceedingly languid, and have nothing of the isochronism 

 so notable in the others. 



Of the cause of these movements I fear I can give but a very 

 imperfect account. It appears certain that they do not arise 

 from any external organs of motion. The more accurate instru- 

 ments now in the hands of the observer have enabled him con- 

 fidently to affirm, that all statements resting upon the reve- 

 lations of more imperfect object-glasses, which have assigned 

 motile cilia, or feet, to the Diatomaceous frustule, have been 

 founded upon illusion and mistake. Among the hundreds of 

 species which I have examined in every stage of growth and 

 phase of movement, aided by glasses which have never been sm'- 

 passed for clearness and definition, I have never been able to 

 detect any semblance of a motile organ ; nor have I, by colour- 



