54 



GENERAL HISTOEY OF THE INFrSOEIA. 



tures would be sufficient to produce moYements in bodies of so little specific 

 gravity. 



" If the motion be produced by the exosmose taking place alternately at one 

 and the other extremity, while endosmose is proceeding at the other, an 

 alternating movement would be the result in fnistules of a linear form, — 

 while in others of an elliptical or orbicular outline, in which foramina exist 

 along the entire line of suture, the movements, if any, must be irregular, or 

 slowly lateral. 



" Such is precisely the case. The backward and forward movements of 

 the Naviculece have been already described ; in Sninrella and Campylodiscus 

 the motion never proceeds farther than a languid roll from one side to the 

 other ; and in Gomphonema, in which a foramen, fulfilling the nutritive office, 

 is foimd at the larger extremity only, the movement is a hardly perceptible 

 advance in intermitted jerks in the dii-ection of the narrow end. The subject 

 is, however, one involved in much obscurity, and is probably destined to 

 remain, for some time to come, among the mysteries of Nature, which baffie 

 while they excite inquiry." 



The last clause of this quotation expresses the unsatisfactoiy state of the 

 question; yet the foregoing examination will, we think, leave only three 

 hypotheses desei'ving further inquiiy : viz., 1. the existence of cilia, or, 2. of 

 an undulating membrane ; and 3. the operation of endosmose and exosmose, 

 as a mechanical cause. To our apprehension, the presence of cilia, perhaps 

 ranged only along the sutural lines, has not been completely disproved ; and, 

 on the other hand, considered as locomotive organs, cilia have the great 

 advantage of analogy over the presumed undulatory membrane. Do not, 

 indeed, the experiments with indigo, recoimted by Siebold, suggest cilia to 

 be the active agents of the movements recorded ? 



The rate of motion of the Diatomeae is exceedingly languid and slow; 

 sometimes it amounts to no more than an oscillating movement, with Httle 

 or no change of place ; and at anothei*, the backward and forward movements 

 are so nearly equal, that the fnistule makes no appreciable advance. Prof. 

 Smith has measured the rate of motion of some species, and remarks that, 

 however vivacious and rapid they may at first sight seem, yet, when con- 

 sidered vdth reference to the high magnifying powers employed, and the 

 consequent amplification of their movements, they are very slow. " I have 

 noted the movements of several species with the aid of an eye-piece micro- 

 meter and a seconds watch, and found that one of the most rapid, viz. Bacil- 

 laria paradoxa, moved over g- ott^^ ^^ ^^ ^^^^ ^^ ^ second ; Pinnularia radiosa, 

 one of the slowest, over ., .^.... th of an inch in the same time ; and that the 

 same period was occupied by Pinmdaria ohlonga m traversing -ginroth of an 

 inch, Nitzschia linearis y-V^th of an inch, and Pleurosigma strigosum ytwo^^ 

 of an inch. Or, expressing the spaces and times by other units, we find that 

 the most active required somewhat more than three minutes to accomplish 

 movements whose sum would make one inch, and the slowest nearly an hour 

 to perform the same feat." 



Before quitting the subject of the movements of the Diatomeae, we would 

 briefly advert to the peculiar motion of some species, especially of BaciUaria 

 paradoxa. The movements of this organism, as the specific name implies, 

 are paradoxical, or very strange in character. Mr. Thwaites essayed to 

 describe what indeed can be rightly apprehended only by personal ob- 

 servation, in the following words (Proc. of Linn. Soc. i. p. 311): — "When 

 the filaments have been detached from the plants to which they adhere, a 

 remarkable motion is seen to commence in them. The first indication of this 

 consists in a shght movement of a terminal fnistule, which begins to slide 



