288 MOVEMENTS OF DIATOMACE^. 



the Naviculce generally. The motion is of a peculiar kind, being 

 usually a series of jerks, which carry forward the frustule in the 

 direction of its length, and then carry it back through nearly the 

 same path. Sometimes, however, the motion is smooth and 

 equable ; and this is especially the case with the curious Bacillaria 

 paradoxa (Fig. 125), whose frustules slide over each other in one 

 direction until they are ail-but detached, and then slide as far in 

 the opposite direction, repeating this alternate movement at very 

 regular intervals.* In either case, the motion is obviously quite of 

 a different nature from that of beings possessed of a power of self- 

 direction. " An obstacle in the path," says Prof. W. Smith, "is 

 not avoided, but pushed-aside ; or, if it be sufficient to avert the 

 onward course of the frustule, the latter is detained for a time equal 

 to that which it would have occupied in its forward progression, 

 and then retires from the impediment as if it had accomplished its 

 full course." The character of the movement is obviously similar 

 to that of those motile forms of Protophyta which have been already 

 described ; but it has not yet been definitely traced to any organ of 

 impulsion ; and the cause of it is still obscure. + By Prof. W. Smith 

 it is referred to forces operating within the frustule, and originating 

 in the vital operations of growth, &c, which may cause the sur- 

 rounding fluid to be drawn -in through one set of apertures, and 

 expelled through the other. X ''If," as he remarks, "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 frustules of a linear 

 form ; whilst 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 Naviculce have been 

 already described ; in Surirella (Fig. 129) and Campylodiscus 

 (Fig. 130), the motion never proceeds further than a languid roll 



* This curious phenomenon the Author has himself repeatedly had the 

 opportunity of witnessing. 



t Prof. Smith says:— "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 surpassed for clearness and definition, I 

 have never been able to detect any semblance of a motile organ ; nor 

 have I, by colouring the fluid with carmine or indigo, been able to detect 

 in the ^coloured particles surrounding the Diatom, those rotatory move- 

 ments, which indicate, in the various species of true Infusorial animal- 

 ciiles, the presence of cilia." ("Synopsis of British Diatomaceaj," In- 

 troduction, p. xxiv.) 



J It has been objected to this view, by the authors of the " Micro- 

 graphic Dictionary," that, if such were the case, the like movements 

 would be frequently met with in other minute unicellular organisms. 

 They seem to have forgotten, however, that there are no other such 

 organisms in which the cell is almost entirely enclosed in an impermeable 

 envelope, the imbition and expulsion of fluid being thus limited to a 

 small number of definite points, instead of being allowed to take place 

 equally (as in other unicellular organisms) over the entire surface. 



