MOVEMENTS OF DIATOMACE^E. 287 



self-dividing process. Hence a very considerable latitude is to 

 be allowed to the limits of species, when the different forms of 

 Diatomaeere are compared ; and here, as in many other cases, a 

 most important question arises as to what are those limits, a 

 question which can only be answered by such a careful study of 

 the entire life-history of every single type, as may advantage- 

 ously occupy the attention of many a Microscopist, who is at 

 present devoting himself to the mere detection of differences, 

 and to the multiplication of reputed species. 1 



179. Most of the Diatoms which are not fixed by a stipes, pos- 

 sess some power of spontaneous movement ; and this is especially 

 seen in those, whose frustules are of a long narrow form, such as 

 that of the Naviculse 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 Ba- 

 cillaria paradoxa (Fig. 92, B), whose frustules slide over each 

 other in one direction, until they are all but detached, and then 

 slide as far in the opposite direction, repeating this alternate 

 movement at very regular intervals. 2 In either case, the motion 

 is obviously quite of a different nature from that of beings pos- 

 sessed 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. 3 By Prof. W. Smith it is referred to 



1 See on this subject a valuable Paper by Prof. W. Smith " On the Determination of 

 Species in the Diatomacece" in the "Quart. Jouro. of Microsc. Science," vol. iii, p. 130: 

 a Memoir by Prof. W. Gregory " On shape of Outline as a specific character of Diato- 

 macecE" in "Trans, of Microsc. Soc.," 2d Series, vol. iii ; and the Author's Presidential 

 Address in the same volume, pp. 44-50. 



2 This curious phenomenon, the Author has himself more than once had the oppor- 

 tunity of witnessing. 



3 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 coloring the fluid with carmine or indigo, been able to 

 detect in the colored particles surrounding the Diatom, those rotary movements, which 

 indicate, in the various species of true Infusorial animalcules, the presence of cilia." 

 (Synopsis of British Diatomaceae, Introduction, p. xxiv.) Mr. Jabez Hogg, however, has 

 recently stated ("Quart. Journ. of Microsc. Science," vol. iii, p. 235) that by the employ- 

 ment of the same mode of illumination as that by which the ciliary action may be dis- 

 cerned in Closterium, &c. ( 164), a ciliary movement may be detected at the orifices 

 which have already been described as existing in the siliceous envelope of the Diato- 

 maceous frustule ( 174). It may be questioned, however, whether this be anything 

 else than an optical illusion, arising from the existence of currents at these orifices, pro- 

 duced by the vital actions going on within the cell, as noticed above. 



