288 MICROSCOPIC FORMS OF VEGErABLE LIFE. 



forces operating within the frustule, and originating in the vital 

 operations of growth, &c., which may cause the surrounding fluid 

 to be drawn in through one set of apertures, and expelled through 

 the other/ "If," as he remarks, "the motion be produced by 

 the exosmose taking place alternately at one and the other ex- 

 tremity, while endosmose is proceeding at the other, an alterna- 

 ting 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 Naviculse 

 have been already described ; in Surirella (Fig. 88) and Campylo- 

 discus (Fig. 87), the motion never proceeds further than a languid 

 roll from one side to the other ; and in Q-ompJionema (Fig. 89), in 

 which a foramen, fulfilling the nutritive office, is found at the 

 larger extremity only, the movement (which is only seen when 

 the frustule is separated from its stipes) is a hardly perceptible 

 advance in intermitted jerks in the direction of the narrow end." 

 180. The principles upon which this interesting group should 

 be classified, cannot be properly determined, until the history of 

 the Generative process, — of which nothing whatever is yet known 

 in a large proportion of Diatoms, and very little in any of them, — 

 shall have been thoroughly followed out. As already stated, 

 there is a strong probability that many of the forms which are at 

 present considered as distinct from each other, would prove to be 

 but different states of the same, if their whole history were ascer- 

 tained. On the other hand, it is by no means impossible that 

 some which appear to be nearly related in the structure of their 

 frustules and in their mode of growth, may prove to have quite 

 ditferent modes of reproduction. At present, therefore, any clas- 

 sification must be merely provisional ; and in the notice now to 

 be taken of some of the most interesting forms of the Diatomacese, 

 the method of Prof. W. Smith, which is based upon the degree 

 of connection that remains between the several frustules after 

 self-division, will be adopted, since it possesses the advantage of 

 being in accordance with the general "physiognomies" of these 

 organisms, as it brings together those forms which correspond 

 most closely in plan of growth; but it cannot be regarded as a 

 truly natural classification, since it often separates genera which 

 are closely allied in the structure of the individual frustules, as, 

 for example, Coscinodiscus and Meloseira. The whole order is 

 thus grouped, in the first instance, under two Tribes ; the fint 

 including those which have the frustules naked, that is, neither 



' It has been objected to this view, by the authors of the " Micrographio 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, which limits the imbibition and expulsion of fluid to a small number of defi- 

 nite points, instead of allowing it to take place equally (as in other unicellular organisms) 

 over the entire surface. 



