284: THE MICROSCOPE AND ITS REVELATIONS. 



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. 175) and Campylodiscus (Fig. 

 176), the motion never proceeds further than a languid roll from one 

 side to the other; and in Gomphonema (Fig. 187), 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 hardly a perceptible advance in intermitted jerks in the direc- 

 tion of the narrow end." 



283. The principles upon which this interesting group should bo 

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

 erative process of which nothing whatever is yet known in a large pro- 

 portion of Diatoms, and very little in any of them shall have been 

 thoroughly followed out. The observations of Focke 1 render it highly 

 probable that many of the forms at present considered as distinct from 

 each other, would prove to be but different states of the same, if their 

 whole history were ascertained. 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 

 different modes of reproduction. At present, therefore, any classification 

 must be merely provisional; and in the notice now to be taken of some 

 of the most interesting forms of Diatomacece, the method of Prof. Kiit- 

 zing, which is based upon the characters of the individual frustules, is 

 followed in preference to that of Prof. W. Smith, which was founded on 

 the degree of connection remaining between the several frustules after 

 self-division. 2 In each Family the frustules may exist under four con- 

 ditions, (a) free, the self -division being entire, so that the frustules sep- 

 arate as soon as the process has been completed; (b) stipitate, the 

 frustules being implanted upon a common stem (Fig. 172), which keeps 

 them in mutual connection after they have themselves undergone a com- 

 plete self-division; (c) united in a filament, which will be continuous 

 (Fig. 177) if the cohesion extend to the entire surfaces of the sides of the 

 frustules, but may be a mere zigzag chain (Fig. 173) if the cohesion be- 

 limited to their angles; (d) aggregated into a frond (Fig. 188), which 

 consists of numerous frustules more or less regularly inclosed in a 

 gelatinous investment. It is not in every family, however, that these 

 four conditions are at present known to exist; but they have been 

 noticed in so many, that they may be fairly presumed to be capable of 

 occurring in all. Excluding the family Actiniscew (of whose silicified 

 skeletons we have examples in Fig. 191, c, d), which seem to have no 

 adequate title to rank among Diatoms (their true alliance being appar- 

 ently with the Polycystina), the entire group may be divided into two 



1 According to this observer ("Ann. of Nat. Hist.," 2d Ser., Vol. xv., 1855, p. 

 237), Navicula bifrons forms, by the spontaneous fission of its internal substance, 

 spherical bodies which, like gemmules, give rise to Surirella microcora. These 

 by conjugation produce ^V. splendida, which gives rise to N. bifrons by the same 

 process. He is only able to speak positively, however, as to the production of N. 

 bifrons from N. splendida ; that of Surirella microcora from jy. bifrons, and that 

 of N. splendida from Surirella microcora, being matters of inference from the 

 phenomena witnessed by him. 



''The method of Kiitzing is the one followed, with some modification, by Mr. 

 Ralfs in his revision of the group for the 4th Edition of Pritchard's " Infusoria;" 

 and to his systematic arrangement the Author would refer such as desire more, 

 detailed informati9n. 



