120 Bacillariex 



to the wall of diatoms it consists mostly of cellulose and is not silicih'ed. In 

 both groups the chromatophores exhibit great diversity in form, but their 

 normal disposition in diatoms is parietal, whereas in the vast majority of 

 Conjugates they are axile. Moreover, no desmids possess the small parietal, 

 disc-like chromatophores which are characteristic of such immense numbers 

 of centric diatoms. The difference of pigment is also important, and so is 

 the general storage of oil as a reserve of food by diatoms. The details of 

 conjugation have been argued as a proof of affinity, but while conjugation 

 of similar aplanogametes results in each case in a zygote, the latter remains 

 as a resting zygospore in the Conjugate, whereas in diatoms there is an 

 immediate rejuvenescence to form an auxospore. In the Conjugate con- 

 jugation is both usual and normal, and except in a few instances the spore is 

 always a zygospore ; but in diatoms the majority of auxospores are partheno- 

 genetic, and nothing comparable with microspores exists at all in the 

 Conjugates. The fact that Conjugates have never been able to adapt 

 themselves to a marine life is also significant. On the whole, there is no 

 satisfactory evidence that the Bacillariese and Conjugate are in any way 

 nearly related. One must regard the similarity of the unicellular and 

 colonial habit of diatoms and desmids, and also the resemblance between 

 their cell-division, merely as a parallelism of modification brought about 

 by the adaptation of two phylogenetically distinct groups to similar con- 

 ditions of environment. 



Up to the present time little if any light has been thrown on the 

 affinities of diatoms. Their characters are so distinctive, and the group 

 as a whole exhibits such a uniformity of structure, that there is every 

 justification for treating them as a distinct class, the affinities of which 

 are very obscure. 



CLASSIFICATION. Space does not allow even an outline of the various 

 proposals which have been put forward for the classification of the numerous 

 genera and species of diatoms. Those systems based upon the disposition 

 and mode of division of the chromatophores, which have been proposed by 

 Pfitzer (71), Petit (77), Pelletan ('92), and Ott ('00), are impracticable 

 owing to the fact that so many genera and species are unknown in the living 

 state, and that considerable diversity in the structure of chromatophores has 

 been found within the limits of a single genus. The classification which 

 was suggested by H. L. Smith, and subsequently adopted by Van Heurck 

 ('85) in his classical systematic work on the Diatomacese, was based upon 

 the presence or absence of a raphe or pseudoraphe ; and all diatoms were 

 grouped under the three divisions of the Raphidese, Pseudoraphidea?, and 

 Cryptoraphidese. More recently, Van Heurck ('09) has remodelled this 

 classification, but he has most inaptly used two of H. L. Smith's divisional 



