DIATOMACE^E 599 



who are applying themselves to the study of diatoms to so important 

 MII argument, on which may depend the possibility of establishing a 

 really good classification of diatoms which will at length satisfy 

 diatomists. At present preference is generally accorded to the 

 classification proposed by H. L. Smith, which establishes the class of 

 Raphidece from the presence of a raphe in the plane of the valves. 

 If there is, on the valves, in place of the raphe, a simple line of 

 division, the forms thus characterised are termed Pseudoraphidece ; 

 while those in which the valves have neither raphe nor its equivalent 

 are called Cryptoraphidece, or, better, Anaraphidece. While, there- 

 fore, in the present state of our>knowledge of diatoms, any classifica- 

 tion can only be regarded as provisional, we do not propose any 

 innovation on this point, although we are disposed to accord our 

 preference to that suggested by H. L. Smith. 



Conjugation, so far as is at present known, takes place among, 

 the ordinary Diatomacece almost exactly as among the De&midiacece, 

 except that it sometimes results in the production of two ' zygo- 

 spores ' instead of a single one. Thus in Surirella (fig. 453), the 

 valves of two free and adjacent frustules separate from each other, 

 and the two endochromes (probably included in their parietal 

 utricles) are discharged ; these coalesce to form a single mass, 

 which becomes enclosed in a gelatinous envelope, and in due time 

 this zygospore shapes itself into a frustule resembling that of its 

 parent, but of larger size. But in Epithemia (fig. 446, A, B), the 

 first diatom in which the conjugating process was observed by 

 Mr. Thwaites, 1 the endochrome of each of the conjugating frustules 

 (C, D) appears to divide at the time of its discharge into two halves ; 

 each half coalesces with half of the other endochrome ; and thus 

 two zygospores (E. F) are formed, which, as in the preceding case, 

 become invested with a gelatinous envelope, and gradually assume 

 the form and markings of the parent frustules, but grow to a very 

 much larger size, the sporangial masses having obviously a power of 

 self-increase up to the time when their envelopes are consolidated. 

 It seems to be in this way that the normal size is recovered, after 

 the progressive diminution which is incident to repeated binary 

 multiplication. Of the subsequent history of the zygospores much 

 remains to be learnt ; and it may not be the same in all cases. 

 Appearances have been seen which make it almost certain that the 

 contents of each zygospore break up into a brood of gonids, and 

 that it is from these that the new generation originates. These 

 gonids, if each be surrounded (as in many other cases) by a distinct 

 cyst, may remain undeveloped for a considerable period ; and they 

 must augment considerably in size before they obtain the dimensions 

 of the parent frustule. It is in this stage of the process that the 

 modifying influence of external agencies is most likely to exert its 

 effects ; and it may be easily conceived that (as in higher plants and 

 animals) this influence may give rise to various diversities among 

 the respective individuals of the same brood ; which diversities, as 

 we have seen, will be transmitted to all the repetitions of each 



1 See Annals of Natural History, vol. xx. ser. i. 1847, pp. 9, 343 and vol. i. 

 ser. ii. 1848, p. 161. 



