456 



ANIMAL NATURE OF DIATOME^. 



manner^ a box which opens, and the two primary sur- 

 faces coming into contact, the disengagement or separa- 

 tion becomes more or less complete. If these deduc- 

 tions be legitimate, as to me they appear to be, the 

 result is that the successive growth of the entire frond 

 is due to a dou4ile element ; the elongation of each series 

 by incessant deduplication, and the intercalation of new 

 series between those that pre-existed. The comparative 

 examination of the extremities is important in respect 

 to the elongation. In various species the single partial 

 tubes tend to detach themselves from their reciprocal 

 union, either uniformly from the base {Jlaffdlifornie, 

 medtisinum, Zanardinii), or merely towards the summit, 

 in a sort of fan-shape [helmintosmn, lacinatum). Again, 

 we find the multiplication of the series principally 

 effected near the apices, and accomphshed more quickly 

 from the elongation of the pre-existing ones, on which 

 account the extremities are clavate {Arbuscula, davatmn.) 

 Where these two processes advance, pari passu, the re- 

 sulting extremities are obtuse {patens, Bryopsis, intri- 

 catimi). Where the elongation of the first series always 

 precedes the appearance and formation of the new ones, 

 the apices are acute {Jloccosum, Kiitz. ramosissimum, 

 Hyalopus, araneosum, Smithii, lorquatwn). It is cha- 

 racteristic, too, of some species (ckondroides, trichocepha- 

 liim, capitaiiim, corymbosum, spjinescens), to have the 

 prolification terminal, which manifests a succession of 

 distinct periods of vegetation. Finally, the particular 

 condition of the 8. myxacantlium demonstrates a contem- 

 poraneous elongation of the series, which the resistance 

 of the external envelope compels to flow towards the 

 apex, until, that resistance being overcome, a palmate 

 disposition of the branches is produced. 



As to the Naviculse, we know still less of them than in 

 the preceding genera. Ehrenberg says that some of them 

 are striated, but this is not confirmed by Kiitzing, nor 

 have I seen striae in any species. Yet in some 1 have 

 seen the two longitudinal lines of the primary surfaces doy- 



