OF THE DIATOME^. 



37 



towards the extremities, where they are somewhat crowded" \vithin the 

 distinctly tubular filaments, and enjoy a certain latitude of movement ; and, 

 lastly, in Colletonema and Schizonema (X. 207, 208) they are arranged 

 in one or more files according to the stage of growth, within less per- 

 fect tubes than in the genus last mentioned, and retained in situ by the 

 mucus around. 



Ehrenberg recognized this tribe of compound Diatomese, and introduced it as 

 one of the sections of his great family Bacillaria, under the name of Lacerifuita, 

 or Naviculce with a double lorica. His acquaintance with the group was, how- 

 ever, very imperfect, and he appears to have comprehended in it organisms 

 quite foreign to it, and to have failed to give that precision to his classifi- 

 cation of the included beings which could alone confer a high scientific value 

 and permanence upon it. 



Of the Eitvelopes of the Feustules of DiATOiiEJE. — The Silicioiis Shell or 

 Lorica ; its Divisions and Structural Comiwsition, Markings, Strict, Canali- 

 culi, Pumta, ^c. — Sufficient has been said of the mucous coat which at 

 certain times, and always in certain genera, surrounds the frustules of 

 Diatomese. The frustules themselves remain to be described: they are 

 hollow variously-shaped cells having but one cavity — unicellular, and a 

 siUcious outer wall, unafi'ected by a red heat and by strong acids, which 

 would corrode and dissolve every other substance belonging to a living being 

 except silex. This silex is stated not to polarize light, as does the mineral 

 silex not in combination with organic beings ; and the erroneous statement 

 made by some authors, of the polarizing effect of some Diatomaceous shells 

 is due to the circumstance, that they did not take care to thoroughly remove 

 the organic carbonaceous matter with which the silex is in union in the 

 fnistides in question. 



The silex, besides being united with organic matter, deposited it may be 

 within a cellular tissue, is contaminated by iron, which Professor Frankland 

 of Manchester states {Smith, Synops. p. xxi) *' exists in the state of a 

 silicate or protoxide . . . . " and he attributes to its presence " the brown colour 

 which is assumed upon exposing the Diatoms to the influence of a moderate 

 heat; the protoxide of iron, by the gradual absorption of oxygen, being 

 converted into brown peroxide of iron, which assumes a redder tinge upon 

 being more strongly heated." 



The relative proportion of silica varies within considerable limits in different 

 genera of Diatomeae. In several genera, perhaps in marine ones exclusively, 

 it is very deficient, and the wall of the frustule is httle more than horny, or it 

 may be even flaccid, as for example in Dichieia and Schizonema. The frustules 

 of Fragilaria, Striatella, and Poclosira are less firmly sihcious than those of 

 many others of the filamentous Diatomese. In some genera (those, viz., 

 which produce tubular processes) silex is deficient or absent from the pro- 

 duced wall ; in Poclosira this deficiency occurs at the apex of the valves, and 

 in Prof. Smith's opinion is probably intended "to allow a free secretion of 

 the mucus which unites the frustules and provides a pedicle for their attach- 

 ment to the plant on which they grow, as it does not occur on the non- 

 attached valve of the fii^st- formed frustule. In the Hving state the absence 

 of silex is not perceived; but when the frustules have been macerated in 

 acid, these portions of the valves appear as perforations, owing to the dis- 

 appearance of the ceU-membrane." 



The frustules of the Diatomeae are composed, as before stated, of two 

 usually more or less convex valves, enclosing a single cavity, which becomes 

 augmented by the growth of a third segment interposed between them, pro- 

 duced preparatory to the process of self-division. Meneghini asserts that 



