OF THE DIATOME^. 39 



believe to be of precisely the same nature, and I am much mistaken if we do 

 not find it in many other species of the Diatomacea." 



In the accompanying part of the Jom-nal {J. M. S. 1858, p. 162), Prof. 

 "Walker- Arnott refers with approval to this opinion of Mr. Shadbolt, and 

 appends some most important remarks bearing on the presence or absence of 

 this membrane in the determination of species. He observes that " There 

 can be no doubt that these discs (i. e. of A^mchnoidiscus) have a horny 

 vegetable outer covering, in addition to the siHcious one, and that by too 

 long boiling in acid, as is necessary for guano, the marks are much obli- 

 terated or entirely removed. This, however, is not peculiar to the present 

 genus, but may be observed, more or less, in all Diatoms, although sometimes 

 the vegetable pellicle is very thin and may be removed by a few seconds' 

 immersion in boiling nitric acid. It is this circumstance which gives a quite 

 different appearance to the same species, according as the preparation is 

 made. Thus, in Actinocydus the vegetable epidermis is cellular, while the 

 silicious part is striated like a Pleurosigma ; and when the vegetable part is 

 removed, we often find nodules or knobs along the margin (forming, then, 

 the genus Omj^halojyelta), not previously visible. Those who describe Diatoms 

 from slides are thus liable to commit great errors, and indeed no certainty can be 

 obtained, except by getting the recent or growing Diatom and examining it, 

 1st, after being immersed for a short time in cold acid, or simply washed in 

 boiling water ; 2ndly, after being boiled in acid for about half a minute, or a 

 whole minute at most ; and Srdly, after being boiled for a considerable time : 

 we shall then see that many of the supposed distinct species of authors are 

 the same, prepared in a different way. Of course, deposits or guanos can yield 

 little or no information, although, when once a species has been determined 

 by the way I have indicated, we may be able to refer forms occurring in 

 guano or deposits to it with tolerable certainty." 



Mr. Brightwell, speaking of the lorica or silicious epiderm of Triceratmnij 

 states that the valves are resolvable into " several distinct layers of silex, 

 dividing like the thin divisions of talc, and frequently found of such exquisite 

 delicacy as to be difficult of detection" (J. M. S. i. 248). The siHcious lamina 

 is generally looked upon as a production or secretion fi'om the subjacent 

 organic membrane, the true cell-waU. NageU (B. S. Rejports, 1846, p. 220) 

 says, " it lies outside the membrane, and must be regarded, from analogy 

 Avith aU other similar structures, as an extra- ceUular substance excreted from 

 the ceU;" and, as Meneghini {op. cit. p. 360) adds, "in fact, anorganic mem- 

 brane ought to exist, for the silica could not become solid except by crystal- 

 lizing or depositing itself' on some pre-existing substance." Prof. Smith 

 moreover states (A. N. H. 1851) that, apart from analogy, he has direct evi- 

 dence of the independence of the silicious coat, having in his possession 

 numerous specimens of a Stauroneis (probably S. aspera, Kiitz.), in which the 

 valves, after slight maceration of the fnistules in acid, have, in part or wholly, 

 become detached fi'om the ceU- membrane, leaving a scar on its walls bearing 

 the distinct impression of the numerous and prominent valvular markings of 

 this beautiful species. The same observer adds that he has in some cases 

 noticed this organic membrane to contract around the ceU-contents, upon the 

 death of the ceU. Again, the application of hydrofluoric acid, proposed by 

 Prof. Bailey, to recent, and sometimes even to fossil shells, proves the same 

 fact, by leaving a distinct internal flexible ceU-membrane retaining the 

 general form, after the dissolution of the siHca by the acid. Further support, 

 if needed, is furnished by the phenomena of cell-division, in which the lining 

 membrane takes the initiative, and is foUowed by the doubling-in of the 

 external coat upon it. 



