A history of Infusoria : including the Dismidiaceoe and Diatomaceoe, British and foreign / by Andrew Pritchard.
- Date:
- 1861
Licence: Public Domain Mark
Credit: A history of Infusoria : including the Dismidiaceoe and Diatomaceoe, British and foreign / by Andrew Pritchard. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![believe to bo of precisely the same natm-e, 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-Amott 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 Arachnoidiscus) have a homy vegetable outer covering, in addition to the silicious one, and that by too long boUing 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 obsei-ved, 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 Actinocyclus the vegetable epidermis is eeUular, while the sDicious 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 Omphalopelta), 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, Ist, 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 3rdly, 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 Triceratiwm, 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 M. S. i. 248). The silicious lamina is generally looked upon as a production or secretion from the subjacent organic membrane, the true ceU-waU. NageH {B. S. Reports, 1846, p. 220) says, it lies outside the membrane, and must be regarded, fr'om analogy with aU other similar structures, as an extra-cellular substance excreted from the ceU; and, as MenegMni {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. K 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 whoUy, become detached from the cell-membrane, leaving a scar on its walls bearing the distinct impression of the nimierous 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 coU-contents, upon the death of the cell. Again, the application of hydi'ofluorie acid, proposed by Prof. Bailey, to recent, and sometimes even to fossil shells, proves the same fact, by leaving a distinct internal flexible cell-membrane retaining the general form, after the dissolution of the silica by the acid. Fiu-ther su])port if needed, is famished by the phenomena of cell-division, in which tlie Uning membrane takes the initiative, and is followed by the doubling-in of the external coat upon it.](https://iiif.wellcomecollection.org/image/b21910224_0055.jp2/full/800%2C/0/default.jpg)
