172. Diatomaceæ.-Notwithstanding the very close affinity, which, as will be presently shown, exists between this group and the preceding, many Naturalists who do not hesitate in regarding the Desmidiaceæ as Plants, persist in referring the Diatomaceæ to the Animal kingdom. For this separation, no valid reason can be assigned; the curious movements which the Diatomaceæ exhibit, being certainly not of a nature to indicate the possession of any truly animal endowment; and all their other characters being unmistakably vegetable. Like the Desmidiaceæ, they are simple cells, having a firm external coating, within which is included a mass of endrochrome whose superficial layer seems to be consolidated into a sort of primordial utricle. The external coat, however, though it seems to have a basis of organic membrane, is consolidated by silex; the presence of which in this situation is one of the most distinctive characters of the group. The endochrome, instead of being bright green, is of a yellowishbrown; and its peculiar color seems to be in some degree dependent upon the presence of iron, which is assimilated by the plants of this group, and which may be detected even in their colorless silicified envelopes. The coloring substance appears to be a modification of ordinary chlorophyll; it takes a green or greenish-blue tint with sulphuric acid; and often assumes this hue in drying. The endochrome consists, as in other Plants, of a viscid protoplasm, in which float the granules of coloring matter. In the ordinary condition of the cell, these granules are diffused through it with tolerable uniformity, except in the central spot which is occupied by a nucleus; round this nucleus they commonly form a ring, from which radiating lines of granules may be seen to diverge into the cell-cavity. At certain times, oil-globules are observable in the protoplasm; these seem to represent the starch-granules of the Desmidiaceae (§ 163) and the oil-globules of other Protophytes (§ 151). A distinct movement of the granular particles of the endochrome, closely resembling the circulation of the cell-contents of the Desmidiacea (§ 164), has been noticed by Prof. W. Smith in some of the larger species of Diatomaceæ, such as Surirella biseriata, Nitzschia scalaris, and Campylodiscus spiralis; and although this movement has not the regularity so remarkable in the preceding group, yet its existence is important, as confirming the conclusion that each Diatom is a single cell (the endochrome moving freely from one part of its interior to another), and that it does not contain in its

A membrane bearing all the markings of the siliceous envelope has been found by Prof. Bailey to remain, after the removal of the silex by hydrofluoric acid; and this membrane seems to have been presumed by him, as also by Prof. W. Smith, to lie beneath the siliceous envelope, and to secrete this on its surface as a sort of epidermis. The Author agrees, however, with the authors of the "Micrographic Dictionary" (p. 200), in considering it much more likely that this membrane is the proper "cellulose coat" interpenetrated by silex; especially since it has been found by Schmidt, that after removing the protoplasm of Frustulia salina by potash, and the oil by ether, a substance remained identical in composition with the cellulose of Lichens.

interior the aggregation of separate organs which have been imagined to exist in it.

173. The Diatomaceæ seem to have received their name from the readiness with which those forms that grow in coherent masses (which were those with which Naturalists first became acquainted) may be cut or broken through; hence they have been also designated by the vernacular term "brittle-worts." Of this we have an example in the common Diatoma (Fig. 94), whose component cells (which in this tribe are usually designated as frustules) are sometimes found adherent side by side (as at b) so as to form filaments, but are more commonly met with in a state of partial separation, remaining connected at their angles only (usually the alternate angles of the contiguous frustules) so as to form a zigzag chain. A similar cohesion at the angles is seen in the allied genus Grammatophora (Fig. 95), in Isthmia (Fig. 96), and in many other Diatoms; in Biddulphia (Fig. 81), there even seems to be a special organ of attachment at these points. In some Diatoms, however, the cells produced by successive acts of binary subdivision, habitually remain adherent one to another; and thus are produced filaments or clusters of various shapes. Thus it is obvious that, when each cell is a short cylinder, an aggregation of such cylinders, end to end, must form a rounded filament, as in Meloseira (Figs. 97, 98); and whatever may be the form of the sides of the cells, if they be parallel one to the other, a straight filament will still be produced, as in Achnanthes (Fig. 93). But if, instead of being parallel, the sides be somewhat inclined towards each other, a curved band will be the result; this may not continue entire, but may so divide itself as to form fan-shaped expansions, as those of Lichmophora flabellata (Fig. 91); or the cohesion may be sufficient to occasion the band to wind itself (as it were) round a central axis, and thus, not merely to form a complete circle, but a spiral of several turns, as in Meridion circulare (Fig. 92, B). Many Diatoms, again, possess a stipes or stalk-like appendage, by which they are attached to other plants or to stones, pieces of wood, &c., and this may be a simple foot-like appendage, as in Achnanthes longipes (Fig. 93), or it may be a composite plant-like structure, as in Lichmophora (Fig. 91), Gomphonema (Fig. 89), and Mastogloia (Fig. 99). Little is known respecting the nature of this stipes; it is, however, quite flexible; and may be conceived to be an extension of the cellulose coat unconsolidated by silex, analogous to the prolongations which have been seen in the Desmidiaceae (§ 163), and to the filaments which sometimes connect the cells of the Palmellacea (§ 194). Some Diatoms, again, have a mucous or gelatinous investment, which may even be so substantial that they lie as it were in a bed of it, as in Mastogloia (Figs. 99, 100), or which may form a sort of tubular sheath, as in Schizonema. In a large proportion of the group, however, the frustules are always met with entirely free; neither remaining in the least degree coherent

one to another, after the process of duplicative subdivision has once been completed; nor being in any way connected, either by a stipes, or by a gelatinous investment. This is the case, for example, with Triceratium (Fig. 79), Pleurosigma (Fig. 80), Actinocyclus (Figs. 84, 101, b b), Heliopelta (Fig. 85), Arachnoidiscus (Fig. 86), Campylodiscus (Fig. 87), Surirella (Fig. 88), Coscinodiscus (Fig. 101, a, a, a), and many others. The discoid forms, however, when obtained in their living state, are commonly found cohering to the surface of seaweeds.

174. We have now to examine more minutely into the curious structure of the siliceous envelope, which constitutes the characteristic feature of the Diatomaces, and the presence of which imparts a peculiar interest to the group, not merely on account of the elaborately marked pattern which it often exhibits, but also through the perpetuation of the minutest details of that pattern, in the specimens obtained from fossilized deposits (Figs. 101, 102). The siliceous envelope of every Diatomaceous "frustule" or cell, consists of two valves or plates, usually of the most perfect symmetry, closely applied to each other, like the two valves of a Mussel or other bivalve shell, along a line of fracture or suture; and each valve being more or less concavo-convex, a cavity is left between the two, which is occupied by the cell-contents. The form of this cavity, however, differs very greatly; for sometimes each valve is hemispherical, so that the cavity is globular; sometimes it is a smaller segment of a sphere, resembling a watch-glass, so that the cavity is lenticular; sometimes the central portion is completely flattened, and the sides abruptly turned up, so that the valve resembles the cover of a pill box, in which case the cavity will be cylindrical; and these and other varieties may coexist with any modifications of the contour of the valves, which may be square, triangular (Fig. 79), heart-shaped (Fig. 87), boat-shaped (Fig. 88, A), or very much elongated (Fig. 80, A), and may be furnished (though this is rare among the Diatomacea) with projecting outgrowths (Fig. 81). In all instances, the frustule is considered to present its "front" view, when its suture is turned towards the eye, as in Fig. 88, B, c; whilst its "side" view is seen, when the centre of either valve is directly beneath the eye (A). Although the two valves meet along the suture, in those newly formed frustules which have been just produced by binary subdivision (as shown in Fig. 81, A, e), yet as soon as they begin to undergo any increase, the valves separate from one another, and the cell-membrane which is thus left exposed, immediately becomes consolidated by silex, and thus forms a sort of hoop that intervenes between the valves (as seen at B); this hoop becomes broader and broader with the increase of the cell in length; and it sometimes attains a very considerable width (Fig. 81, A, b). As growth and self-division are continually going on when the frustules are in a healthy vigorous condition, it is rare to find a specimen in which the valves are not in some degree separated by the interposition of

the "hoop." The impermeability of the siliceous envelope renders necessary some special aperture, through which the surrounding water may communicate with the contents of the cell. Such apertures are found along the whole line of suture in disklike frustules; but when the Diatom is of an elongated form, they are found at the extremities of the frustules only. They do not appear to be absolute perforations in the envelope, but are merely points at which its siliceous impregnation is wanting; and these are usually indicated by slight depressions of its surface. In some Diatoms, as Surirella (Fig. 88) and Campylodiscus (Fig. 87), these interruptions are connected with what seem to be minute canals hollowed out between the siliceous envelope and the membrane investing the endochrome. In many genera, the surface of each valve is distinguished by the presence of a longitudinal band, on which the usual markings are deficient; and this is widened into small expansions at the extremities, and sometimes at the centre also, as we see in Pleurosigma (Fig. 80) and Gomphonema (Fig. 89). This band seems to be merely a portion in which the siliceous envelope is thicker than it is elsewhere, forming a sort of rib that seems designed to give firmness to the valve; and its expansions are solid nodules of the same substance.1

175. The nature of the delicate and regular markings, with which probably every Diatomaceous valve is beset, has been of late years a subject of much discussion among Microscopists, and cannot be said to be even yet settled, although (in the Author's opinion) the weight of evidence now decidedly preponderates on

FIG. 78.

one side. In the first place it may be remarked, that there is a much greater uniformity in the general character of these markings, than was supposed when attention was first directed to them; for what were at first supposed to be lines, are now resolved by objectives of large angular aperture into rows of dots; and these dots, when sufficiently magnified, are found to bear a close resemblance to the coarser It is to the latter, therefore.

1 These nodules were mistaken by Prof. Ehrenberg for apertures; and in this error he has been followed by Kützing. There cannot any longer, however, be a doubt as to their real nature. As Prof. W. Smith has justly remarked:-" The internal contents of the frustule never escape at these points when the frustule is subjected to pressure. but invariably at the suture or at the extremities, where the foramina already described exist. Nor does the valve, when fractured, show any disposition to break at the expansions of the central line, as would necessarily be the case were such points perforations and not nodules."

FIG. 79.

that we should have recourse for the determination of the nature of these markings; and we cannot resort to better illustrations, than those which are afforded by Isthmia (Fig. 78), Triceratium (Fig. 79), and Biddulphia (Fig. 81), in all of which the structure of the valve can be distinctly seen under a low magnifying power and with ordinary light. In each of these instances, we see a number of areola, rounded, oval, or hexagonal, with intervening spaces symmetrically disposed; and the idea at once suggests itself, that these areola are portions of the surface either elevated above or depressed below the rest. That the latter is their true condition, is suggested by the appearances they present when the light is obliquely directed; and it may also be inferred from the aspect presented by these objects when viewed by the black ground illumination (§ 61), since the areola are then less bright than the intervening spaces, less light being stopped by their thinner substance. Moreover, when a valve is broken, the line of fracture corresponds to what, on this supposition, is its weakest portion; since it passes through the areolæ, instead of through the intervening spaces, which last would be the weaker portions if the areola were prominences. But the most satisfactory proof that the areola are depressions, is perhaps that which is afforded by a side view of them, such as may be obtained by examining the curved edges of the valves in Isthmia; this, it may be safely affirmed, can leave no doubt in the mind of any competent and unprejudiced observer, as to the nature of the markings in that genus; and analogy would seem to justify the extension of the same view to the other cases in which the microscopic appearances correspond.' Now, it would not be difficult to bring together a connected series of Diatomaceæ, in which the markings, still exhibiting the same general aspect, become more and more minute, requiring for their resolution the use of oblique

It is considered by Prof. W. Smith, that this areolation is indicative of a cellular structure in the siliceous envelope. But when it is borne in mind that the entire frustule constitutes a single cell, such an hypothesis seems altogether inadmissible. The Author would rather consider the markings in question as analogous to those which are presented by the surface of many pollen-grains (Fig. 189), of whose single-celled nature no doubt can exist; and in his researches on the Foraminifera, he has met with several instances, in which the calcareous investments of those segments of sarcode which must be considered as the representatives of single cells, are marked with a like areolation, the areolae being here unquestionably depressions, formed by the thinning away of the envelope at those parts.