The general correctness of Dujardin’s description of the animals that form Foraminiferal shells has been fully confirmed by subsequent researches; but the larger knowledge since obtained of the nature of the Protozoa has led to a more correct apprehension of the relations of the Rhizopoda to the other components of that group. What is now regarded as the simplest type of an animal, designated a Moner (see Animal Kingdom, vol. ii. p. 50), consists of an independent particle of the elementary form of living matter known as “protoplasm” (the “sarcode” of Dujardin), which is capable of growth and maintenance by the assimilation of nutrient material, and of multiplying itself either by subdivision or by some modification of that process. Now the first stage of differentiation of this apparently homogeneous substance into histogenetic elements consists in the appearance of certain rounded bodies termed endoplasts, which appear to be the equivalents of the nuclei of the “cells” whose individuation marks a higher stage of differentiation. And the next stage (well seen in Amoeba) consists in the differentiation of a more consistent external layer, or “ectosarc,” from the less consistent substance of the interior, or “endosarc,” in which are observable “vacuoles” containing fluid, of which one or more (that seem bounded by a definite pellicle, and are known as “contractile vesicles ”) contract and dilate rhythmically. As yet, however, there is no definite point of entrance for alimentary particles, or of exit for excrementitious matter,—nutrient τngesta being admitted, and their unassimilable residue being got rid of, through any spot of the ectosarc. It is in the presence of a definite mouth, and usually of an anus also, that even the simplest of the true ciliated Infusoria show a decided advance upon the “rhizopodal” type,—an advance which is still more marked in the higher Infusoria by the complexity of their internal organization.

Now the animal bodies of Foraminifera, notwithstanding the regularity and complexity of the shells they form, show but a very slight advance on the simplest moneral type. For their protoplasmic substance does not seem to be differentiated into “ectosarc” and “endosarc,”—every part of it alike projecting itself into pseudopodial extensions, and these extensions being capable, not only of dividing and ramifying indefinitely, but also of reuniting by mutual fusion when they come into contact with each other, so as to constitute an irregular network (figs. 2, 3, 4). It is on account of this last peculiarity that the writer has distinguished the reticulose forms of the Rhizopod type from the lobose (consisting of Amoeba and its allies) on the one hand, and from the radiolarτan (of which Actinophrys is the type) on the other. The sarcodic bodies of Foraminifera were believed until recently to have neither “nuclei” nor “contractile vesicles”; but as the observations of Hertwig and F. E. Schulze have established the presence of these in some instances (the writer also having observed “endoplasts” in Orbitolites), it is probable that they exist universally.

The attention which has been given in recent years to the study of Foraminifera, has invested the group—formerly considered as comparatively insignificant—with a new interest and importance. For (1) these minute testaceous Rhizopods, instead of having mere local habitats, are diffused abundantly through all save polar seas, and seem to do the first work of collecting by imbibition, and of converting into living substance, the organic matter which is contained, however sparingly, in all oceanic water, and the restoration of which, as fast as it is thus withdrawn, is effected by the various forms of marine vegetation. Again (2), without any thing that can be called organization, the protoplasmic bodies of these animals give origin to protective casings of marvellous regularity of form, and often of great complexity of structure,—these being sometimes “tests " built up by the apposition of sand-grains or other particles collected from the bottom on which they live, the animals only furnishing the cement by which they are held together, but being more often true “shells,” formed (like the skeletons of higher animals) by an interstitial deposit of carbonate of lime drawn from the surrounding medium, in the substance of living tissue. Notwithstanding (3) the absence of any perceptible differences in the character of the animals they respectively contain, these protective casings, whether sandy “tests” or calcareous “shells,” present a wide diversity of fundamental form, which is almost indefinitely augmented by subordinate modifications; and these modifications are generally so gradational as to render it impossible (when a sufficient number of specimens are compared) to draw any lines of separation between what appear, when only the extremes are regarded, to be clearly differentiated types. And this is true, not merely of species (which in the sense of constantly differentiated races cannot be said to have any existence among Foraminifera), but also of what would elsewhere be accounted genera·, whilst even families cannot, as a rule, be sharply defined, many of them containing aberrant forms that defy all attempts at strict limitation. In fact it would be scarcely a figure of speech to say that, within each of the three primary subdivisions of the group to be presently marked out, every form passes gradationally into every other. Hence the study of Foraminifera— whose small (but not too small) size and numerical abundance are peculiarly favourable to the comparison of large series of individuals—affords a most valuable lesson in taxonomy,—giving the best illustration that the whole animal kingdom can afford of the production of almost endless varieties of conformation by “descent with modification.” Looking (4) to the almost universal diffusion of existing Foraminifera, and to the continuous accumulation of their calcareous exuviae over vast areas of the ocean-bottom, there can be little doubt that they are at present doing more than any other tribe of marine animals to separate carbonate of lime from its solution in sea-water, so as to restore to the solid crust of the earth what is beiùg continually withdrawn from it by the solvent action exerted by rain and rivers upon the calcareous material of the upraised land. And when (5) we look back into the past history of our globe, we not only find conclusive proof of the enormous contributions which Foraminifera have made to the calcareous strata of Tertiary and Secondary epochs (Nummulitic Limestone and Chalk having been almost entirely formed by their agency), but encounter strong reason for the belief that the principal mass of the Palaeozoic Carboniferous Limestone had a similar origin. And when finally (6) we go still further back in geological time, and bring our knowledge of this type to bear on the very earliest calcareous formation that has yet been found in stratified rocks (the Serpentine Limestone of the Canadian Laurentians), we find distinct evidence that this also had its origin in Foraminiferal growth; which has thus stamped the impress of its existence even upon the oldest of those Archaic rocks, which, through a long subsequent succession, have as yet yielded no other distinct evidence of the existence of either animal or vegetable life on our planet. And thus in all parts of the geological series we not only recognize the constructive work of these humble organisms, but, interpreting the past by the present, find ever-increasing reason to believe that it has exceeded that of all other marine animals taken collectively.

A concise statement of the evidence in support of each of these positions will be embodied in the following sketch of the life-history, present and past, of this group; which will be so drawn as rather to bring into view its great features of general interest, than those details with which the systematist only is concerned.

Although the testaceous Foraminifera are all marine, certain shell-less animals essentially similar in nature occur in fresh water; and these, of which Liaeberkühnia (fig. 2) is a characteristic example, afford the best means of studying the life-history of the “reticulose” type. In its inactive state Lieberkühnia is an ovate corpuscle of granular protoplasm about l-400th of an inch in diameter, not presenting either any distinct differentiation of “endosarc’’ and “ectosarc” or “contractile vesicle,” or any ev ident “nucleus,” but including a large number of vacuoles filled with a homogeneous liquid. In its active state, on the other hand, the body puts forth a sort of stem, which soon branches irregularly in to pseudopodia; and these subdivide and ramify with great minuteness to an extent two or three times as great as that represented in the figure, the ramifications coalescing whenever they come into contact with each other, so as to form an irregular network which may be compared to an animated spider’s-web. This network serves to entangle either minute particles (such as chlorophyll granules) or larger bodies (such as Infusoria) that come in its way,—the former being transmitted to the central body by a sort of circulation (closely resembling the cyclosis of vegetable cells) which is constantly going on through the whole protoplasmic network, and the latter becoming enclosed in a kind of sheath formed by the blending of the neighbouring pseudopodia, which apply themselves to its surface and draw it into the central mass by their gradual contraction. Although destitute of any protective envelope, Lieberkühnia does not put forth its pseudopodia indifferently from any part of its surface,—their primary stem being enveloped in a transparent sheath, which may be traced as a thin pellicle over the whole body. And in Gromia (fig. 3), of which some forms inhabit fresh water (attaining a size that renders them visible to the naked eye), whilst others are marine, this pellicle is thickened into a distinct casing or “test” of ovoidal shape (probably composed of chitin), with a single round orifice of moderate size, through which the sarcodic body puts forth its pseudopodial extensions. When the animal is in a state of repose, these are entirely retracted into the test; but when its activity recommences, they are put forth from its orifice, as from the stem of Lieberkühnia, and form a sarcodic network which is in a state of incessant change—new centres of radiation often arising where two or more pseudopodia coalesce, by the flowing of the protoplasm towards those points. It is specially noticeable in this type that the sarcodic body extends itself over the entire surface of the test, so as completely to enclose it, and that pseudopodia are put forth from every part of this extension, being especially numerous at the posterior end, where they probably serve to fix the test, and thus to enable the animal to put forth more power in seizing the larger creatures that serve as its food. For whilst it is partly nourished by the minute granules that adhere to its outspread network, it lays hold of the smaller Infusoria and Diatoms, the zoospores of Confervae, &c., and draws them entire into the interior of the test, within which their indigestible remains may frequently be seen. Now, if the transparent pellicle of Lieberkühnia were consolidated by calcareous instead of by chitinous deposit, a monothalamous (single-chambered) “shell” would be produced (9 in fig. 1), such as constitutes the permanent form of the simplest Foraminifera, and the primordial form of even the most complex. This shell, like the test of Gromia, may have but a single aperture, from which the pseudopodial extensions are put forth, and through which alone nutriment is received into the contained body. But in a large proportion of the polythalamous (many-chambered) Foraminifera (fig. 4), the shell-wall is perforated with minute pores over its entire surface, through which pseudopodia extend themselves in a radial direction; and while the coalescence of these at their origin forms a continuous sarcodic layer that covers the exterior of the shell, the coalescence of their extremities forms new centres of radiation at a distance from it. The aperture or mouth in these types is much smaller than in the preceding, and appears to serve rather for the projection of the stolon-process, by which new chambers are formed (in the manner to be presently described), than for the introduction of nutriment. And when it is considered that the diameter of the pores of the shell never exceeds l-5000th of an inch (being often much less than 1-10,OOOth), and that the chlorophyll-granules, zoospores, &c., that nourish the fresh-water Rhizopods are altogether wanting in the sea-depths inhabited by the higher types of Foraminifera, there seems additional ground for the doctrine already propounded, that the nutrition of the animals of this group is mainly drawn rather from the organic matter that is dissolved in the medium they inhabit, than from solid particles suspended in it.

In regard to the propagation of Foraminifera, little is as yet certainly known. The growth of the individual may proceed, as will be presently shown, to an almost indefinite extent, by the gemmiparous multiplication of the segments or divisions of its body, with a corresponding multiplication of the chambers of its shell; but it more frequently happens that when the organism has attained a certain limited size, the new segments detach themselves, each one growing into the likeness of its parent; and this is probably the ordinary way in which the continuance of the race is provided for. But it would also seem that under certain circumstances the sarcodic body of the parent breaks itself up into segments, each containing an “endoplast” or “nucleus,” and that around each of these a shelly covering is formed while it is still enclosed within the original shell, the offspring finally escaping by its rupture. Whether this mode of propagation is preceded by any process of “conjugation,” and is thus related to sexual generation, is still undetermined.

By far the greater number of Foraminifera are composite fabrics, evolved from a simple protoplasmic body by a process of continuous gemmation, each bud remaining in connexion with the stock from which it was put forth; and according to the plan on which this gemmation takes place, will be the configuration of the composite fabric thereby produced. Thus if the bud be put forth from the aperture of the monothalamous Lagena (9 in fig. 1) in the direction of the axis of its body, if a second chamber be formed around this bud in continuity with the first (receiving the neck of the latter into its own cavity), and if this process be successionally repeated, it is obvious that a straight rod-like shell (10 in fig. 1) will be produced, having a series of chambers communicating with each other by the apertures that originally constituted their mouths, and opening externally by the mouth or aperture of the last formed chamber. The successive segments may be all of the same size, or nearly so; in which case the entire series may either resemble a string of beads, or may approach the cylindrical form, according to the shape of the chambers. But it more frequently happens that each segment somewhat exceeds its predecessor in size, so that the entire shell has a somewhat conical shape. If, on the other hand, the axis of growth should be slightly curved instead of straight, the resulting composite shell will be arcuate, while a more rapid deflection gives it a spiral curvature (5 and 11 in fig. 1). The form of the spire will depend in the first place upon whether its convolutions lie in the same plane, like those of a nautilus, so that the shell is equilateral (16 and 19 in fig. 1), or pass obliquely round a vertical axis, as in a snail, so that the shell becomes “inequilateral” (18 in fig. 1), having a more or less conical form, with the primordial chamber at its apex. In other cases, again, the vertical axis is greatly elongated, and the number of chambers forming each revolution around it is reduced to four, three, or even two (13 and 14 in fig. I). But further, the spiral plan of growth may give place to the cyclical, successive circles of new chambers being formed around the interior growth, so as to give the shell a discoidal shape (6 and 17 in fig. 1). And sometimes all regularity of plan disappears in the later stages of growth, new chambers being added in various directions, so that the fabric becomes “acervuline.”

In the older classifications of Foraminifera, these differences of plan of growth were adopted as characters of primary importance in the subdivision of the group. But it has now become obvious that comparatively little value is to be attached to them. For to associate together all rectilineal, all spiral, and all cyclical Foraminifera, is not only to run counter to the indications of natural affinity that are furnished by the intimate structure of the shell, the conformation of the individual chambers, and the mode of their intercommunication, but to set at nought two general facts of fundamental significance,— first, that one plan of growth often graduates insensibly into another, as does the straight into the spiral in the group of which 10 and 11 of fig. 1 are extreme forms; and, second, that it is not at all uncommon for the plan to change during the growth of one and the same individual,—the spire, for example, either straightening itself out, so as to revert to the rectilinear type, or returning into itself so as to make a complete circle, round which a succession of concentric annuli is then produced on the cyclical type, while the original regularity is sometimes lost altogether in the “acervuline” piling up of the later-formed chambers. On the other hand, the fundamental importance of the perforation or imperforateness of the external envelope, as affecting the physiological condition of the contained animal, is now universally admitted. For where (as in Peneroplis, 5 in fig. 1) that envelope, whether composed of calcareous shell or built up by the cementation of sand-grains, has no other communication with the exterior than by the single or multiple aperture of the last chamber through which the whole pseudopodial apparatus of the contained animal is put forth, the nutrition of the entire segmental body that occupies the previously formed chambers must be carried on by a continual interchange of protoplasmic substance extending through the entire mass, however great may be the multiplication of its segments. Where, on the other hand (as in Rotalia, fig. 4), the wall of each chamber is perforated for the passage of pseudopodia, the segment it contains is thereby placed in direct communication with the surrounding medium from which it derives its sustenance, so as to be independent of the remainder of the series. The “imperforate” calcareous shells of Foraminifera are also termed “porcellanous,” from the opaque-white aspect they present when viewed by reflected light; though, when thin, they show a rich brown hue by transmitted light. Even under ⅜ high magnifying power, their substance appears entirely homogeneous. The perforated shells, on the other hand, having an almost glassy transparence (except where this is interrupted by tubulation), are known as “vitreous” or “hyaline.” Where the shells are thin, the perforations are simply pores (fig. 4); but where they are thick, the perforations are tubules usually running straight and parallel from surface to surface (fig. 24¾ Thus, then, the Foraminifera are naturally divisible in the first instance into two groups, the Imperforata and the Perforata; the former of which is again divisible into the Arenacea,...