Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/392

 378 FOBAM.INIFEBA nected by a passage with those on cither side of it ; and it is from these intermediate passages, not from the chamberlets themselves, that the radiating passages proceed which lead to the chamberlets of the next ring, or which, in the case of the outermost ring, open at the margin between the protuberant walls of the chamberlets. So, in looking at the sarcodic body (tig. 12) of the animal which occupied the shell, we see that it consists of a succession of neck lace-like rings, each consisting of an annular cord with bead-like enlargements at regular intervals, and giving off from the intervals between these beads the radiating stolon-processes which give origin to the sub-segments of the next annulus, as shown at the margin cf the figure. These, extending themselves laterally at the growing edge of the shelly disk, will coalesce into a continuous cord, and a new ring of shell-structure will be formed by an excretion of calcareous matter from its surface. There is here no differentiation of parts whatever, every ring being a precise repetition (except as to number of parts) of that which preceded it ; and in the living condition the semi-fluidity of the sarcode-body will allow of a perfectly free communication being kept up throughout the disk, so that nutrient matter taken in at the margin may be readily conveyed towards the centre. The uniformity in the endowments of the segments is shown by the fact, of which accident has repeatedly furnished proof, that a small portion of a disk entirely separated from the remainder, will not only continue to live, but will so increase as to form a new disk (fig. 13) whose concentric annuli form themselves regularly around the fragment, obviously by the extension of the annular sarcodic band from its proper growing margin around its whole periphery, Fits. 13. OrbitolHe disk formed around a fragment of a previous disk, by the extension of the growing margin cf the fragment along its broken edges. a fact of extreme significance, as showing that even in this elabo rately constructed shell the animal body retains its original protoplasmic homogeneousness. A yet greater complexity of structure presents itself in the large Orbitolite-disks of tropical seas, as also in those of whose fossil remains many Tertiary Limestones are chiefly composed. This com plexity depends, in the first place, on the vertical elongation of the sub-segments into a columnar form, so as to produce a progressive thickening of the disk from the centre towards the margin, by which it comes to acquire a somewhat biconcave form ; and with this there is an increase in the number of radiating stolon-processes, showing itself in a vertical multiplication of the ranges of marginal pores. The ends of the columns of each ring are connected laterally by two annular cords ; and from each of these is given off a series of sub- segments, that forms a layer of chamberlets on cither surface of the disk, having no communication with those of the intermediate col umnar structure except through the annular passages. Even this morphological differentiation does not seem connected with any diversity of function ; and a careful comparison of a sufficiently extensive series of forms, from the most simple to the most complex, renders it clear that they are gradationally connected, the transition being a process of &quot;evolution,&quot; on which heat (probably coupled with a more copious supply of food) exerts an important influence. III. VITREA. The vitreous-shelled Foraminifera consti tute the most elevated division of the group, in regard alike to the high organization of their shell-substance, and to the elaborate construction shown in the shells of its most developed types. And it is in this division that we find the multiplication of individuals accomplishing by far the greater part of those two vast operations now in progress (as they seem to have been through all Geological time of which we know anything), which have been already specified as the great functions of the group: the extraction from sea-water (1) of the minute quantity of organic matter universally diffused through it, which is converted into a protoplasmic substance fit to serve as food for higher animals, and (2) of the minute quantity of carbonate of lime held by it in solution, which is solidified into the sub stance of the shells whose accumulation gives rise to calcareous deposits on the ocean-bottom. The Vitreous Foraminifera may be grouped into three families, Layenida, Globigerinida, and Nummulinida, the typical forms of which are distinguished by well-marked characters. The chief feature of interest in the Family Lagcnida is the unbroken succession of forms it presents, from the simple monotha- lamous Lagena to the complex polythalamous Cristdlaria, all characterized by the very minute tubulation of the shell-substance, and by the circular form of the aperture of each chamber, which has usually an everted lip marked by radiating furrows. The Lagena (9 in fig. 1) is a minute flask-shaped shell, with a prolonged neck, having the mouth at its extremity ; and its external surface usually presents some form of sculpture, such as prolonged ribs, separate tubercles, or honeycomb areolation. In Nodosaria (10 in fig. 1) we find a rectilinear succession of similar chambers, the neck of each re ceived into the cavity of the next ; wliile in Cristdlaria (11 in fig. 1) a series of progressively enlarging chambers, still presenting the characteristic radiate aperture, is arranged in a nautiloid spiral. These composite types, which are connected by a series of inter mediate degrees of curvature, show also the characteristic sculptur ing of the simple Lagena with an endless variety of modifications ; so that there is no group of Foraminifera which presents a more complete gradation between extremes so extremely diverse in forn tional term, the whole ought to constitute a single species. But governed by the exigencies of a partially artificial arrangement, the modifications embraced in this single unbroken series constitute thirteen genera, or almost two entire families, in Professor Reuss .s classification ; while how many hundreds, if not thousands, of so- called species have been founded upon the trivial characters above enumerated, it would m-ed much patience to ascertain.&quot; This variability shows itself also in the curious &quot; polymorphism &quot; sometimes exhibited during the growth of one and the same individual, which is especially characteristic of the genus Polymorphina, The young shell of this type is obviously made up of lagenifonn chambers regularly arranged in a double series alternating on the two sides of a rectilinear axis (13 in fig. 1) ; but from this plan there are often very wide de partures in later stages of growth, both in the form and in the arrangement of the chambers, one of the most curious con sisting in the backward prolongation of the last chamber over all that preceded it, and its extension into &quot; stags-horn processes,&quot; which completely disguise the original conformation. In this aber rant type the characteristic &quot; sculpturing &quot; of the typical Lagcnida is generally wanting, the shell in its young state being peculiarly thin and glassy ; whilst in the large &quot; wild-growing &quot; forms the wall of the outer chamber is usually smooth, and has a porcellanous opacity. The fundamental characters of the Family Gloligcrinida, which essentially consist in the coarse porosity of the shell (the pores being often -Ernnrth of an inch in diameter), and in the fissured, cresoen- tic, or semioval form of the aperture of each chamber, are well seen in the typical genus Globigerina, the principal agent in that pro duction of globigerina-ooze which is nonrepeating (rather, perhaps, continuing) the formation of the old Chalk. As obtained from the bottom by sounding or dredging, the Globigerina presents itself in the form shown in fig. 14, that is, as an aggregation of nearly spher ical chambers, having coarsely porous walls like those of Rotalia FIG. 14.Globiyerina bulloidts, as seen in three positions. (fig. 4), and cohering externally into a more or less regular turbinoid spire, each turn of which consists of four chambers, progressively increasing in size. These chambers, whose number seldom exceeds 1 Monograph of Carboniferous and Permian Foraminifera, p. 11.