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THE ENCYCLOPÆDIA BRITANNICA
A DICTIONARY OF ARTS, SCIENCES, LITERATURE AND GENERAL INFORMATION
ELEVENTH EDITION
VOLUME X SLICE VI
Foraminifera to Fox, Edward
Articles in This Slice
FORAMINIFERA FORT LEE FORBACH FORT MADISON FORBES, ALEXANDER PENROSE FORTROSE FORBES, ARCHIBALD FORT SCOTT FORBES, DAVID FORT SMITH FORBES, DUNCAN FORTUNA FORBES, EDWARD FORTUNATIANUS, ATILIUS FORBES, JAMES DAVID FORTUNATUS FORBES, SIR JOHN FORTUNATUS, VENANTIUS HONORIUS CLEMENTIANUS FORBES (town) FORTUNE, ROBERT FORBES-ROBERTSON, JOHNSTON FORTUNY, MARIANO JOSE MARIA BERNARDO FORBIN, CLAUDE DE FORT WAYNE FORCELLINI, EGIDIO FORT WILLIAM (Ontario, Canada) FORCHHAMMER, JOHANN GEORG FORT WILLIAM (Scotland) FORCHHAMMER, PETER WILHELM FORT WORTH FORCHHEIM FORTY FORD, EDWARD ONSLOW FORUM FORD, JOHN FORUM APPIIFORAMINIFERA, in zoology, a subdivision of Protozoa, the name selected for this enormous class being that given by A. D’Orbigny in 1826 to the shells characteristic of the majority of the species. He regarded them as minute Cephalopods, whose chambers communicated by pores (foramina). Later on their true nature was discovered by F. Dujardin, working on living forms, and he referred them to his Rhizopoda, characterized by pseudopodia given off from the sarcode (protoplasm) as organs of prehension and locomotion. W.B. Carpenter in 1862 differentiated the group nearly in its present limits as “Reticularia”; and since then it has been rendered more natural by the removal of a number of simple forms (mostly freshwater) with branching but not reticulate pseudopods, to Filosa, a distinct subclass, now united with Lobosa into the restricted class of Rhizopoda.
Anatomy.—Protista Sarcodina, with simple protoplasmic bodies of granular surface, emitting processes which branch and anastomose freely, either from the whole surface or from one or more elongated processes (“stylopods”); nucleus one or more (not yet demonstrated in some little known simple forms), usually in genetic relation to granules or strands of matter of similar composition, the “chromidia” scattered through the protoplasm; body naked, or provided with a permanent investment (shell or test), membranous, gelatinous, arenaceous (of compacted or cemented granules), calcareous, or very rarely (in deep sea forms) siliceous, sometimes freely perforated, but never latticed; opening by one or more permanent apertures (“pylomes”) or crevices between compacted sand-granules, often very complex; reproduction by fission (only in simplest naked forms), or by brood formation; in the latter case one mode of brood formation (A) eventuates in amoebiform embryos, the other (B) in flagellate zoospores which are exogamous gametes, pairing but not with those of their own brood; the coupled cell (“zygote”) when mature in the shelled species gives rise to a very small primitive test-chamber or “microsphere.” The adult microspheric animal gives rise to the amoebiform brood which have a larger primitive test (“megalosphere”); and megalospheric forms appear to reproduce by the A type a series of similar forms before a B brood of gametes is finally borne, to pair and reproduce the microspheric type, which is consequently rare.
Fig. 1B.—Protomyxa aurantiaca, Haeck. (After Haeckel.)1, Adult, containing two diatom frustules, and three Tintinnid ciliates, with a large Dinoflagellate just caught by the expanded reticulate pseudopodia.
2, Adult encysted and segmented.
3, Flagellate zoospore just freed from cyst.
4, Zoospore which has passed into the amoeboid state.
Fig. 2.—Allogromiidea.1, Diplophrys archeri, Barker.
a, Nucleus.
b, Contractile vacuoles.
c, The yellow oil-like body. Moor pools, Ireland.
2, Allogromia oviformis, Duj.
a, The numerous nuclei; near these the elongated bodies represent ingested diatoms. Freshwater. Figs. 2, 3, 11, 12 belong to Rhizopoda Filosa, and are included here to show the characteristic filose pseudopodia in contrast with the reticulate spread of the others.
3, Shepheardella taeniiformis, Siddall (Quart. Jour. Micr. Sci., 1880).
Marine. The protoplasm is retracted at both ends into the tubular case.
a. Nucleus.
5, Shepheardella taeniiformis; with pseudopodia fully expanded.
6-10, Varying appearance of the nucleus as it is carried along in the streaming protoplasm within the tube.
11, Amphitrema wrightianum, Archer, showing membranous shell encrusted with foreign particles. Moor pools, Ireland.
12, Diaphorophodon mobile, Archer.
a. Nucleus. Moor pools, Ireland.
The shells require special study. In the lowest forms they are membranous, sometimes encrusted with sand-grains, always very simple, the only complication being the doubling of the pylome in Diplophrys (fig. 2, 1), Shepheardella (fig. 2, 3-5), Amphitrema (fig. 2, 11), Diaphorophodon (fig. 2, 12). The marine shells are, as we have seen, of cemented particles, or calcareous, glassy, and regularly perforated, or again calcareous, but porcellanous and rarely perforate. These characters have been used as a guide to classification; but some sandy forms have so large a proportion of calcareous cement that they might well be called encrusted calcareous genera, and are also not very constant in respect of the character of perforation. The porcellanous genera, however, form a compact group, the replacement of the shell by silica in forms dwelling in the red clay of the ocean abysses, where calcium carbonate is soluble, not really making any difficulty. Moreover, the shells of this group show a deflected process or neck of the embryonic chamber (“camptopyle”) at least in the megalospheric forms, whereas when such a neck exists in other groups it is straight. The opening of the shell is called the pylome. This may be a mere hole where the lateral walls of the body end, or there may be a diaphragmatic ingrowth so as to narrow the entrance. It may be a simple rounded opening, oblong or tri-multi-radiate, or branching (fig. 4, 1); or replaced by a number of coarse pores (“ethmopyle”) (fig. 3, 5a). Again, it may lie at the end of a narrowed tube (“stylopyle”), which in Lagena (fig. 3, 9) may project outwards (“ectoselenial”), or inwards (“entoselenial”). In most groups the stylopyle is straight; but in the majority of the porcellanous shells it is bent down on the side of the shell, and constitutes the “flexopyle” of A. Kemna, which being a hybrid term should be replaced by “camptopyle.” The animal usually forms a simple shell only after it has attained a certain size, and this “embryonic chamber” cannot grow further. In Spirillina and Ammodiscus there is no pylomic end-wall, and the shell continues to grow as a spiral tube; in Cornuspira (fig. 3, 1) there is a slight constriction indicating the junction of a small embryonic chamber with a camptopyle, but the rest of the shell is a simple flat spiral of several turns. In the majority at least one chamber follows the first, with its own pylome at the distal end. This second chamber may rest on the first, so that the part on which it rests serves as a party-wall bounding the front of the newer chamber as well as the back of the older; and this state prevails for all added chambers in such cases. In the highest vitreous shells, however, each chamber has its complete “proper wall”; while a “supplementary skeleton,” a deposit of shelly matter, binds the chambers together into a compact whole. In all cases the protoplasm from the pylome may deposit additional matter on the outside of the shell, so as to produce very characteristic sculpturing of the surface.
Fig. 3.—Various forms of Calcareous Foraminifera.1, Cornuspira.
2, Spiroloculina.
3, Triloculina.
4,
5, Peneroplis.
6, Orbiculina (cyclical).
7, Orbiculina (young).
8, Orbiculina (spiral).
9, Lagena.
10, Nodosaria.
11, Cristellaria.
12, Globigerina.
13, Polymorphina.
14, Textularia.
15, Discorbina.
16, Polystomella.
17, Planorbulina.
18, Rotalia.
19, Nonionina.
Fig. 4.—Modifications of Peneroplis. 1, Dendritina; 2, Eu-Peneroplis.Compound or “polythalamic” shells derive their general form largely from the relations of successive chambers in size, shape and direction. This is well shown in the porcellanous Miliolidae. If we call the straight line uniting the two ends of a chamber the “polar axis,” we find that successive chambers have their pylomes at alternate poles; but they lie on different meridians. In Spiroloculina (fig. 3, 2) the divergence between the meridians is 180°, and the chambers are strongly incurved, so that the whole shell forms a flat spiral, of nearly circular outline. In the majority, however, the chambers are crescentic in section, their transverse prolongations being termed “alary” outgrowths, so that successive chambers overlap; when under this condition the angle of successive meridians is still 180° we have the form Biloculina (fig. 3, 4), or with the alary extensions completely enveloping, Uniloculina; when the angle is 120° we have Triloculina, or 144°, Quinqueloculina. Again in Peneroplis (figs. 3, 5, and 4) the shell begins as a flattened shell which tends to straighten out with further growth and additional chambers. In some forms (Spirolina, fig. 22, 3) the chambers have a nearly circular transverse section, and the adult shell is thus crozier-shaped. In others (which may have the same sculpture, and are scarcely distinguishable as species) the chambers are short and wide, drawn out at right angles to the axis, but in the plane of the spiral, and the growing shell becomes fan-shaped or “flabelliform” (figs. 3, 5, 4, 2). This widening may go on till the outer chambers form the greater part of a circle, as in Orbiculina (fig. 3, 6-8) where, moreover, each large chamber is subdivided by incomplete vertical bulkheads into a tier of chamberlets; each chamberlet has a distinct pylomic pore opening to the outside or to those of the next outer zone. In Orbitolites (figs. 5, 6) we have a centre on a somewhat Milioline type; and after a few chambers in spiral succession, complete circles of chambers are formed. In the larger forms the new zones are of greater height, and horizontal bulkheads divide the chamberlets into vertical tiers, each with its own pylomic pore.
Fig. 5.—Shell of simple type of Orbitolites, showing primordial chamber a, and circumambient chamber b, surrounded by successive rings of chamberlets connected by circular galleries which open at the margin by pores. Fig. 6.—Animal of simple type of Orbitolites, showing primordial segment a, and circumambient segment b, surrounded by annuli of sub-segments connected by radial and circular stolon-processes.The Cheilostomellidae (fig. 3, 13) reproduce among perforate vitreous genera what we have already seen in the Miliolida: Orbitoides (fig. 10, 2) and Cycloclypeus, among the Nummulite group, with a very finely perforate wall, recall the porcellanous Orbiculina and Orbitolites.
In flat spiral forms (figs. 22, 1, 7; 3, 2, 16, 19, &c.) all the chambers may be freely exposed; or the successive chambers be wider transversely than their predecessors and overlap by “alary extensions,” becoming “nautiloid”; in extreme cases only the last turn or whorl is seen (fig. 11). When the spiral axis is conical the shell may be “rotaloid,” the larger lower chambers partially concealing the upper smaller ones (fig. 3, 12, 15, 17, 18); or they may leave, as in Patellina, a wide central conical cavity—which, in this genus, is finally occupied by later formed “supplementary” chambers. When the successive chambers are disposed around a longitudinal central axis they may be said to “alternate” like the leaves of a plant. If the arrangement is distichous we get such forms as Polymorphina, Textularia and Frondicularia (fig. 3, 13, 14), if tristichous, Tritaxia. Such an arrangement may coexist with a spiral twist of the axis for at least part of its course, as in the crozier-shaped Spiroplecta.
Fig. 7.—Section of Rotalia beccarii, showing the canal system, a, b, c, in the substance of the intermediate skeleton; d, tubulated chamber-wall. Fig. 8.—Internal cast of Polystomella craticulata.a, Retral processes, proceeding from the posterior margin of one of the segments.
b, b¹, Smooth anterior margin of the same segment.
c, c¹, Stolons connecting successive segments and uniting themselves with the diverging branches of the meridional canals.
d, d¹, d², Three turns of one of the spiral canals.
e, e¹, e², Three of the meridional canals.
f, f¹, f², Their diverging branches.
Fig. 9.—Operculina laid open, to show its internal structure.a, Marginal cord seen in cross section at a’.
b, b, External walls of the chambers.
c, c, Cavities of the chambers.
c′, c′, Their alar prolongations.
d,
g, g, Non-tubular columns.
Two phenomena interfere with the ready availability of the characters of form for classificatory ends—dimorphism and multiformity.
Dimorphism.—The majority of foraminiferal shells show two types, the rarer with a much smaller central chamber than that of the more frequent. The chambers are called microsphere and megalosphere, the forms in which they occur microsphaeric and megalosphaeric forms, respectively. We shall study below their relation to the reproductive cycle.
Fig. 10.—1, Piece of Nummulitic Limestone from the Pyrenees, showing Nummulites laid open by fracture through the median plane; 2, vertical section of Nummulite; 3, Orbitoides. Fig. 11.—Vertical section of portion of Nummulites, showing the investment of the earlier whorls by the alar prolongations of the later.a, Marginal cord.
b, Chamber of outer whorl.
c, c, Whorl invested by a.
d, One of the chambers of the fourth whorl from the margin.
e, e′, Marginal portions of the enclosed whorls.
f, Investing portion of the outer whorl.
g, g, Spaces left between the investing portions of successive whorls.
h, h, Sections of the partitions dividing these.
Fig. 12.—Internal surface of wall of two chambers, a, a, of Nummulites, showing the orifices of its minute tubuli.b, b, The septa containing canals.
c, c, Extensions of these canals in the intermediate skeleton.
d, d, Larger pores.
Multiformity.—Many of the Polythalamia show different types of chamber-succession at different ages. We have noted this phenomenon in such crozier forms as Peneroplis, as well as in discoid forms; it is very frequent. Thus the microspheric Biloculina form the first few chambers in quinqueloculine succession. The microspheric forms attain to a greater size when adult than the megalospheric; and in Orbitolites the microsphere has a straight outlet, orthostyle, instead of the deflected camptostyle one, so general in porcellanous types; and the spiral succession is continued for more turns before reaching the fan-shaped and finally cyclic stage. Globigerina, whose chambers are nearly spherical, is sometimes seen to be enclosed in a spherical test, perforate, but without a pylome, and known as Orbulina; the chambered Globigerina-shell is attached at first inside the wall of the Orbulina, but ultimately disappears. The ultimate fate of the Orbulina shell is unknown; but it obviously marks a turning-point in the life-cycle.
Fig. 13.—Internal cast of two chambers, a, a, of Nummulites, the radial canals between them passing into b, marginal plexus.