Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V10, Page 632 of the 1911 Encyclopedia Britannica.
Spread the word: it!
FORAMINIFERA, 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 "l; 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'broodformation (A) eventuates in moebiform 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. 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, I), Shepheardella (fig. 2, 3-5), Amphitrema (fig. 2, ir), 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 4 z, Adult, containing two diatom a, Adult encystedandsegmented. frustules,andthreeTintinnid 3, Flagellate zoospore just freed ciliates, with a large Dino- from cyst. flagellate just caught by the 4, Zoospore which has passed expanded reticulate pseudo. into the amoeboid^state. podia. 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 porcellanbus 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 Marine. The protoplasm is retracted at both ends into the tubular case. a, Nucleus. 5, Shepheardella taeniiformis; with pseudopodia fully expanded. 6-lo, Varying appearance of the nucleus as it is carried along in the streaming protoplasm within the tube. zr, Amphitrema wrightianum, Archer, showing membranous shell encrusted with foreign particles. Moor pools, Ire-land. 12, Diaphorophodon mobile, Archer. [land. a, Nucleus. Moor pools, Ire- the newer chamber as well as the back of the older; and this state prevails for all added chambers in such cases. In the r, 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, t I, 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. Mice. Sci., 1880) . 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. 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 1, Cornuspira. 8, Orbiculina (spiral). 14, Textularia. 2, Spiroloculina. 9, Lagena. 15, Disconbina. 3, Triloculina. 10, Nodosaria. 16, Polystomella. 4, Biloculina. 11, Cristellaria. 17, Planorbulina. 5, Peneroplis. 12, Globigerina. 18, Rotalia. 6, Orbiculina (cyclical). 13, Polymorphina. 19, Nonionina. 7, Orbiculina (young). have their pylomes at alternate poles; but they lie on different meridians. In Spiroloculina (fig. 3, 2) the divergence between the meridians is 18o°, 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 18o° 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 sub-divided 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 .r '- %' / • ~r -ems\ g( N^.10is-r-I ,e ',kw jl 1110 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. The Cheilostomellidae (fig. 3,13) re-produce among perforate vitreous genera what we have already seen in the Miliolida: Orbitoldes (fig. 1(3,2) 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, 1.9, &c.) all the chambers may be freely exposed; or the successive chambers be wider transversely than their predecessors 2 1, Dendritina; 2, Eu-Peneroplis. a, an and overlap by " alary extensions," becoming " nautiloid "; in extreme cases only the last turn or whorl is seen (fig. II). 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 " supplement- ary" chambers. When Retral processes, proceeding from the posterior margin of one of the segments. b, b', Smooth anterior margin of the same segment. c, c', Stolons connecting successive segments and uniting themselves with the di- is distichous we get such forms as Polymorphina, Textularia and Frondicularia (fig. 3, 13, 14), if tristichous, Tritaxia. Such interseptal canals, the general distribution of which is seen in the septa e, e; the lines radiating from e, e point to the secondary pores. at d", so as to lay open the g, g, Non-tubular columns. arrangement may coexist with a spiral twist of the axis for at least part of its course, as in the crozier-shaped Spiroplecta. 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 2 M, GS Ss and megalosphere, the forms in which they occur microsphaeric and megalosphaeric forms, respectively. We shall study below their relation to the reproductive cycle. Multiformity.—Many of the Polythalamia show different types of chamber-succession at different ages. We Have noted Fm. 11.—Vertical section of portion of Nummulites, showing the investment of the earlier whorls by the alar prolongations of the later. a, Marginal cord., f, Investing portion of the b, Chamber of outer whorl. outer whorl. c, c, Whorl invested by a. g, g, Spaces left between the in- d,, One of the chambers of the vesting portions of succes- fourth whorl from the sive whorls. margin. [closed whorls. h, h, Sections of the partitions e, e', Marginal portions of the en- dividing these. 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 t yp es; 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. Protoplasmic Body and Reproduction.—The protoplasm is not differentiated into ecto- and endosarc, although it is often denser a c e, e2, f,ft,f, verging branches of the meridional canals. d, d', d2, Three turns of one of the spiral canals. Three of the meridional canals. Their diverging branches. a, b, b, c, c, c' c' , d, d, Marginal cord seen in cross section at a'. [chambers. External walls of the Cavities of the chambers. Their alar prolongations. Septa divided at d', d', and b, b, The septa containing canals. c, c, Extensions of these canals in the intermediate skeleton. d, d, Larger pores. in the central part within the shell, and clearer in the pseudopodial ramifications and the layer (or stalk in the monothalamic forms) from which it is given off. In pelagic forms like Globigerino the external layer is almost if not quite identical in structure with the extracapsular protoplasm of Radiolaria(q.v.), being differentiated into granular strands traversing a clear jelly. rich in large vacuoles (alveoli), and uniting outside the jelly to form the basal layer of the pseudopods; these again are radiolarian in character. Hence E. R. Lankester justly enough compares the shell here to the central capsule of the Radiolarian,` though the comparison Must not be pushed too far. The cyto- om© , plasm contains granules of 6 a' - ate ¢ various kinds, and the in- 1 sees ternal protoplasm Is some-times pigmented. The Chrysotnonad Flagellate, Zooxanthella, so. abundant,.; in its resting state—the, so-called " yellow cells "—in the extracapsular protoplasm of Radiolaria (q.v.) also occurs in the outer protoplasm of many Foraminifera, not only pelagic but also bottom-( dwellers, such as Orbitoliles. The nucleus is single in the 4 / Nuda .and Allogromidia and in the megalospheric forms d pairing to pass into the microspheric form, is only occasional, and possibly seasonal. This life-history we owe to the researches of Schaudinn and J. J. Lister. In several species (notably Patellina) plastogamy, the union of the cytoplasmic bodies without nuclear fusion, has been noted, as a prelude to the resolution of the conjoined protoplasm into uninucleate amoebulae. Calcituba, a porcellanous type, which after forming the embryonic chamber with its deflected pylome grows into branching stems, may fall apart into sections, or the protoplasm may escape and break up into small amoebulae. Of the reproduction of the simplest forms we know little. In Mikrogrdtnia the dell undergoes fission within the test, and on its completion the daughter-cells may emerge as biflagellate zoospores. The sandy shells. are a very interesting series. In Asirorhiza the sand grains are loosely agglutinated, without mineral cement; they. leave numerous pores for the exit of the protoplasm, and there are no true pylomes. In other forms the union of the grains by a calcareous or ferruginous cement necessitates the existence of distinct pylomes. 'Many of the species reproduce the 'varieties of farm found in calcareous tests; some are finely perforated, others not. Many of the larger ones have their walls thickened internally and traversed by complex passages; this structure is called laby- rinthic (fig. 19, g, h). The shell of Endothyra, a form only known to Us by its abundance in Carboniferous and Triassic strata, is largely composed of calcite: and is sometimes perforated. It is noteworthy that though of similar habitat each species selects its own sizgg,orsort of sand, some utilizing the siliceous spicules of sponges. L7espitethe roughness of the materials, they are often so laid as to yield a perfectly smooth inner wall; and sometimes the outer wall may be as eimpie. As we can find no record of a deflected stylopyle to the primitive chamber of the polythalamous Arenacei, it is safe to conclude that they have no close alliance with the Porcellanea. Classification. I. NtJBA.-Protoplasmic body without any Pellicle or shell save in the resting encysted condition, sometimes forming colonial aggregates by coalescence of pseudopods (Myxo-,d-ictyurn), or even plasmodia (Protomyxa). Brood-cells at first uniflagellate or amoeboid from birth. Fresh-water and marine genera Protogenes (Haeckel), Biomyxa (Leidy), Afyxod ctyum (Haeckel), Prototnyxa (Haeckel) (fig. Is). This group of very simple forms includes many of Haeckel's Monera, defined as " cytodes," masses of protoplasm without a nucleus. A nucleus (or nuclei) has, however, been demonstrated by improved methods of staining in so many that it is probable that this distinction will fall to the ground. II. ALLOGROMIDIACEAE (figs. I A, 2) .—Protoplasmic body protected in adult state by an imperforate test with one or two openings (pylomes) for the exit of the stylopod; test. simple, gelatinous, membranous, sometimes incrusted with foreign bodies, never • calcareous nor arenaceous; reproduction by fission alone 'krroysn; Fresh-water or marine nera Aliegronud ZRhumbl.)', M.~yxothecq (Schaud.), Cieberktlhnia (Cl. & L.) (fig. IA), She jiheardelia (Siddall)' (fig:., a, -zo); Diplophrys'' (Barker"), Amphilrema (Arch.) (fig. a, ix), Diaphorophodon (Areh.)ffig 2; irzlo),sare. apossiTbly FIG. 16,—Heterostegina. Fhis group differs from the preceding in its simple test, but, like it, includes many fresh-water species, which possess contractile vacuoles.
End of Article: FORAMINIFERA

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.