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.
| Fig. 1A.—Lieberkühnia, with reticulate pseudopodia. |
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.
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| Fig. 1B.—Protomyxa aurantiaca, Haeck. (After Haeckel.) | |
1, Adult, containing two diatomfrustules, and three Tintinnidciliates, with a large Dinoflagellatejust caught by theexpanded reticulate pseudopodia. | 2, Adult encysted and segmented. 3, Flagellate zoospore just freedfrom cyst. 4, Zoospore which has passedinto the amoeboid state. |
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| Fig. 2.—Allogromiidea. | |
1, Diplophrys archeri, Barker. 2, Allogromia oviformis, Duj. 3, Shepheardella taeniiformis,Siddall (Quart. Jour. Micr.Sci., 1880). | 5, Shepheardella taeniiformis;with pseudopodia fullyexpanded. 6-10, Varying appearance of thenucleus as it is carried alongin the streaming protoplasmwithin the tube. 11, Amphitrema wrightianum,Archer, showing membranousshell encrusted with foreignparticles. Moor pools, Ireland. 12, Diaphorophodon mobile,Archer. |
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.
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| Fig. 3.—Various forms of Calcareous Foraminifera. | ||
1, Cornuspira. 2, Spiroloculina. 3, Triloculina. 4, Biloculina. 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.