 | |
| Fig. 11.—Vertical section of portion of Nummulites, showing theinvestment of the earlier whorls by the alar prolongations of thelater. | |
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. |
Protoplasmic Body and Reproduction.—The protoplasm is not differentiated into ecto- and endosarc, although it is often denser 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 Globigerina 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 cytoplasm contains granules of various kinds, and the internal protoplasm is sometimes pigmented. The Chrysomonad 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 Orbitolites.
The nucleus is single in the Nuda and Allogromidia and in the megalospheric forms of higher Foraminifera; but microspheric forms when adult contain many simple similar nuclei. The nucleus in every case gives off granules and irregular masses (“chromidia”) of similar reactions, which play an important part in reproduction. During the maturation of the microsphere the nuclei disappear; and the cytoplasm breaks up into a large number of zoospores, each of which is soon provided with a single nucleus, whether entirely derived from the parent-nucleus or from the coalescence of chromidia, or from both these sources is still uncertain. These zoospores are amoeboid; they soon secrete a shell and reveal themselves as megalospheres, the original state of the megalospheric forms. In the adult megalosphere the solitary nucleus disappears and is replaced by hosts of minute vesicular nuclei, formed by the concentration of chromidia. Each nucleus aggregates around it a proper zone of dense protoplasm; by two successive mitotic divisions each mass becomes quadri-nucleate, and splits up into four biflagellate, uninucleate zoospores. These are pairing-cells or gametes, though they will not pair with members of the same brood. In the zygote resulting from pairing two nuclei soon fuse into one; but this again divides into two; an embryonic shell is secreted, and this is the microspheric type, which is multinuclear from the first. F. Schaudinn compares the nuclei of the adult Foraminifera with the (vegetative) meganucleus of Infusora (q.v.) and the chromidial mass with the micronucleus, whose chief function is reproductive.
| Fig. 14.—Vertical section of tubulated chamber-walls, a, a, of Nummulites. b, b, Marginal cord; c, cavity of chamber; d, d, non-tubulated columns. |
Since megalospheric forms are by far the most abundant, it seems probable that under most conditions they also give rise to megalospheric young like themselves; and that the production of zoospores, 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 Mikrogromia the cell undergoes fission within the test, and on its completion the daughter-cells may emerge as biflagellate zoospores.