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.
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Fig. 13.—Internal cast of two chambers, a, a, of Nummulites, the radial canals between them passing into b, marginal plexus. |
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.
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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.
The sandy shells are a very interesting series. In Astrorhiza 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 form 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 labyrinthic (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.
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| Fig. 15.—Cycloclypeus. |
It is noteworthy that though of similar habitat each species selects its own size or sort of sand, some utilizing the siliceous spicules of sponges. Despite the 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 simple. As we can find no record of a deflected stylopyle to the primitive chamber of the polythalamous Arenacea, it is safe to conclude that they have no close alliance with the Porcellanea.
Classification.
I. Nuda.—Protoplasmic body without any pellicle or shell save in the resting encysted condition, sometimes forming colonial aggregates by coalescence of pseudopods (Myxodictyum), or even plasmodia (Protomyxa). Brood cells at first uniflagellate or amoeboid from birth. Fresh-water and marine genera Protogenes (Haeckel), Biomyxa (Leidy), Myxodictyum (Haeckel), Protomyxa (Haeckel) (fig. 1B).
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.
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| Fig. 16.—Heterostegina. |
II. Allogromidiaceae (figs. 1A, 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 known. Fresh-water or marine genera Allogromia (Rhumbl.), Myxotheca (Schaud.), Lieberkühnia (Cl. & L.) (fig. 1A), Shepheardella (Siddall) (fig. 2, 3-10), Diplophrys (Barker), Amphitrema (Arch.) (fig. 2, 11), Diaphorophodon (Arch.) (fig. 2, 12), are possibly Filosa. This group differs from the preceding in its simple test, but, like it, includes many fresh-water species, which possess contractile vacuoles.
III. Astrorhizidiaceae.—Simple forms, rarely polythalamous (some Rhabdamminidae), but often branching or radiate; test arenaceous, loosely compacted and traversed by chinks for pseudopodia (Astrorhizidae), or dense, and opening by one or more terminal pylomes at ends of branches. Marine, 4 Fam. The test of some Astrorhizidae is so loose that it falls to pieces when taken out of water. Haliphysema is remarkable for its history in relation to the “gastraea theory.” Pilulina has a neat globular shell of sponge-spicules and fine sand. Genera, Astrorhiza (Sandahl)
