Foraminifera

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Foraminifera

 

an order of Protozoa of the subclass Rhizopoda of the class Sarcodina. There are more than 1,500 species of foraminifers. The cytoplasmic body of most species is clad in a limestone shell (an external skeleton). A few species have chitinous shells or shells consisting of foreign particles (grains of sand, sponge spicules) cemented together by secretions of the cytoplasm. The unicameral or polycameral shells are sometimes branching. The variation in arrangement and shape of the chambers (in one or two rows, in a spiral) results in a diversity of skeletal shapes. The shell is usually 0.1–1.0 mm long but occasionally reaches 20 cm. Stomata connect the interior of the shell with the external environment; in many species there also are numerous pores in the wall of the shell. Through the stomata and the pores protrude slender branching and anastomosing pseudopodia, which serve to capture food and for locomotion.

All foraminifers are characterized by the alternation of haploid and diploid generations. The asexual diploid generation—the agamont—develops from the zygotes. The nucleus repeatedly divides as it grows and the organism becomes multinuclear. The two final nuclear divisions involve meiosis, after which the agamont decomposes into numerous (according to the number of nuclei) agametes. The agametes give rise to the haploid sexual generation—the gamont—whose growth and development are completed by the formation of gametes. Some foraminifers have motile flagellate isogametes, which enter the water and copulate, forming diploid zygotes. In plastogamic species the gamonts first join (most often in pairs) in syzygy and then form amoeboid or drop-shaped gametes. The diploid zygote gives rise to an agamont.

Foraminifers are marine, predominantly benthic organisms. Only two families—Globigerinidae and Globorotalidae—are planktonic. Foraminiferal shells form a substantial portion of oceanic slimes. Fossils are known from the Cambrian, although the organisms probably appeared in the Precambrian. In the first floraminifers the shell was organic and unicameral; species with multicameral limestone shells appeared later. Foraminifers reached the height of their development in the Carboniferous-Permian, with the appearance of the Fusulina and closely related forms whose shells formed substantial layers of limestones after the organisms died. These groups of foraminifers became extinct at the end of the Paleozoic. During the Mesozoic-Cenozoic new groups appeared, of which the best known are the Nummulites, which had a large, coin-shaped shell. The new groups played a role in rock formation. Paleontologists consider the Foraminifera a subclass of Protozoa; the subclass includes 13 orders, with many families and genera. Fossil foraminifers have great significance in Paleozoic, Mesozoic, and Cenozoic stratigraphy.

REFERENCES

Dogel’,.V. A., Iu. I. Polianskii, and E. M. Kheisin. Obshchaia protozoologiia. Moscow-Leningrad, 1962.
Osnovy paleontologii: Obshchaia chast’, Prosteishie. Moscow, 1959.

IU. I. POLIANSKII and V. N. SHIMANSKII

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