Foraminifera

Foraminifera, a group of Rhizopoda (q.v.) in which the body is protected by a shell or "test," through openings in which numerous fine filaments (pseudopodia) are emitted. The shell is composed either of carbonate of lime, silica, chitin, or grains of sand cemented together by the same chitinous or horny substance. The openings through the shell are either minute and numerous or larger and confined to one particular area or mouth; the pseudopodia are long, and interlace to form a network around the test, which is often completely covered by a layer, formed by the fusion of the bases of the pseudopodia. The protoplasm is uniform and is not differentiated into ectoplasm and endoplasm, as in the Ameeba (q.v.)., A nucleus is, of course, present, and is at first always single : when reproduction is about to take place it subdivides. The shell is the most important structure in the group, and the various classifications proposed have been based upcn it. Of these, the most important was one that divided the foraminifera into two groups; the Perforata, and the Imperforata. The former included all those forms in which the test is perforated by numerous minute pores and of which the substance appeared hyaline or glassy. The Imperforata included those in which the shell substance is chitinous, porcellanous or composed of sand grains, and which is not penetrated by pores, the pseudopodia all issuing at the mouth. It has, however, now been found that such an arrangement is quite artificial, and perforate and imperforate forms are now included in the same family, as in the Litnolidec. The test may be simple as in Orbuliiia, or composed of many simple chambers united into a single (polythalamic) individual as Glohigerina. In many of the complex forms, such as the Nummulites, the separate chambers are separated by an "intermediate skeleton" through which a "canal system" ramifies, and across which various "stolen passages" maintain the connection between the chambers. Most of the Foraminifera have either a straight, coiled, spiral, crozier, or disc-shaped test, but in the arenaceous forms it is often very irregular; thus in the recent Hgperammina it is tubular, and in the allied fossil Girvanella it is a much-coiled tube. With very rare exceptions, the Foraminifera are all marine; they are universally distributed through the oceans, living on the surface or at great depths. Their tests accumulate at the bottom as a chalky ooze which covers great tracts of the ocean floor at depths of from 250 to 3,000 fathoms. As rock builders they have played an equally important part in the past, as Fitsulina in the Carboniferous period, Globiyerina-, etc., in the chalk and

Nummulites in the Eocene have helped to build up great sheets of limestone. The earliest known fossil form occurs in the Ordovician. Eozoon (q.v.) was once regarded as an earlier representative of the group, but the theory of its organic origin is now discredited.