Rhizopod Protozoa with long branching and anastomosing psendopodia which show well-marked granule-streams. The protoplasm is of a uniform character throughout the body; the nucleus single; contractile and non-contractile vacuoles are very rare. A test or shell is invariably present either chitinoid, calcareous or adventitious; it presents many varieties of shape and structure. Reproduction is typically effected by the multiplicationi of the nucleus, the separation of a portion of protoplasm round each nucleus so formed, and its inch sure by a test. Chiefly marine; a few freshwater.
The test is the most prominent feature of the Foraminifera. It is chitinoid in the Gromidae; very delicate in Lieberkiihnia, stouter but flexible in Gromia itself, resistent in Microgromia, and in Diaphoropodon incrusted with diatom shells, etc. It is hyaline or somewhat yellow; ovate with a single terminal aperture, except in Shepheardella, where it is a tube with an aperture at both ends. There is reason to suppose that it is porous in Diaphoropodon 1.
A calcareous test is characteristic of the Miliolidae, Texttdaridae, Cheilostomellidae, Lagenidae, Globigerinidae, Rotalidae and Nummulinidae. In the Miliolidae its substance is compact in texture, homogeneous with a polished white or 'porcellanous' appearance by reflected light, in thin shells, or in thin sections viewed by transmitted light, amber-coloured. Young specimens are opalescent and diaphanous. In the other families above-named the substance of the test is traversed by fine vertical pores, varying in diameter and sometimes of two different sizes in the same test, e. g. in Orbidina and some other Globigerinidae, often laminated and in thin tests or sections transparent or hyaline. When the test is thin, and its pores relatively far apart and wide, it has a vitreous aspect; when thick, the pores fine and close set, it is milky and semiopaque. Some are perfectly opaque like Calcarina, and this is always the case if they are dead and have been lying long in sea-water. Owing to the absence of pores the Miliolid test is often termed 'imperforate,' that of the other families where pores are present, 'perforate' designations sometimes employed as classificatory.
There is an organic basis, more or less plentiful, visible after careful decalcification, and a distinct chitinoid membrane of varying thickness lines the cavity of the test, its passages and pores1 .The principal mineral constituent is Calcium carbonate, as Calcite in the perforate test, but possibly in the form of Arragonite in the imperforate. Among Miliolidae, Bilocidina has been found to contain as much as 7-10 per cent, of Silica, and Orbitolites about 10 per cent, of Magnesium carbonate with a trace of Silica. In the perforate series it has been observed that the tests of pelagic Globigerinae are entirely soluble in acid; and that those of two Nummulids, Amphistegina Lessonii and Operculina complanata, contain 5 per cent, of Magnesium carbonate, with traces of phosphates, Silica, Ferric and Aluminic oxides. Grains of sand are in some instances attached to the outside of the test, e. g. frequently in the Miliolid Nnbe-cularia; and in the larger Textularinae the quantity of sand thus added may be very great, the perforate calcareous basis sometimes disappearing. The perforate Polytrema and Carpenteria, especially the latter, use sponge-spicules as a foundation for the calcareous crust.
In species which are able to live in brackish water the amount of calcareous matter diminishes until in one instance, Miliolina (= Quinqiieloculina) fusca, the test becomes chitinoid with minute impacted sand-grains; so too in Entzia from a salt-pool at Deva in Transylvania. The tests of Miliolina from abyssal depths (3950 fathoms) in the North Pacific, where the bottom is a Radiolarian ooze, are reduced to delicate homogeneous siliceous films.
1The test of Microgromia withstands the action of concentrated acids and alkalies; it is perhaps silicified. The short hyaline simple processes fringing the body of Diaphoropodon appear to be pseudopodial (Archer, Q. J. M. ix p. 396).
The calcareous test affords many varieties of shape. It may consist of a single or of many chambers; in other words, it is mono- or poly-thalamous, a distinction which has been used for classificatory purposes, but has no significance so far as the intimate structure of the organism is concerned (infra, p. 891). It is nearly always free, seldom attached, and its growth as a rule follows a definite plan. Sometimes however it is really irregular, though outwardly symmetrical, as in the lenticular Nummuline Archaediscus, which consists of a tube coiled on itself in varying directions, but inclosed by a thick and finely perforated calcareous mass; or it becomes irregular, and this is especially the case when it is attached, e. g. in the imperforate Nnbecitlaria, the perforate Carpenteria, where in C. raphidodendron it is arborescent. But in most instances of this kind growth is at first regular. Exceptions to this rule however occur among the Rotalid Tinoporinae, e. g. in the encrusting or arborescent Polytrema, or in the Lagenid Ramulina, with chambers connected to one another by long tubes.
The single-chambered test may be more or less spherical, as in the imperforate Squamula, the perforate Orbulina, vasi-form, as in the perforate Lagena, or piano-spiral, as in the imperforate Comuspira and the perforate Spirillina. When the test is polythalamous, its chambers may follow one another in a more or less straight or arcuate and single series, e. g. Nodosaria, seldom in a bi- or tri-serial series, as is sometimes the case in Polymorphina, or they are disposed in a spiral with the elements lying either in the same plane, e. g. in Biloculina, the nautiloid Polystomellinae and Ntimmulites, or in a more or less helicoid or trochoid manner as is generally the case. Successive chambers or coils may inclose their predecessors, i. e. the shell becomes involute, e. g. in the imperforate Bilocirtina and the perforate Hastigerina1. Spiral growth may give way to a cyclical, due to the excessive widening in a planospiral test of the peripheral chambers, which meet sooner or later round the first formed portion of the shell, as in some species of Orbicidina and the genus Orbi-tolites, among imperforate forms, and the Cycloclypeinae among perforate 2. Or the axis of convolution may be lengthened out, as in the Alveolinae and Fusulininae respectively, in the imperforate and perforate series, the chambers consequently attaining a great width and the shell a fusiform aspect, successive coils inclosing their predecessors completely.