The Foraminifera may be defined as Rhizopoda in which the body is protected by a shell or "test," composed of carbonate of lime, or of sand-grains cemented together, or, rarely, of chitine ; there is no distinct separation of the sarcode of the body into ectosarc and endosarc, and a nucleus and contractile vesicle are present in at any rate some cases. The pseudopodia are long and filamentous, and interlace with one another to form a network.
The Foraminifera are specially characterised by the possession of a "test" or external shell, which is usually composed of carbonate of lime, but is often composed of grains of sand or other adventitious solid particles cemented together by animal matter, or which, as in Gromia, may be simply chitin-ous. The test may be composed of an aggregation of chambers or "loculi" (fig. 11, c), or of a single chamber only, and its walls are usually pierced by numerous pores or "foramina" through which the pseudopodia are protruded ; the place of these being in other forms supplied by the large size of the terminal, or "oral" aperture of the shell (fig. 10, b), the walls themselves being imperforate. The presence or absence of foramina in the shell-walls is believed to constitute a genuine structural distinction, and the Foraminifera may be thereby divided into two great groups (Perforata and Imperforata).
As regards the soft parts of the Foraminifera, the body is composed of extensile and contractile sarcode - usually reddish or yellowish in colour - which not only fills the interior of the shell, but generally invests its outer surface also with a thin film, from which the pseudopodia are emitted (fig. 10, b). The test, therefore, in this case, is not a true cuticular secretion, like that of the Mollusca, but it is truly immersed within the sarcode of the body. The sarcode is not differentiated into a distinct ectosarc and endosarc, and until recently was believed to be devoid of a nucleus and contractile vesicle, and, indeed, of any organs or specialised parts of any kind. Recent researches, by Hertwig and F. E. Schultze, have, however, shown the presence of a nucleus and of contractile vesicles in, at any rate, some of the Foraminifera; and these structures are, therefore, probably present in all. Even in the polythalamous forms there seems to be, as a rule, only one nucleus, so that the organism morphologically may be regarded as a single cytode.
Fig. 10 - Foraminifera. a The animal of Nonionina, after the shell has been removed by a weak acid; b Gromia (after Schultze), showing the shell surrounded by a network of filaments derived from the body-substance.
The pseudopodia in all the Foraminifera (fig. 11, b, c) are filamentous and protrusible to a great length and they possess the singular property of uniting together in various directions so as to form a kind of network, like an "animated spider's web." (Hence the name Reticulosa applied to the order by Dr Carpenter.) This property, however, is not peculiar to members of this order, but is seen also in Actinophrys and in the Thalassicollida, though to a less extent. Further, throughout the entire network formed by the inosculating pseudopodia there is a constant circulation of minute protoplasmic granules in different directions.
Fig. 11 - Morphology of Foraminifera. a Lagena vulgaris, a monothalamous Fora-minifer ; b Miliola (after Schultze), showing the pseudopodia protruded from the oral aperture of the shell; c Discorbina (after Schultze), showing the nautiloid shell with the foramina in the shell-wall giving exit to pseudopodia; d Section of Nodosaria (after Carpenter) ; e Nodosaria hispida; f Globigerina bulloides.
The shells of Foraminifera may be classed, according to their composition, in three divisions, termed respectively the "porcellanous," the "hyaline" or "vitreous," and the "arenaceous." The porcellanous shell is calcareous and quite homogeneous in its composition, is opaque-white when seen by reflected light, and is not perforated by pseudopodial foramina. In these forms (e.g., Miliola, fig. 11, b) the pseudopodia are emitted solely from the mouth of the last-formed segment of the shell. The vitreous shell is also calcareous in composition, but is transparent and glassy in texture, and its walls are perforated by numerous pseudopodial apertures. The arenaceous Foraminifera (fig. 12) are among the largest of the living types, the test being sometimes half an inch or more in length. In its nature, the test is normally composed of siliceous particles embedded in an apparently chitinous matrix, with a notable proportion of peroxide of iron and a small percentage of carbonate of lime (H. B. Brady). It should be noted, however, that the test of Miliola, though normally calcareous and porcellanous, has been shown by Mr H. B. Brady to occasionally assume arenaceous characters, or even, when obtained from very great depths, to be composed of pure hyaline silica. It would appear, therefore, that the composition of the shell is liable to variation, in accordance with the nature of the materials obtainable at any particular station by the organism, so that too great stress cannot be laid upon this character in classification. As a rule, the arenaceous test is imperforate, and the pseudopodia are emitted by the terminal aperture of the shell; but cases are not unknown in which the walls are porous. Finally, there is a group of forms in which the test (as in Gromia) is composed simply of chitine. In some of the Foraminifera, hence called "simple" or "unilocular" (Monothalamia), the shell consists of a single chamber, and the animal is, in fact, nothing more than a little mass of sarcode enveloped in a calcareous covering. Lagena (fig. 11, a) with its beautiful flask-shaped shell, may be taken as the type of this division. Another well-known unilocular form is Entosolenia, which is like Lagena in shape, but has the tubular neck reversed, so as to be inserted into the interior of the test. In the more complex Foraminifera, the sarcode of the body undergoes a subdivision into partially separated segments, which may be produced by a process of budding, or, perhaps, by the occurrence of constrictions in the growing protoplasm, and each of these segments becomes more or less completely divided off from its neighbours, or enclosed by a wall of shell. In these "multilocular" or "polythalamous" Foraminifera, therefore, the shell ultimately comes to consist of a series of chambers, separated by partitions of the test, and filled with sarcode. The partitions, however, or "septa," between the different chambers, are perforated by one or more apertures, through which pass connecting bands, or "stolons," of sarcode; so that the sarcode occupying the different chambers is united into a continuous and organic whole. Each segment may give out its own pseudopodia through perforations in its investing wall (fig. 11, c), or the pseudopodia may be simply emitted from the mouth of the shell by the last segment only (fig. 11, b). In any case the direction in which the segments are developed is governed by a determinate law, and differs in different species, the form ultimately assumed by the shell depending wholly upon this. The forms, however, assumed by the shells of Foraminifera are extremely variable, even within the limits of a single species, and it would be impossible to notice even the chief types in this place. There are, however, two or three important variations which may be noticed. If the buds are thrown out from the primitive spherule in a linear series so as to form a shell composed of numerous chambers arranged in a straight line, we get such a type as Nodosaria (fig. 11, e). When the new chambers are added in a spiral direction, each being a little larger than the one which preceded it, and the coils of the spiral lying in the same plane, we get such a form as Discorbina (fig. 11, c), or Robulina. These are the so-called "nautiloid" Foraminifera, from the resemblance of the shell, in figure, to that of the Pearly Nautilus. From this resemblance the nautiloid Foraminifera were originally placed in the same class as the Ammonites (Cephalopoda), but their true position was shown by the examination of their soft parts. In the typical nautiloid shell the convolutions of the spiral all lie in one plane; but in other cases, as in Rotalia, the shell becomes turreted or top-shaped, in consequence of the coils of the spiral passing obliquely round a central axis.