We have already said that the shells of these minute zoophytes vary much in form. They are generally many-chambered, each chamber communicating by pores in the walls; the different gelatinous parts of the animalcules are, in this manner, placed in continual communication with each other. Alcide d'Orbigny, to whom we owe almost all that is known of the class, has distributed them into six families, making the form of the shell the basis of their arrangement. These six families include sixty genera, and more than sixteen hundred species, the families being as follows:-- I. Monostega.--Animals consisting of a single segment. Shell of a single chamber. II. Stichostega.--Animal in segments, arranged in a single line. Shell in chambers, superimposed linearly on a straight or curved axis. III. Helicostega.--Animal in segments, spirally arranged. Chambers piled or superimposed on one axis, forming a spiral erection. In Fig. 21 we have a horizontal section of _Faujasina_, in which the spiral convolutions are visible on the truncated half of the shell. IV. Entomostega.--Animal composed of alternating segments forming a spiral. Chambers superimposed on two alternating axes, also forming a spiral. V. Enallostega.--Animal formed of alternate segments. Non-spiral chambers disposed alternately along two or three axes, also non-spiral. VI. Agathistega.--Animal formed of segments wound round an axis. Chambers formed round a common axis, each investing half the circumference. The simplest form of Foraminifera is illustrated by Fig. 14 (_Orbulina universa_), which is a small spherical shell, having a lateral aperture, the interior of which has been occupied by the living jelly, to which the shell owes its existence. In the second order, the shell (Fig. 15), _Dentalina communis_, advances beyond this simple type by a process of linear budding, the first cell being spherical, with an opening through which a second segment is formed, generally a little larger than the first. This new growth is successively followed by others developed in the same way, until the organism attains its maturity, when it exhibits a series of cells arranged end on end, in a slightly curved line. In the next group the gemmation takes a spiral bias, producing the nautilus shape which misled the earlier naturalists. In some cases all the convolutions are visible, as in _Operculina_ (Fig. 16). In others, the external convolute conceals those previously formed, as in _Nummulitis lenticularis_ (Fig. 17), _Cassidulina_ (Fig. 18), _Textilaria_ (Fig. 19), and _Alveolina oblonga_, d'Orbigny (Fig. 25), the latter forming part of the eocene formation in the quartz and greystone rocks of the neighbourhood of Paris; one figure representing the shell entire, and the other a vertical section, while the small figure between represents it in its natural size. [Illustration: Fig. 14. Orbulina universa.] [Illustration: Fig. 15. Dentalina communis.] [Illustration: Fig. 16. Operculina.]