The foraminifera are marine animals of low organization, and, with but few exceptions, extremely minute: in an ounce of sea-sand between three and four millions have been distinctly enumerated. When living, they are not aggregated, but always individually distinct; they are composed of a body (or vital mass) of a gelatinous consistence, which is either entire, and round, or divided into segments, placed either on a simple or alternate line, or coiled spirally, or involuted round an axis. This body is covered with an envelope or shell, which is generally testaceous, rarely cartilaginous, and is modelled on the segments, and follows all the modifications of form and contour of the body. From the extremity of the last segment, there issue, sometimes from one, sometimes from several openings of the shell, or through numerous pores or foramina, very elongated, slender, contractile, colourless filaments, more or less divided and ramified, serving for prehension, and capable of entirely investing the shell. The body varies in colour, but is always identical in individuals of the same species,--it is yellow, fawn-coloured, red, violet, blue, &c. Its consistence is variable; it is composed of minute globules, the aggregation of which determines the general tint. It is sometimes entire, round, and without segments, as in _Gromia_, _Orbulina_, &c., which represent, at all ages, the embryonic state of all the other genera. They increase, without doubt, by the entire circumference. When the body is divided by lobes or segments, the primary lobe, as in the permanent condition of the Gromia, is at first round or oval, according to the genus; once formed it never enlarges, but is enveloped externally by testaceous matter; it may be compared to a ball on which is applied a second larger one, then a third still larger, and so on during the life of the animal. The annexed figure of the animal of _Nummulina_ (as given by MM. Joly and Leymerie) will serve to convey a general idea of the living Foraminifera. [Illustration: THE ANIMAL OF THE NUMMULINA.]