[115] The best scientific account of these animals will be found in M. D'Orbigny's work on the "Foraminifères Fossiles du Bassin Tertiaire de Vienne, (Autriche)." Paris, 1846. 1 vol. 4to, with plates.
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.
THE ANIMAL OF THE NUMMULINA.]
The segments, as the body increases, are agglomerated in six different ways, and these modifications are the basis of M. D'Orbigny's classification. The discoidal forms, as the Rotalia, Rosalina, Cristellaria, &c. are involuted like the nautilus, and divided by septa or partitions, the different lobes of the body occupying contemporaneously every chamber, and being connected by a tube or canal that extends through the entire series. In the spiral forms, the Textilaria, &c. the same structure is apparent. These two groups are the most abundant in the cretaceous strata; many beds of the white chalk consist almost wholly of the aggregated shells of the Rosalinæ, Rotaliæ, and Textilariæ.[116] Whatever the form of the body, the filaments always consist of a colourless matter as transparent as glass; they elongate from the base to six times the diameter of the shell. They often divide and subdivide, so as to appear branched. Though alike in form in the different genera, they vary much in their position. In some they form a bundle which issues from a single opening, and is withdrawn into the same by contraction; in others the filaments project only through each of the pores in the shell which covers the last segment; in others they issue from both the large aperture and the foramina. In fine, these filaments or pseudopodia fulfil in the foraminifera the functions of the numerous tentacula in the Asteriadæ, or Star-fishes, serving as instruments of locomotion and attachment.
[116] See Wonders of Geology, p. 299
Neither organs of nutriment nor of reproduction have been detected. In the genera having one large aperture from which the filaments issue and retract, we can conceive nutriment to be absorbed by that opening; but this cannot be the case in the species which have the last cell closed up; in these the filaments issuing through the foramina are probably also organs of nutrition. M. D'Orbigny considers the Foraminifera as constituting a distinct class in zoology; less complicated than the Echinoderms and the Polypiaria in their internal organization, they have by their filaments the mode of locomotion of the first, and by their free, individual existence—not aggregated and immovably fixed—they are more advanced in the scale of being than the latter. To me they appear to be merely hydra-form polypes of the most simple structure, protected by shells;[117] those composed of different segments, I conceive to be a single aggregated individual, and not a successive series of beings.
[117] An admirable paper on the "Polystomella crispa," by Mr. Williamson, of Manchester, (Trans. Micros. Society of London, vol. ii.) should be consulted on this question.
The white chalk is well known to be largely composed of a few kinds of foraminifera, but the occurrence of the soft bodies of these animalcules in a fossil state was first discovered by me, in 1845, in chalk-flints, and was announced in a paper, read before the Geological Society, entitled, "Notes of a Microscopical Examination of Chalk and Flint."[118] This statement was regarded by some eminent palæontologists as so "startling and unsatisfactory," that I resumed the investigation, and communicated the result to the Royal Society, in a memoir "On the Fossil Remains of the Soft Parts of Foraminifera discovered in the Chalk and Flint of the South-East of England;"[119] and with the kind assistance of that able chemist and microscopist, Mr. Henry Deane, of Clapham Common, I obtained, by immersing chalk in dilute hydrochloric acid, and mounting the residue in Canada balsam, several specimens of the entire integuments of the bodies of Rotaliæ, as distinct as if recent! This fact is now admitted; and the experiment has been successfully repeated in India, by Mr. Carter, on the limestones of that country;[120] and in America, by Dr. Bailey, &c.[121] In some limestone recently collected by my eldest son, Mr. Walter Mantell, in the Middle Island of New Zealand, and which, like our cretaceous strata, is almost entirely made up of foraminifera, I have detected the soft parts of the bodies of Rotaliæ in the cells of the fossil shells, as distinctly as in the chalk of England; and two of the species appear to be identical with European forms.