The concentrically-laminated mass of Eozoön is composed of numerous calcareous layers, representing the original skeleton of the organism (fig. 23, b). These calcareous layers
Fig. 23.—Diagram of a portion of Eozoön cut vertically. A, B, C, Three tiers of chambers communicating with one another by slightly constricted apertures: a a, The true shell-wall, perforated by numerous delicate tubes; b b. The main calcareous skeleton ("intermediate skeleton"); c, Passage of communication ("stolon-passage") from one tier of chambers to another; d, Ramifying tubes in the calcareous skeleton. (After Carpenter.) serve to separate and define a series of chambers arranged in successive tiers, one above the other (fig. 23, A, B, C); and they are perforated not only by passages (fig. 23, c), which serve to place successive tiers of chambers in communication, but also by a system of delicate branching canals (fig. 23, d). Moreover, the central and principal portion of each calcareous layer, with the ramified canal-system just spoken of, is bounded both above and below by a thin lamina which has a structure of its own, and which may be regarded as the proper shell-wall (fig. 23, a a). This proper wall forms the actual lining of the chambers, as well as the outer surface of the whole mass; and it is perforated with numerous fine vertical tubes (fig. 24, a a), opening into the chambers and on to the surface by corresponding fine pores. From the resemblance of this tubulated layer to similar structures in the shell of the Nummulite, it is often spoken of as the "Nummuline layer." The chambers are sometimes piled up one above the other in an irregular manner; but they are more commonly arranged in regular tiers, the separate chambers being marked off from one another by projections of the wall in the form of partitions, which are so far imperfect as to allow of a free communication between contiguous chambers. In the original condition of the organism, all these chambers, of course, must have been filled with living-matter; but they are found in the present state of the fossil to be generally filled with some silicate, such as serpentine, which not only fills the actual chambers, but has also penetrated the minute tubes of the proper wall and the branching canals of the intermediate skeleton. In some cases the chambers are simply filled with crystalline carbonate of lime. When the originally porous fossil has been permeated by a silicate,
Fig. 24.—The animal of Nonionina, one of the Foraminifera, after the shell has been removed by a weak acid; b, Gromia, a single-chambered Foraminifer (after Schultze), showing the shell surrounded by a network of filaments derived from the body substance. it is possible to dissolve away the whole of the calcareous skeleton by means of acids, leaving an accurate and beautiful cast of the chambers and the tubes connected with them in the insoluble silicate.
The above are the actual appearances presented by Eozoön when examined microscopically, and it remains to see how far they enable us to decide upon its true position in the animal kingdom. Those who wish to study this interesting subject in detail must consult the admirable memoirs by Dr W. B. Carpenter and Principal Dawson: it will be enough here to indicate the results which have been arrived at. The only animals at the present day which possess a continuous calcareous skeleton, perforated by pores and penetrated by canals, are certain organisms belonging to the group of the Foraminifera. We have had occasion before to speak of these animals, and as they are not conspicuous or commonly-known forms of life, it may be well to say a few words as to the structure of the living representatives of the group. The Foraminifera are all inhabitants of the sea, and are mostly of small or even microscopic dimensions. Their bodies are composed of an apparently structureless animal substance of an albuminous nature ("sarcode"), of a gelatinous consistence, transparent, and exhibiting numerous minute granules or rounded particles. The body-substance cannot be said in itself to possess any definite form, except in so far as it may be bounded by a shell; but it has the power, wherever it may be exposed, of emitting long thread-like filaments ("pseudopodia"), which interlace with one another to form a network (fig. 25, b). These
Fig. 25.—The animal of Nonionina, one of the Foraminifera, after the shell has been removed by a weak acid; b, Gromia, a single-chambered Foraminifer (after Schultze), showing the shell surrounded by a network of filaments derived from the body substance. filaments can be thrown out at will, and to considerable distances, and can be again retracted into the soft mass of the general body-substance, and they are the agents by which the animal obtains its food. The soft bodies of the Foraminifera are protected by a shell, which is usually calcareous, but may be composed of sand-grains cemented together; and it may consist of a single chamber (fig. 26, a), or of many chambers arranged in different ways (fig. 26, b-f). Sometimes the shell has but
Fig. 26.—Shells of living Foraminifera. a, Orbulina universa, in its perfect condition, showing the tubular spines which radiate from the surface of the shell; b, Globigerina bulloides, in its ordinary condition, the thin hollow spines which are attached to the shell when perfect having been broken off; c, Textularia variabilis; d, Peneroplis planatus; e, Rotalia concamerata; f, Cristellaria subarcuatula. [Fig. a is after Wyville Thomson; the others are after Williamson. All the figures are greatly enlarged. one large opening into it—the mouth; and then it is from this aperture that the animal protrudes the delicate net of filaments with which it seeks its food. In other cases the entire shell is perforated with minute pores (fig. 26, e), through which the soft body-substance gains the exterior, covering the whole shell with a gelatinous film of animal matter, from which filaments can be emitted at any point. When the shell consists of many chambers, all of these are placed in direct communication with one another, and the actual substance of the shell is often traversed by minute canals filled with living matter (e.g., in Calcarina and Nummulina). The shell, therefore, may be regarded, in such cases, as a more or less completely porous calcareous structure, filled to its minutest internal recesses with the substance of the living animal, and covered externally with a layer of the same substance, giving off a network of interlacing filaments.