The Amœba and Actinophrys are fresh water animals, and are destitute of any shell or covering. But in the sea there exist swarms of similar creatures, equally simple in organization, but gifted with the power of secreting around their soft bodies beautiful little shells or crusts of carbonate of lime, having one orifice, and often in addition multitudes of microscopic pores through which the soft gelatinous matter can ooze, and form outside finger-like or thread-like extensions for collecting food. In some cases the shell consists of a single cavity only, but in most, after one cell is completed, others are added, forming a series of cells or chambers communicating with each other, and often arranged spirally or otherwise in most beautiful and symmetrical forms. Some of these creatures, usually named Foraminifera, are locomotive, others sessile and attached. Most of them are microscopic, but some grow by multiplication of chambers till they are a quarter of an inch or more in breadth. ([Figs. 14 to 17.])
The original skeleton or primary cell-wall of most of these creatures is seen under the microscope to be perforated with innumerable pores, and is extremely thin. When, however, owing to the increased size of the shell, or other wants of the creature, it is necessary to give strength, this is done by adding new portions of carbonate of lime to the outside, and to these Dr. Carpenter has given the appropriate name of “supplemental skeleton;” and this, when covered by new growths, becomes what he has termed an “intermediate skeleton.” The supplemental skeleton is also traversed by tubes, but these are often of larger size than the pores of the cell-wall, and of greater length, and branched in a complicated manner. ([Fig. 20.]) Thus there are microscopic characters by which these curious shells can be distinguished from those of other marine animals; and by applying these characters we learn that multitudes of creatures of this type have existed in former periods of the world’s history, and that their shells, accumulated in the bottom of the sea, constitute large portions of many limestones. The manner in which such accumulation takes place we learn from what is now going on in the ocean, more especially from the result of the recent deep-sea dredging expeditions. The Foraminifera are vastly numerous, both near the surface and at the bottom of the sea, and multiply rapidly; and as successive generations die, their shells accumulate on the ocean bed, or are swept by currents into banks, and thus in process of time constitute thick beds of white chalky material, which may eventually be hardened into limestone. This process is now depositing a great thickness of white ooze in the bottom of the ocean; and in times past it has produced such vast thicknesses of calcareous matter as the chalk and the nummulitic limestone of Europe and the orbitoidal limestone of America. The chalk, which alone attains a maximum thickness of 1000 feet, and, according to Lyell, can be traced across Europe for 1100 geographical miles, may be said to be entirely composed of shells of Foraminifera imbedded in a paste of still more minute calcareous bodies, the Coccoliths, which are probably products of marine vegetable life, if not of some animal organism still simpler than the Foraminifera.
Lastly, we find that in the earlier geological ages there existed much larger Foraminifera than any found in our present seas; and that these, always sessile on the bottom, grew by the addition of successive chambers, in the same manner with the smaller species. To some of these we shall return in the sequel. In the meantime we shall see what claims Eozoon has to be included among them.
Let us, then, examine the structure of Eozoon, taking a typical specimen, as we find it in the limestone of Grenville or Petite Nation. In such specimens the skeleton of the animal is represented by a white crystalline marble, the cavities of the cells by green serpentine, the mode of whose introduction we shall have to consider in the sequel. The lowest layer of serpentine represents the first gelatinous coat of animal matter which grew upon the bottom, and which, if we could have seen it before any shell was formed upon its surface, must have resembled, in appearance at least, the shapeless coat of living slime found in some portions of the bed of the deep sea, which has received from Huxley the name Bathybius, and which is believed to be a protozoon of indefinite extension, though it may possibly be merely the pulpy sarcode of sponges and similar things penetrating the ooze at their bases. On this primary layer grew a delicate calcareous shell, perforated by innumerable minute tubuli, and by some larger pores or septal orifices, while supported at intervals by perpendicular plates or pillars. Upon this again was built up, in order to strengthen it, a thickening or supplemental skeleton, more dense, and destitute of fine tubuli, but traversed by branching canals, through which the soft gelatinous matter could pass for the nourishment of the skeleton itself, and the extension of pseudopods beyond it. ([Fig. 10.]) So was formed the first layer of Eozoon, which seems in some cases to have spread by lateral extension over several inches of sea bottom. On this the process of growth of successive layers of animal sarcode and of calcareous skeleton was repeated again and again, till in some cases even a hundred or more layers were formed. (Photograph, [Plate III.], and nature print, [Plate. V.]) As the process went on, however, the vitality of the organism became exhausted, probably by the deficient nourishment of the central and lower layers making greater and greater demands on those above, and so the succeeding layers became thinner, and less supplemental skeleton was developed. Finally, toward the top, the regular arrangement in layers was abandoned, and the cells became a mass of rounded chambers, irregularly piled up in what Dr. Carpenter has termed an “acervuline” manner, and with very thin walls unprotected by supplemental skeleton. Then the growth was arrested, and possibly these upper layers gave off reproductive germs, fitted to float or swim away and to establish new colonies. We may have such reproductive germs in certain curious globular bodies, like loose cells, found in connection with irregular Eozoon in one of the Laurentian limestones at Long Lake and elsewhere. These curious organisms I observed some years ago, but no description of them was published at the time, as I hoped to obtain better examples. I now figure some of them, and give their description in a note. (Fig. 18). I have recently obtained numerous additional examples from the beds holding Eozoon at St. Pierre, on the Ottawa. They occur at this place on the surface of layers of the limestone in vast numbers, as if they had been growing separately on the bottom, or had been drifted over it by currents. These we shall further discuss hereafter. Such was the general mode of growth of Eozoon, and we may now consider more in detail some questions as to its gigantic size, its precise mode of nutrition, the arrangement of its parts, its relations to more modern forms, and the effects of its growth in the Laurentian seas. In the meantime a study of our illustration, [Plate. IV.], which is intended as a magnified restoration of the animal, will enable the reader distinctly to understand its structure and probable mode of growth, and to avail himself intelligently of the partial representations of its fossilized remains in the other plates and woodcuts.
Fig. 18. Minute Foraminiferal forms from the Laurentian of Long Lake.
Highly magnified. (a.) Single cell, showing tubulated wall. (b, c.) Portions of same more highly magnified. (d.) Serpentine cast of a similar chamber, decalcified, and showing casts of tubuli.
With respect to its size, we shall find in a subsequent chapter that this was rivalled by some succeeding animals of the same humble type in the Silurian age; and that, as a whole, foraminiferal animals have been diminishing in size in the lapse of geological time. It is indeed a fact of so frequent occurrence that it may almost be regarded as a law of the introduction of new forms of life, that they assume in their early history gigantic dimensions, and are afterwards continued by less magnificent species. The relations of this to external conditions, in the case of higher animals, are often complex and difficult to understand; but in organisms so low as Eozoon and its allies, they lie more on the surface. Such creatures may be regarded as the simplest and most ready media for the conversion of vegetable matter into animal tissues, and their functions are almost entirely limited to those of nutrition. Hence it is likely that they will be able to appear in the most gigantic forms under such conditions as afford them the greatest amount of pabulum for the nourishment of their soft parts and for their skeletons. There is reason to believe, for example, that the occurrence, both in the chalk and the deep-sea mud, of immense quantities of the minute bodies known as Coccoliths along with Foraminifera, is not accidental. The Coccoliths appear to be grains of calcareous matter formed in minute plants adapted to a deep-sea habitat; and these, along with the vegetable and animal debris constantly being derived from the death of the living things at the surface, afford the material both of sarcode and shell. Now if the Laurentian graphite represents an exuberance of vegetable growth in those old seas proportionate to the great supplies of carbonic acid in the atmosphere and in the waters, and if the Eozoic ocean was even better supplied with carbonate of lime than those Silurian seas whose vast limestones bear testimony to their richness in such material, we can easily imagine that the conditions may have been more favourable to a creature like Eozoon than those of any other period of geological time.