Fig. 45.—Tubular Structure of Cœnostroma, Silurian.
The oldest that we at present know, and consequently the nearest in time to Eozoon, impress us rather with the latter affinity. They are the fossils of the genus Cryptozoon of Hall ([Fig. 7][36]), which form great masses filling certain beds of Upper Cambrian age, and which, when sliced and studied microscopically, are found to consist of concentric thin laminæ filled in between with a porous mass of calcareous matter penetrated by an infinity of tortuous tubes. Forms of this kind have been traced downward into pre-Cambrian beds in Colorado, and as we shall find in New Brunswick, into the Upper Laurentian itself.
[36] See Figs. [7] and [7a], pp. [37], [38]; also [Fig. 8] and Microscopic slice, [Fig. 59], at end.
They present, however, structural differences from Eozoon, which rather conforms to the arrangements found in some Protozoa of smaller size, and which, under the name of Foraminifera, have abounded in all geological periods, and are excessively abundant in the modern ocean. They may be defined as animals composed of a soft and apparently homogeneous animal jelly known as protoplasm or sarcode. When carefully examined, however, it is found to have a granular texture and to be divisible into two layers, an outer and an inner, while it possesses a little hollow vessel capable of expanding and absorbing the liquid matter of the enclosing protoplasm, and of contracting so as to expel its contents. This seems to be the only organ of circulation and excretion. There are, however, small cells or reproductive bodies in the interior, varying in number, size, and development in different forms. The most remarkable property of these creatures is that of stretching out from the surface of the body threads or projections of the protoplasm,[37] often of considerable length, and which serve at once as organs of locomotion and prehension.
[37] Known as Pseudopodia.
Amœba. |
Actinophrys. |
| From original sketches. | |
Biloculina. A many-chambered Foraminifer. |
Polystomella. A spiral Foraminifer. |
Fig. 46.—Recent Protozoa. | |
These creatures are in some respects the simplest of animals, yet in other respects they present strange complexities. This is more especially evident in their tests or coverings, made for the most part of limestone or calcium carbonate, but sometimes of grains of fine sand cemented together. These coverings are always perforated with at least one orifice for the emission of the thread-like processes or pseudopods, and often with a vast number of small pores for the same purpose. Sometimes the test or shell is smooth, sometimes beautifully sculptured externally. Sometimes it consists of a single chamber like a ball or vase. More often, as the animals increase in size, they form additional chambers, and the body thus becomes divided into lobes connected with each other by necks passing through orifices in the partitions. The chambers are arranged in rows or in spirals, and in other ways, giving a vast variety of forms, often presenting the most beautiful patterns executed in the purest white marble, and the ornamental parts constitute thickenings of the walls giving greater strength, and are penetrated with microscopic canals communicating with the soft substance of the animal.
These creatures abound in all parts of the ocean, from the surface to the greatest depths. The Foraminifera have also existed from the earliest geological times, and in all the long ages of the earth's history seem to have retained the same structures and even ornamentation; so that species from very old geological formations are often scarcely distinguishable from those now living, and must have played precisely the same parts in the system of nature. One of these functions is that of accumulating great thicknesses of calcareous matter in the sea-bottom.
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 1,000 feet, and, according to Lyell, can be traced across Europe for 1,100 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.
There are, however, some sessile examples of these animals which attain to larger dimensions than the free and locomotive forms. As an example of these we may take the Polytrema, which forms little hard red lumps on West Indian corals. Such a creature, beginning life as a little round spot of protoplasm, almost invisible, and protected with a little dome of carbonate of lime for the extension of its pseudopods as it grows in size, adds chamber to chamber in successive tiers till it assumes an appreciable size, all the chambers communicating with each other, while the outer ones are perforated with pores for extension of the pseudopods. In one form (Carpenteria) the same end is secured by leaving an open space in the middle of the conical mass like the crater of a small volcano. It is with these larger and sessile forms that we must compare Eozoon, though some of its minute structures rather resemble those of some smaller types.