Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/386

372 FOKAMINIFJ3RA for cxcrcmentitious matter, nutrient ingcsta being ad mitted, and their unassimilable residue being got rid of, through any spot of the ectosarc. It is in the presence of a definite mouth, and usually of an anus also, that even the simplest of the true ciliated Infusoria show a decided advance upon the "rhizopodal" type, an advance which is still more marked in the higher Infusoria by the complexity of their internal organization. Now the animal bodies of Foraminifera, notwithstand ing the regularity and complexity of the shells they form, show but a very slight advance on the simplest numeral type. For their protoplasmic substance does not seem to be differentiated into "ectosarc" and "endosarc," every part of it alike projecting itself into pseudopodial exten sions, and these extensions being capable, not only of dividing and ramifying indefinitely, but also of reuniting by mutual fusion when they come into contact with each other, so as to constitute an irregular network (figs. 2, 3, 4). It is on account of this last peculiarity that the writer has distinguished the reticulose forms of the Bhizopod type from the lobose (consisting of Amoeba and its allies) on the one hand, and from the radioiarian (of which Actino- plirys is the type) on the other. The sarcodic bodies of Foraminifera were believed until recently to have neither " nuclei " nor " contractile vesicles"; but as the observa tions of Hertwig and F. E. Schulze have established the presence of these in some instances (the writer also having observed " endoplasts " in Orbitolites), it is probable that they exist universally. The attention which has been given in recent years to the study of Foraminifera, has invested the group formerly considered as comparatively insignificant with a new interest and importance. For (1) these minute testaceous Rhizopods, instead of having mere local habitats, are diffused abundantly through all save polar seas, and seem to do the first work of collecting by imbibition, and of converting into living substance, the organic matter which is contained, however sparingly, in all oceanic water, and the restoration of which, as fast as it is thus withdrawn, is effected by the various forms of marine vegetation. 1 Again (2), without 1 That all marine animal life must ultimately depend upon marine vegetation is as certain as that all the animal life of the land ultimately depends upon terrestrial vegetation. And looking to the very large proportion which, not only among Fishes, but also in the higher Mol- lusca, Crustacea, and Echinodermata, the carnivorous bear to the phytophagous types, and to the abundance of the former on bottoms far too deep for the growth of the Alga; required for the sustenance of the latter, and on which there is no "raining down" of Diatoms from the surface (as in polar areas), it seems obvious either that there must be animals capable of generating organic compounds for themselves out of the gases contained in ocean-water, or that it must itself supply nutrient material in the liquid form to animals specially adapted to imbibe and assimilate it. Of the possibility of the former hypothesis we have no evidence whatever ; and in the absence of light at great depths, any new production of organic compounds seems almost incon ceivable. On the other hand, the analyses made by Dr Frankland of the specimens of ocean-water brought home in the " Porcupine" ex pedition of 1869 have shown that it always contains an appreciable proportion of nitrogenous matter ; while in Foraminifera there seems to be a special capability of imbibing and assimilating such matter by the extension of the soft body into a protoplasmic network, exposing a very large surface. Of the importance of this provision (first sug gested by Sir Wyville Thomson) in the economy of nature the following is an apt illustration : Large quantities of cod are taken by fishermen on the Faroe banks, attracted thither by the abundance of star-fish, on which they greedily feed ; and the stomachs of these star-fish are found to be filled with Globigerince. Thus man is dependent for this "harvest of the sea" in the first place upon the star-fish that feed the cod ; secondly, on the foraminifers that feed the star-fish ; thirdly, on the organic material (in very weak solution) which ocean- water supplies to the foraminifers ; and finally, on the various forms of marine vegetation by which this supply is continually being renewed. The writer deems it probable that the same function is performed by Sponyes, whose ramifying internal canal-system provides a vast extent of absorbent surface, and that this is the explanation of the otherwise singular fact, that the " cold-area " (temp. 30 Fahr. ), not far from the Faroe banks, swarms with various types of boreal Echinoderms, any thing that can be called organization, the protoplasmic bodies of these animals give origin to protective casings of marvellous regularity of form, and often of great com plexity of structure, these being sometimes " tests " built up by the apposition of sand-grains or other particles collected from the bottom on which they live, the animals only furnishing the cement by which they are held together, but being more often true " shells," formed (like the skele tons of higher animals) by an interstitial deposit of car bonate of lime drawn from the surrounding medium, in the substance of living tissue. Notwithstanding (3) the absence of any perceptible differences in the character of the animals they respectively contain, these protective casings, whether sandy "tests" or calcareous "shells," present a wide diversity of fundamental form, which is almost indefinitely augmented by subordinate modifications: and these modifications are generally so gradational as to render it impossible (when a sufficient number of specimens are compared) to draw any lines of separation between what appear, when only the extremes are regarded, to be clearly differentiated types. And this is true, not merely of species (which in the sense of constantly differentiated races cannot be said to have any existence among Foraminifera), but also of what would elsewhere be accounted genera; whilst even families cannot, as a rule, be sharply defined, many of them con taining aberrant forms that defy all attempts at strict limitation. In fact it would be scarcely a figure of speech to say that, within each of the three primary subdivisions of the group to be presently marked out, every form passes gradationally into every other. Hence the study of Foraminifera whose small (but not too small) size and numerical abundance are peculiarly favourable to the com parison of large series of individuals affords a most valuable lesson in taxonomy, giving the best illustration that the whole animal kingdom can afford of the produc tion of almost endless varieties of conformation by " descent with modification." Looking (4) to the almost universal diffusion of existing Foraminifera, and to the continuous accumulation of their calcareous exuvice over vast areas of the ocean-bottom, there can be little doubt that they are at present doing more than any other tribe of marine animals to separate carbonate of lime from its solution in sea-water, so as to restore to the solid crust of the earth what is being continually withdrawn from it by the solvent action exerted by rain and rivers upon the calcareous material of the upraised land. And when (5) we look back into the past history of our globe, we not only find conclusive proof of the enormous contributions which Foraminifera have made to the calcareous strata of Tertiary and Secondary epochs (Nummulitic Limestone and Chalk having been almost entirely formed by their agency), but encounter strong reason for the belief that the principal mass of the Palaeozoic Carboniferous Limestone had a similar origin. And when finally (6) we go still further back in geological time, and bring our knowledge of this type to bear on the very earliest calcareous formation that has yet been found in stratified rocks (the Serpentine Limestone of the Canadian Laurentians), we find distinct evidence that this also had its origin in Foramiuiferal growth ; which has thus stamped the impress of its existence even upon the oldest of those Archaic rocks, which, through a long subse quent succession, have as yet yielded no other distinct evidence of the existence of either animal or vegetable life on our planet. And thus in all parts of the geological series we not only recognize the constructive work of these humble organisms, but, interpreting the past by the notwithstanding the almost entire absence of Foraminiftra caused by the depression of temperature, their place being taken by a particular type of Sponge which there finds a congenial habitat, and probably supplies food to higher marine animals.