Small weathered specimen of Eozoon.
Fig. 15(Nos. 1 to 4).—Small weathered specimen ofEozoon. From Petite Nation.
1, Natural size; showing general form, and acervuline portion above and laminated portion below. 2, Enlarged casts of cells from upper part. 3, Enlarged casts of cells from the lower part of the acervuline portion. 4, Enlarged casts of sarcode layers from the laminated part.
The best specimens ofEozoonoccur as rounded, flattened, or more or less irregular lumps or masses in certain layers of the Laurentian limestone. When weathered on the surface of the rock, these lumps show a regular concentric lamination, caused by thin fibres of limestone, alternating with other mineral substances, filling up the spaces between them. When these intervening layers are composed of such minerals as Serpentine, Loganite, Pyroxene, or Dolomite, which are moreresisting than the limestone, they project when weathered, or when the limestone is etched by an acid, so as to show the lamination very distinctly. At the lower surface of the masses the layers are seen to be thicker than they are above, and in perfect specimens they are seen toward the surface to break up into small rounded vesicles of calcite, like little bubbles, which constitute the so-called acervuline condition ofEozoon(Fig. 15, No. 2). Slices of the fossil etched with an acid show these appearances very perfectly, and can even be printed from, so as to present perfect nature-prints of the structure (Fig. 16).
Nature-printed specimen of Eozoon slightly etchedFig. 16.—Nature-printed specimen ofEozoonslightly etched with acid. It shows the lamination, and at one side fragmentalEozoon(Life’s Dawn on Earth).
Fig. 16.—Nature-printed specimen ofEozoonslightly etched with acid. It shows the lamination, and at one side fragmentalEozoon(Life’s Dawn on Earth).
On etching a small fragment or slice with very dilute acid,so as to dissolve away the calcite slowly, if the specimen be well preserved, we find that the calcite layers have a very curious structure. This is indicated by the appearance of little white or transparent threads of Serpentine, Dolomite, or Pyroxene, which ramify throughout the substance of the limestone layers, and are left intact when they have been dissolved. These little processes must originally have been pores in the limestone layers, which have been filled with the substance which constitutes the alternate laminæ. In addition to this, if we use a somewhat high microscopic power, and especially if we study the structures as seen in thin transparent slices, we can perceive a still finer tubulation along the sides of the calcite layers, represented by extremely minute parallel rods of mineral matter (Figs. 17, 18).
Now if we regard these structures as those of an infiltrated fossil, as described in last chapter, their interpretation will not be difficult. The original organism was composed of calcareous matter in thin concentric laminæ, connected with each other by pillars and plates of similar material. Between these laminæ was lodged the soft, jelly-like substance of a marine animal, growing by the addition of successive layers, each protected by a thin calcareous crust. The layers were originally traversed by very numerous parallel tubuli, permitting the soft protoplasm to penetrate them; and when, in the progress of growth, it was necessary to strengthen these layers, they were thickened by a supplemental deposit traversed by larger and ramifying canals. When the animal was dead, and its soft parts removed by decay, the chambers between the laminæ, as well as the minute canals and tubuli, became infiltrated with mineral matter, in the manner described in the last chapter, and when so preserved became absolutely imperishable under any circumstances short of absolute fusion.
Magnified group of canals in supplemental skeleton of Eozoon.
Fig. 17.—Magnified group of canals in supplemental skeleton ofEozoon.
Taken from the specimen in which they were first recognised (Life’s Dawn on Earth).
Portion of Eozoon magnified 100 diameters.Fig. 18.—Portion ofEozoonmagnified 100 diameters, showing the original cell-wall with tubulation, and the supplemental skeleton with canals.—After Carpenter.
Fig. 18.—Portion ofEozoonmagnified 100 diameters, showing the original cell-wall with tubulation, and the supplemental skeleton with canals.—After Carpenter.
a, Original tubulated wall or “Nummuline layer.” More magnified in Fig. A.b,c, Intermediate skeleton, with canals.
This interpretation leads to the conclusion, at which I arrived from the study of the first well-preserved specimen ever submitted to microscopic examination, that the animal whichproduced the calcareous skeleton ofEozoonwas a member of that lowest grade of Protozoa known as Foraminifera; and which, after living through the whole of geological time, still abound in the sea. The main differences are, thatEozoonpresents a somewhat generalised structure, intermediate between two modern types, and that it attained to a gigantic size compared with most of these organisms in later periods. How near it approaches in structure to some modern forms may be seen by comparison of the recent species represented inFig. 19, in which the parts corresponding to the chambers, laminæ, tubuli, and canals ofEozooncan be readily distinguished.
Magnified portion of shell of Calcarina.
Fig. 19.—Magnified portion of shell ofCalcarina.—After Carpenter.
a, Cells.b, Original cell-wall with tubuli.c, Supplementary skeleton with canals.
The modern animals of this group are wholly composed of soft gelatinous protoplasm or sarcode, the outer layer of which is usually somewhat denser than the inner portion; but both are structureless, except that the inner layer may present a moreor less distinct granular appearance. Many of them show a distinct spot or cell, called the nucleus, and some have minute transparent vesicles, which contract and expand alternately, and appear to be of the nature of circulatory or excretory organs. They have no proper alimentary canal, but receive their food into the general mass and digest it in temporary cavities. Their means of locomotion and prehension are soft thread-like or finger-like processes, extended at will from the surface of any part of the body, and known as false feet (pseudopodia). From these processes the whole group has obtained the name of Rhizopods, or rootfooted animals. They may be regarded as constituting the simplest and humblest form of animal life certainly known to us.
The very numerous species of these creatures existing in the waters of the modern world may be arranged under three principal groups. The first and highest includes those which have lobate or finger-like pseudopods, and a well-developed nucleus and pulsating vesicle (Fig. 20,a). They are mostly inhabitants of fresh water, and destitute of a hard crust or shell. A second group, including many inhabitants of the sea as well as of fresh waters, has thread-like radiating pseudopodia4(Fig. 20b). Some of these form beautiful silicious skeletons. A third group, essentially marine, consists of those with reticulated pseudopodia, and usually destitute of distinct nucleus and pulsating vesicle (Fig. 21). They produce beautiful calcareous skeletons, often very complex, or sometimes are content to cover themselves with a crust of agglutinated grains of sand. It is to this last group thatEozoonbelongs, and to the highest division of it—that which has the shell perforated with minute pores, often of two kinds. It is curious that just as we have the chambers and pores ofEozoonfilled with serpentine, so in all geological formations and in the modern seas it is not uncommon to find Foraminifera having their cavitiesfilled with glauconite and other hydrous silicates allied to serpentine.
Amoeba, a fresh-water naked Rhizopod, etc.
Fig. 20.—a,Amœba, a fresh-water naked Rhizopod; andb,Actinophrys, a fresh-water Protozoon of the group Radiolaria, with thread-like pseudopodia.
Nonionina, a modern marine Foraminifer.
Fig. 21.—Nonionina, a modern marine Foraminifer. Showing its chambered shell and netted pseudopodia.—After Carpenter.
If we attempt to trace the Rhizopods onward from the Middle Laurentian, we are met with a great hiatus in the Upper Laurentian. The speciesEozoon Bavaricumhas, however, been found in rocks apparently of Huronian age; but this is the last known appearance ofEozoon, properly so-called. In the Cambrian or Siluro-Cambrian, however, we meet with many gigantic Protozoa, more especially those known asStromatopora,Archæocyathus,Receptaculites, andCryptozoon.
Stromatopora concentrica.Fig. 22.—Stromatopora concentrica.—After Hall.
Fig. 22.—Stromatopora concentrica.—After Hall.
a, Section of the same, magnified.b, Small portion highly magnified, showing laminæ and pillars.
The typical Stromatoporæ, or Layer-corals, consist, likeEozoon, of concentric layers, connected by numerous pillars, which are often, though not always, more definite and regular than in the Laurentian fossil. The laminæ are perforated, but more coarsely than inEozoon, and they are often thickened with supplemental deposit which, in some of the forms, presents canals radiating from vertical tubes or bundles of tubes penetrating the mass (Figs. 22, 23). The mode of growth ofStromatoporamust have closely resembled that ofEozoon, and the forms produced are so similar that it is often quite impossible todistinguish them by the naked eye. LikeEozoon, they form the substance of important limestones, and single masses are sometimes found as much as three feet in diameter. The Stromatoporæ extend from the Upper Cambrian to the Devonian inclusive. In the Carboniferous they are continued by smaller and more regular organisms of the genusLoftusia,5and this genus seems to extend without marked change up to the Eocene Tertiary. Recent students of the Stromatoporæ seem disposed to promote them from the province of Protozoa to that of the Hydroids.6The reasons for this seem cogent in the case of some of the forms, but in my judgment fail in others, more especially in the older forms. It may ultimately be found that the group as now held includes very different types of structure. In modern times I know of no nearer representative than the animal whose skeleton often adheres in red encrusting patches to our specimens of corals, and which is known asPolytrema. In general structure it is not very far from being a very degenerate kind ofStromatopora.
Caunopora planulata.
Fig. 23.—Caunopora planulata.Showing the radiating canals on a weathered surface. Devonian.—After Hall.
It is curious that in the line of succession above stated, the beautiful tubulated cell-wall ofEozoondisappears; and this structure seems, after the Laurentian, to be for ever divorced from the great laminated Protozoans. It reappears in theCarboniferous, in certain smaller organisms of the type of theNummulites, or Money-stone Foraminifers, and is continued in this group of smaller and free animals down to the present time. In the Cretaceous and early Tertiary periods, the Foraminifera of different types have been nearly as great rock-builders as they were in the Laurentian. Some of these later rock-builders, however, have belonged to the lower or imperforate group; others to the higher or Rotaline and Nummuline groups; and, as a whole, they have been individually small, making up in numbers what they lacked in size. Probably the conditions for enabling animals of this type rapidly, and on a large scale, to collect calcareous matter, were more favourable in the Laurentian than they have ever been since.
Archaeocyathus minganensis.
Fig. 24.—Archæocyathus minganensis.A Primordial Protozoon.—After Billings.
a, Pores of the inner wall.
In the Siluro-Cambrian age two other forms of gigantic Foraminiferal Protozoans were introduced, widely different fromEozoon, and destined apparently not to survive the period inwhich they appeared. These wereArchæocyathus, the ancient Cup-corals, and Receptaculites, which may perhaps be called the Sack-corals. Both are quite remote fromEozoonin structure, wanting its complexity in the matter of minute tubules, and having greater regularity and complication on the large scale.Archæocyathushad the form of a hollow inverted cone with double perforated walls, connected by radiating irregular plates, also perforated (Fig. 24). It has been regarded as a sponge, and some species are certainly accompanied with spicules; but these I have ascertained to be merely accidental, and will be referred to in the next chapter. The true structure of Archæocyathus consists of radiating calcareous plates enclosing chambers connected by pores.Archæocyathuscame in with the Later Cambrian, and seems to have died out in the Siluro-Cambrian. The only more modern things which at all resemble it are the Foraminifera calledDactylopora, which belong to the Tertiary period.
Receptaculites. Restored.
Fig. 25.—Receptaculites. Restored.—After Billings.
a, Aperture.b, Inner wall.c, Outer wall.n, Nucleus, or primary chamber.v, Internal cavity.
Receptaculites is a still more complex organism. It has asack-like form, often attaining a large size, and the double walls are composed of square or rhombic plates, connected with each other by hollow tubes from which proceed canals perforating the plates (Fig. 25). This curious structure is confined to the Siluro-Cambrian, and is so dissimilar from modern forms that its affinities have been subject to grave doubts.
Section of Loftusia Persica
Fig. 26.—Section ofLoftusia Persica. An Eocene Foraminifer. Magnified five diameters.—After Carpenter and Brady.
We thus have presented to us the remarkable fact that in the Palæozoic age we have no precise representative ofEozoon, but instead three divergent types, differing from it and from each other, all apparently specialised to particular uses, all temporary in their duration; while in later times nature seems to have returned nearer to the type ofEozoon, though on a smaller scale, and separating some characters conjoined in it. Some portion of this curious result may be due to our ignorance; and it would be interesting to know, what we may know someday, how this type of life was represented in the long interval between the Huronian and the Upper Cambrian, when perhaps there may have been forms that would at least enable us to connectEozoonandStromatopora.
Another link in the chain of being remains to be noticed here. In the Laurentian limestones we meet with numerous minute spherical bodies and groups of spheres with calcareous tubulated tests.7These may either be small Foraminiferæ, distinct fromEozoon, or may be germs or detached cells from its surface. Similar bodies are found in the lower part of the Siluro-Cambrian, in the Quebec group at Point Levis; and there they are filled with a species of glauconite constituting a sort of greensand rock. Still higher, in the Carboniferous, there are very numerous species of Foraminifera, presenting forms very similar to those in the modern seas, so that in the smaller shells of this group we seem to have evidence of a continuous series all the way from the Laurentian to the present time. The greater laminated forms co-exist with these up to the Eocene Tertiary. Throughout the whole of geological time—from the formation of the Laurentian limestones to that of the chalky ooze accumulating in the modern ocean—these humble creatures have been among the chief instruments in seizing on the calcareous matter of the waters and depositing it in the form of limestone.
Foraminiferal Rock Builders.Fig. 27.—Foraminiferal Rock Builders, in the Cretaceous and Eocene.
Fig. 27.—Foraminiferal Rock Builders, in the Cretaceous and Eocene.
a,Nummulites lævigata—Eocene.b, The same, showing chambered interior.c, Milioline limestone, magnified—Eocene, Paris.d, Hard Chalk, section magnified—Cretaceous.
I have said nothing of the development of higher forms of animal life fromEozoon, simply because I know nothing of it. We shall see in the next chapter that these are introduced seemingly in an independent manner. We may be content to trace foraminiferal life along its own line of development, waxing and waning, but ever confined within the same general boundaries, from the Laurentian to the present time. It is likely that if, in any of the ages constituting this vast lapse of time, a dredge had been dropped into the depths of ocean,it would have brought up Foraminifera not essentially different in form and structure. If any one asks to what extent the successive species constituting this almost endless chain may be descendants one of the other, we have no absolutely certain information to give. On the one hand, it is not inconceivable that such forms asStromatoporaorNummulinamay have descended fromEozoon. On the other hand, it is equally conceivable that the same power which producedEozoonat first, whether from dead matter or from some unknown lower form of life, may have repeated the process in later times with modifications. In any case it is probable that the Foraminifera have experienced alternations of expansion and shrinkage,of elevation and decadence, in the lapse of geological time. There were times in which many new forms swarmed into existence, and times in which old forms were becoming extinct without being replaced by others. In so far as the areas of the continents and the adjacent waters are concerned, those periods when the land was subsiding under the ocean must have been their times of prosperity, those in which the crust of the earth shrunk and raised up large areas of land must have been their times of decay. Still this lowest form of animal life has never perished, but has always found abundant place for itself, however pressed by physical change and by the introduction of higher beings.
Paradoxides ReginaParadoxides Regina(Matthews). Lower Cambrian of New Brunswick.1/6th Nat. Size.
Paradoxides Regina(Matthews). Lower Cambrian of New Brunswick.1/6th Nat. Size.
Ifthe middle portion of the Laurentian age was really a time of exuberant and abounding life, either this met with strange reverses in succeeding periods, or the conditions of preservation have been such as to prevent us from tracing its onward history. Certain it is, that according to present appearances we have a new beginning in the Cambrian, which introduces the great Palæozoic age, and few links of connection are known between this and the previous Eozoic.
At the beginning of the Palæozoic we have reason to believe that our continents were slowly subsiding under the sea, after a period of general continental elevation which was consequent on the crumbling of the earth’s crust at the close of the Eozoic; and on the new sea-bottoms formed by this subsidence came in, slowly at first, but in ever-increasing swarms, the abundant and varied life of the early Palæozoic.
In the oldest portion of the Cambrian series in Wales, Hicks has catalogued species of no less than seventeen genera, embracing Crustaceans, the representatives of our crabs and lobsters, bivalve and univalve shell-fishes of different types, worms, sea-stars, zoophytes, and sponges. If we could have walked on the shores of the old Cambrian sea, or cast our dredge or trawl into its depths, we should have found representatives of most of the humbler forms of sea life still extant,though of specific forms strange to us. Perhaps the nearest approach to such experience which we can make is to examine the group of Cambrian animals delineated inFig. 28, and to notice, under the guidance of the geologist above named, the sections seen at St. David’s, in South Wales.
Group of Cambrian Animals.Fig. 28.—Group of Cambrian Animals (from Nicholson).
Fig. 28.—Group of Cambrian Animals (from Nicholson).
a,Arenicolites didymus, worm tubes.b,Lingulella ferruginea.c,Theca Davidii.d,Modiolopsis solvensis.e,Orthis Hicksii.f,Obolella sagittalis.g,Hymenocaris vermicauda.h, Trilobite,Olenus micrurus.
Here we find a nucleus of ancient rocks supposed to be Laurentian, though in mineral character more nearly akin to the Huronian, but which have hitherto afforded no trace of fossils. Resting unconformably on these is a series of partially altered rocks, regarded as Lower Cambrian, and also destitute of organic remains. These have a thickness of almost 1,000 feet, and they are succeeded by 3,000 feet more of similar rocks, still classed as Lower Cambrian, but which have afforded fossils. The lowest bed which contains indications of life is a red shale, perhaps a deep-sea bed, and possiblyitself partly of organic origin, by that strange process of decomposition or dissolution of foraminiferal ooze and volcanic fragments, going on in the depths of the modern ocean, and described by Dr. Wyville Thomson as occurring over large areas in the South Pacific. The species are twoLingulellæ, aDiscinaand aLeperditia. Supposing these to be all, it is remarkable that we have no Protozoa or Corals or Echinoderms, and that the types of Brachiopods and Crustaceans are of comparatively modern affinities. Passing upward through another 1,000 feet of barren sandstone, we reach a zone in which no less than five genera of Trilobites are found, along with Pteropods and a sponge. Thus it is that life comes in at the base of the Cambrian in Wales, and it may be regarded as a fair specimen of the facts as they appear in the earlier fossiliferous beds succeeding the Laurentian. Taking the first of these groups of fossils, we may recognise in theLeperditiaa two-valved Crustacean closely allied to forms still living in the seas and fresh waters. The Lingulellæ, whether we regard them as molluscoids, or, with Professor Morse, as singularly specialised worms, represent a peculiar and distinct type, handed down, through all the vicissitudes of the geological ages, to the present day. The Pteropods and the sponge are very similar to forms now living. The Trilobites are an extinct group, but closely allied to some modern Crustaceans. Had the primordial life begun with species altogether inscrutable and unexampled in succeeding ages, this would no doubt have been mysterious; but next to this is the mystery of the oldest forms of life being also among the newest. Whatever the origin of these creatures, they represent families which have endured till now in the struggle for existence without either elevation or degradation. Yet, though thus vast in their duration, they seem to have swarmed in together and in great numbers, in the Cambrian, without any previous preparation. From the Cambrian onward, throughout the whole Palæozoic, there is no decided break in the continuity of marine life; andalready in the Silurian period the sea was tenanted with all the forms of invertebrate life it yet presents, and these in a teeming abundance not surpassed in any succeeding age. Let us now, in accordance with our plan, select some of these ancient inhabitants of the waters and trace their subsequent history.
Remains of sea-weeds are undoubtedly present in the Cambrian rocks. One of the lowest beds in Sweden has been named from their abundance the Fucoidal Sandstone; and wherever fossiliferous Cambrian rocks occur, some traces, more or less obscure, of these plants may be found. Nearly all that we can say of them, however, is, that, in so far as their remains give any information, they are very like the plants of the same group that now abound in our seas. In the fucoidal sandstone of Sweden certain striated or ribbed bodies have been found, which have even been regarded as land plants;8but they seem rather to be trails or marks left by sea-weeds dragged by currents over a muddy bottom. The plants of the sea thus precede those of the land, and they begin on the same level as to structure that they have since maintained. I agree with Nathorst, however, in holding that the Bilobites and many other forms believed by some to be sea-weeds, are really trails and tracks of animals.9
The Foraminifera of the Palæozoic we have noticed in the last chapter; but we now find a new type of Protozoan—that of the Sponge. Sponges as they exist at present may be defined to be composite animals, made up of a great number of one-celled or gelatinous zoids, provided with vibrating threads or cilia, and so arranged that currents of water are driven through passages or canals in the mass, by the action of the cilia, bringing food and aerated water for respiration. To support these soft sarcodic sponge-masses, they secrete fibres of horny matter and needles (spiculæ) of flint or oflimestone, forming complicated fibrous and spicular skeletons, often of great beauty. They abound in all seas, and some species are found in fresh waters.
Portion of skeleton of Hexactinellid Sponge
Fig. 29.—Portion of skeleton of Hexactinellid Sponge (Cœloptychium). Magnified. After Zittel.
With the exception of a very few species destitute of skeleton, and which we cannot expect to find in a fossil state, the sponges may be roughly divided into three groups: 1, those with corneous or horny skeleton, like our common washing sponges; 2, those with skeletons composed of silicious needles of various forms and arrangement; 3, those with calcareous spicules. Of these, the second or silicious group has precedence in point of time, beginning in the Early Cambrian, and continuing to the present. Two of its subdivisions are especially interesting in their range. The first is that of the Lattice-sponges (Hexactinellidæ), in which the spicules have six rays placed at right angles, and are attached to each other by their points, so as to form a very regular network (Fig. 29). The second is that of the Stone-sponges (Lithistidæ), in whichthe spicules are four-rayed or irregular, and are united by the branching root-like ends of the rays. The most beautiful of all sponges, the Venus Flower-basket (Euplectella), is a modern Hexactinellid, and the wonderful weaving of its spicules is as marvellous a triumph of constructive skill as its general form is graceful. The Lithistids are less beautiful, but are the densest and most compact of sponges, and are represented by several species in the modern seas. Both of these types go back to the Early Cambrian, and have continued side by side to the present day, as representatives of two distinct geometrical methods for the construction of a spicular skeleton.
Protospongia fenestrata.
Fig. 30.—Protospongia fenestrata(Salter). Menevian group.
a, Fragment showing the spicules partially displaced.b, Portion enlarged.
Astylospongia praemorsa.
Fig. 31.—Astylospongia præmorsa(Roemer). Niagara group.—After Hall.
a, Spicules magnified.
Spicules of Lithistid sponge.
Fig. 32.—Spicules of Lithistid sponge (Trichospongiaof Billings). From the Cambrian of Labrador.
Many years ago the keen eye of the late lamented Salter detected in a stain on the surface of a slab of Cambrian slate the remains of a sponge; and minute examination showed that its spicules crossed each other, and formed lattice-work on the hexactinellid plan. Salter boldly named itProtospongia(the first sponge), and it is still the earliest that we know (Fig. 30). Thus the type whose skeleton is the most perfect in a mechanical point of view takes the lead. It is continued in the Silurian in many curious forms, of which the stalkless sponges (Astylospongia) are the most common (Fig. 31). It perhaps attains its maximum in the Cretaceous, from which the beautiful example inFig. 29is taken, and it still flourishes, giving us the most beautiful of all recent forms. Before theclose of the Cambrian there were other sponges of the Lithistid type.Fig. 32represents a group of spicules from the Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,10and which probably belong to a large Lithistid sponge of that early time. The Lithistids have been recognised in the Upper Silurian and Carboniferous, and continuing upward to the Cretaceous, there become vastly numerous, while theirmodern representatives are by no means few. The silicious sponges with simple spicules appear to have existed as far back as the Siluro-Cambrian, and there is believed to be almost as early evidence of horny or corneous sponges. The calcareous sponges have been recognised as far back as the Silurian.11Thus from the close of the Palæozoic all the types of sponges seem to have existed side by side; and in the Cretaceous period, when such large areas of our continents were deeply submerged, they attained a wonderful development, perhaps not equalled in any other era of the earth’s history.
Oldhamia antiqua.
Dictyonema sociale.
Fig. 33.—Oldhamia antiqua(Forbes).
Fig. 34.—Dictyonema sociale. Enlarged.Lingulaflags.—After Salter.
Sponges may be regarded as the highest or most complex of the Protozoa or the lowest of the Coelenterates. We have no links wherewith to connect them with the lower Protozoa of the Eozoic period; and through their long history, though very numerous in genera and species, they show no closer relationship with the Foraminifera below, and the Corals above, than do their successors in the modern seas. They thus stand verymuch apart; and modern studies of their development and minute structures do not seem to remove them from this isolation. Though we are treating here of inhabitants of the sea, it may be proper to mention that Geinitz has described two species from the Permian which he believed to be early precursors of the Spongillæ, or fresh-water sponges; but more recently he seems to regard them as probably Algæ. Young has, however, recently found true spicules ofSpongillain the Purbeck beds.12
Dictyonema Websteri.
Fig. 35.—Dictyonema Websteri(Dn). Niagara formation.
a, Enlarged portion (Acadian Geology).
Group of modern Hydroids.
Fig. 36.—Group of modern Hydroids allied to Graptolites. Magnified, and natural size.
a,Sertularia.b,Tubularia.c,Campanularia.
Silurian Graptolitidae.
Fig. 37.—Silurian Graptolitidæ.
a,Graptolithus.b,Diplograpsus.c,Phyllograpsus.d,Tetragrapsus.e,Didymograpsus.
Central portion of Graptolite.
Ptilodictya acuta.
Fig. 38.—Central portion of Graptolite, with membrane, or float (Dichograpsus octobrachiatus, Hall).
Fig. 39—Ptilodictya acuta(Hall). Bryozoan. Siluro-Cambrian.
A stage higher than the sponges are those little polyp-like animals with sac-like bodies and radiating arms or tentacles, which form minute horny or calcareous cells, and bud out into branching communities, looking to untrained eyes like delicate sea-weeds—the sea-firs and sea-mosses of our coasts (Fig. 36). These belong to a very old group, for in the oldest Cambrian we have a form referred to this type (Fig. 33), and in the Upper Cambrian another still more decided example (Fig. 34).13This style of life, once introduced, must have increased in variety and extended itself with amazing rapidity, for in the Siluro-Cambrian age we find it already as characteristic as in our modern seas, and so abundant that vast thicknesses of shale are filled and blackened with thedébrisof forms allied to the sea-firs, and masses of limestone largely made up of themore calcareous forms of the sea-mosses. As examples of the former we may take theGraptolites, so named from their resemblance to lines of writing, and of which several forms are represented inFig. 37. The little teeth on the sides of these were cells, inhabited probably by polyps, like those represented in the modernSertulariainFig. 36. Some of them were probably attached to the bottom. In others the branches radiated from a central film which may have been a hollow vesicle or float, enabling them to live at the surface of the water (Fig. 38). These Graptolites are specially characteristic of the Upper Cambrian and Lower Silurian. The netted ones (Dictyonema), as may be seen fromFigs. 34 and 35, came in before the close of the Cambrian, and continue unchanged to the Silurian, where they disappear. The branching forms, seen inFig. 37, have scarcely so great a range. They thus form most certain marks of the period to which they belong,and being oceanic and probably floaters, they diffused themselves so rapidly that they appear to indicate the same geological time in countries so widely separated as Europe, North America, and Australia. It is curious, too, that while the Graptolites thus mark a definite geological time, and seem to disappear abruptly and without apparent cause, they are the first link in the long chain of the Hydroids, which, though under different family forms, continue to this day, apparently neither better nor worse than their perished Palæozoic relatives. There is a group of little Stony Corals (Monticuliporidæ), which were possibly alsothe cells of Hydroids, that have a similar history. They are the only known Corals that date so far back as the Upper Cambrian; and they continue under very similar forms all through the Palæozic, and are represented by the millepore corals of the present day.Fig. 40represents a form found at the base of the Siluro-Cambrian, andFig. 41shows forms characteristic of the Carboniferous Limestone.
Fenestella Lyelli.
Fig. 39a.—Fenestella Lyelli(Dawson). A Carboniferous Bryozoan.
If we turn now to the sea-mosses (Bryozoa), we have a group of minute polyp-like animalsChaetetes fibrosa.Fig. 40.—Chaetetes fibrosa. A tubulate coral with microscopic cells. Siluro-Cambrian.inhabiting cells not unlike those of the Hydroids, and which form plant-like aggregates. But the animals themselves are so different in structure that they are considered to be nearer allies of the bivalve shell-fishes than of the Corals. They are, in short, so different, that the most ardent evolutionist would scarcely hold a community of origin between them and such creatures as the Graptolites and Millepores, though an ordinary observer might readily confound the one with the other. These animals appear at the beginning of the Siluro-Cambrian, and such forms as that represented inFig. 39, very closely allied to some now living, are large constituents of some of the limestones of that period. Other forms, like that represented inFig. 39a, are very characteristic of the Carboniferous. These animals, individually small, though complicated in structure and branching into communities, scarcely ever of any great magnitude, humble creatures which have never played any great part in the world, have, nevertheless, been so persistent that, though specific and generic forms have been changed, the group may be said to be in the modern seas exactly what it was in those of the early Palæozoic, nor can it be affirmed to have originated in anything different, or to have produced anything.