Chapter III: The Age of Invertebrates of the Sea

If the 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 PalA|ozoic age, and few links of connection are known between this and the previous Eozoic.

At the beginning of the PalA|ozoic 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 earthaEuro(TM)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 PalA|ozoic.

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 in Fig. 28, and to notice, under the guidance of the geologist above named, the sections seen at St. DavidaEuro(TM)s, in South Wales.

_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 possibly itself 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 two _LingulellA|_, a _Discina_ and a _Leperditia_. 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 the _Leperditia_ a two-valved Crustacean closely allied to forms still living in the seas and fresh waters. The LingulellA|, 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 PalA|ozoic, there is no decided break in the continuity of marine life; and already 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;[8] but 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 PalA|ozoic we have noticed in the last chapter; but we now find a new type of ProtozoanaEuro"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 (spiculA|) of flint or of limestone, forming complicated fibrous and spicular skeletons, often of great beauty. They abound in all seas, and some species are found in fresh waters.

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 (_HexactinellidA|_), 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 (_LithistidA|_), in which the 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.

_a_, Fragment showing the spicules partially displaced. _b_, Portion enlarged.]

_a_, Spicules magnified.]

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 it _Protospongia_ (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 in Fig. 29 is taken, and it still flourishes, giving us the most beautiful of all recent forms. Before the close of the Cambrian there were other sponges of the Lithistid type. Fig. 32 represents a group of spicules from the Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,[10] and 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 their modern 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.[11] Thus from the close of the PalA|ozoic 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 earthaEuro(TM)s history.

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 very much 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 SpongillA|, or fresh-water sponges; but more recently he seems to regard them as probably AlgA|. Young has, however, recently found true spicules of _Spongilla_ in the Purbeck beds.[12]

_a_, Enlarged portion (_Acadian Geology_).]

_a_, _Sertularia_. _b_, _Tubularia_. _c_, _Campanularia._]

_a_, _Graptolithus_. _b_, _Diplograpsus_. _c_, _Phyllograpsus_. _d_, _Tetragrapsus_. _e_, _Didymograpsus_.]

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-weedsaEuro"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).[13] This 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 the _dA(C)bris_ of forms allied to the sea-firs, and masses of limestone largely made up of the more calcareous forms of the sea-mosses. As examples of the former we may take the _Graptolites_, so named from their resemblance to lines of writing, and of which several forms are represented in Fig. 37. The little teeth on the sides of these were cells, inhabited probably by polyps, like those represented in the modern _Sertularia_ in Fig. 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 from Figs. 34 and 35, came in before the close of the Cambrian, and continue unchanged to the Silurian, where they disappear. The branching forms, seen in Fig. 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 PalA|ozoic relatives. There is a group of little Stony Corals (_MonticuliporidA|_), which were possibly also the 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 PalA|ozic, and are represented by the millepore corals of the present day. Fig. 40 represents a form found at the base of the Siluro-Cambrian, and Fig. 41 shows forms characteristic of the Carboniferous Limestone.

If we turn now to the sea-mosses (Bryozoa), we have a group of minute polyp-like animals 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 in Fig. 39, very closely allied to some now living, are large constituents of some of the limestones of that period. Other forms, like that represented in Fig. 39_a_, 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 PalA|ozoic, nor can it be affirmed to have originated in anything different, or to have produced anything.

The true Stony Corals (_Anthozoa_) are as yet unknown in the Cambrian. They entered on the stage in immense abundance in the Siluro-Cambrian, where considerable limestones are largely composed of their remains, mixed, however, and sometimes overpowered with those of Bryozoa and Hydroids. An ordinary coral, such as those of which coral reefs are builtaEuro"the red coral, used for ornament is not quite similaraEuro"is the skeleton of an animal constructed on the plan of a sea anemone; with a central stomach surrounded by radiating chambers, and having above a crown of tentacles. The stony coral surrounds and protects the soft body of the animal, and may either be a single cell, for one animal, or an aggregation of such cells, constituting a rounded or branching mass. The modern star coral, represented in Fig. 42, is an instance of the latter condition. It shows nineteen or twenty animals, each with a central mouth and fringe of short tentacles, aggregated together, and two of them showing the spontaneous division by which the number of animals in the mass is progressively increased. The living coral shows only the soft animals and the animal matter connecting them; but if dead there would be a white stony mass with a star-like cell or depression corresponding to each animal.

In their general plan, the oldest Corals were precisely of this character, but they presented some differences in detail, which have caused them to be divided into two groups, which are eminently characteristic of the PalA|ozoic ageaEuro"the tabulate or floored corals, and the rugose or wrinkled corals. In the former (Fig. 43) the cells are usually small and thin-walled, often hexagonal, like a honeycomb, and are floored across at intervals with tabulA| or horizontal plates. A few modern corals present a similar arrangement,[14] but this kind of structure was far more prevalent in the PalA|ozoic. In the second type the animals are usually larger and often solitary, the cell has strongly marked radiating plates, while the horizontal floors are absent or subordinate, and there is usually a thick external rind or outer coat (Figs. 44, 45). In general plan, these rugose corals closely resemble those of our modern reefs; but they differ in their details of structure, and only a very few modern forms from the deep sea are regarded as actual modern representatives.[15] One curious point of difference is that their radiating laminA| begin with four, and increase by multiples of that number, while in modern corals the numbers are six and multiples of six; a change of mathematical relation not easily accounted for, and which assimilates them to Hydroids on the one hand, and to a higher group, the Alcyonids, on the other, both of which prefer four and eight to six, or have had these numbers chosen for them. In the Mesozoic period the tabulate and rugose corals were replaced by others, the porous and solid corals of the modern seas; but, in so far as we know, the animals producing these, though differing in some details, were neither more nor less elevated than their predecessors, and they took up precisely the same rA'le as reef-builders in the sea, though with probably more tendency to the accumulation of great masses of coral limestone in particular spots.

_a_, _Halisites_, and _b_, _Favosites_. Upper Silurian.]

_a_, _Zaphrentis Minas_ (Dn.), and _b_, _Cyathophyllum Billingsi_ (Dn.). Carboniferous.]

Leaving the corals, we may turn to the sea-stars and sea-urchins. These merely put in an appearance in the Early Cambrian, but become vastly multiplied in the Silurian, where the stalked feather stars (Crinoids) (Fig. 46) seem to have covered great areas of sea-bottom, and multiplied so rapidly that thick sheets of limestone are largely made up of the fragments of their skeletons. The ordinary star-fishes appear first in the Silurian (Fig. 47). The sea-urchins begin in the Upper Silurian, the early species having numerous and loosely attached plates, like some of those now found in the deep sea[16] (Fig. 48).

The most curious history in this group is that of the feather-stars. In the Early Cambrian they are represented by a few species known to us only in fragments, and these belong to a humble group (Cystideans) resembling the larval or immature condition of the higher Crinoids. Fig. 49 shows one of these animals of somewhat later age. They have few or rudimentary arms and short stalks, and want the beautiful radial symmetry of the typical star-fishes. In the Silurian these creatures are reinforced by a vast number of beautiful and perfect feather-stars (Figs. 50, 51). These continue to increase in number and beauty, and apparently culminate in the Mesozoic, where gigantic forms exist, some of them probably having more complicated skeletons, in so far as number of distinct parts is concerned, than any other animals. Buckland has calculated that in a crinoid similar to that in Fig. 52 there are no less than 150,000 little bones, and 300,000 contractile bundles of fibres to move them. In the modern seas the feather-stars have somewhat dwindled both in numbers and complexity, and are mostly confined to the depths of the ocean. On the other hand, the various types of ordinary star-fishes and sea-urchins have increased in number and importance. We thus find in this group a certain advance and improvement from the Cystideans of the Early PalA|ozoic to the sea-urchins and their allies. This advance is not, however, along one line for the Cystideans continue unimproved to the end. The Crinoids culminate in the Mesozoic, and are not known to give origin to anything higher. The star-fishes and sea-urchins commence independently, before the culmination of the Crinoids, and, though greatly increased in number and variety, still adhere very closely to their original types.

The great sub-kingdom of the Mollusca, including the bivalve and univalve shell-fishes, makes its first appearance in the Cambrian, where its earliest representatives belong to a group, the Arm-bearers or Lamp shells (Brachiopods), held by some to be allied to worms as much as to mollusks. The oldest of all these shells are allies of the modern _LingulA|_ (Fig. 54), some of the earliest of which are shown in Fig. 55. The modern _Lingula_ is protected by a delicate two-valved shell, composed, unlike that of most other mollusks, of phosphate of lime or bone earth. It lives on sand-banks, attached by its long flexible stalk, which it buries like a root in the bottom. Its food consists of microscopic organisms, drifted to its mouth by cilia placed on two arm-like processes, from which the group derives its name. In the modern world about one hundred species of Brachiopods are known, belonging to about twenty genera, some of which differ considerably from the LingulA|. The genus _Terebratula_, represented at Fig. 56, is one of the most common modern as well as fossil forms, and has the valves unequal, with a round opening in one of them for the stalk, which is attached to some hard object, and there is an internal shelly loop for supporting the arms.

_a_, _Lingulella Matthewi_ (Hartt). Acadian group. _b_, _Lingula quadrata_ (Hall). Siluro-Cambrian. _c_, _Lingulella prima_ (Hall). Potsdam. _d_, _Lingulella antiqua_ (Hall). Potsdam.]

These curious, and in the modern seas, exceptional shells, were dominant in the PalA|ozoic period. Upwards of three thousand fossil species are known, of which a large proportion belong to the Cambrian and Silurian, nine genera appearing in the Cambrian, and no less than fifty-two in the Silurian. The history of these creatures is very remarkable. The LingulA|, which are the first to appear, continue unchanged and with the same phosphatic shells to the present day. Morse, who has carefully studied an American species, remarks in illustration of this, that it is exceedingly tenacious of life, bearing much change of depth, temperature, etc., without being destroyed. The genus _Discina_, which is nearly as old, also continues throughout geological time. The genus _Orthis_ (Fig. 57), which appears at the same time with the last, becomes vastly abundant in Silurian times, but dies out altogether before the end of the PalA|ozoic. _Rhynchonella_ (Fig. 58), which comes in a little later, near the beginning of the Siluro-Cambrian, continues to this day. _Spirifer_ and _Productus_ (Figs. 59 and 60) appear later, and die out at the close of the PalA|ozoic. So strange and inscrutable are the fortunes of these animals, which on the whole have lost in the battle of life, that their place in nature is vastly less important than it was. It has been suggested that if any group of creatures could throw light upon the theory of descent with modification, it would be these; but Davidson, who has perhaps studied them more thoroughly than any other naturalist, found them as silent on the subject as the sponges or the corals. In a series of papers published in the _Geological Magazine_, a short time before his death, he remarked as follows:

_a_, _O. Billingsi_ (Hartt). Lower Cambrian. _b_, _O. pectinella_ (Hall). Siluro-Cambrian. _c_, _O. lynx_ (Eichwald). Siluro-Cambrian.]

_a_, _b_, Exteriors. _c_, Interior, showing spirals.]

aEurooeWe find that the large number of genera made their first appearance during the PalA|ozoic periods, and since they have been decreasing in number to the present period. We will leave out of question the species, for they vary so little that it is often very difficult to trace really good distinctive characters between them; it is different with the genera, as they are, or should be, founded on much greater and more permanent distinctions. Thus, for example, the family _SpiriferidA|_ includes genera which are all characterised by a calcified spiral lamina for the support of the brachial appendages; and, however varied these may be, they always retain the distinctive characters of the group from their first appearance to their extinction. The Brachiopodist labours under the difficulties of not being able to determine what are the simplest, or which are the highest families into which either of the two great groups of his favourite class is divided; so far, then, he is unable to point out any evidence favouring progressive development in it. But, confining himself to species, he sees often before him great varietal changes, so much so as to make it difficult for him to define the species; and it leads him to the belief that such groups were not of independent origin, as was universally thought before Darwin published his great work on the _Origin of Species_. But in this respect the Brachiopoda reveal nothing more than other groups of the organic kingdoms.

aEurooeNow, although certain genera, such as _Terebratula_, _Rhynchonella_, _Crania_, and _Discina_, have enjoyed a very considerable geological existence, there are genera, such as _Stringocephalus_, _Uncites_, _Porambonites_, _Koninckina_, and several others, which made their appearance very suddenly and without any warning; after a while they disappeared in a similar abrupt manner, having enjoyed a comparatively short existence. They are all possessed of such marked and distinctive internal characters that we cannot trace between them and associated or synchronous genera any evidence of their being either modifications of one or the other, or of being the result of descent with modification. Therefore, although far from denying the possibility or probability of the correctness of the Darwinian theory, I could not conscientiously affirm that the Brachiopoda, as far as I am at present acquainted with them, would be of much service in proving it. The subject is worthy of the continued and serious attention of every well-informed man of science. The sublime Creator of the universe has bestowed on him a thinking mind; therefore all that can be discovered is legitimate. Science has this advantage, that it is continually on the advance, and is ever ready to correct its errors when fresh light or new discoveries make such necessary.aEuroŁ The late Joachim Barrande, the great palA|ontologist of Bohemia, bears similar testimony.

1, _Cucullea opima_. 2, _Nucula oblonga_. 3, _Nucula lineata_. 4, _Cypricardia truncata_. 5, _Tellina ovata_. 6, _Nucula bellatula_. 7, _Modiola concentrica_.]

The ordinary bivalves, like the mussels and cockles, now so very plentiful on our coasts, are rare in the Cambrian and Silurian, and for the first time make a somewhat conspicuous appearance in the Upper Silurian and Devonian. But from the first they resemble very closely their modern successors, though on the whole neither so large nor so ornate (Fig. 61). Their fortunes have thus been precisely the opposite of those of the Brachiopods, though in neither case is there very marked elevation or deterioration in the individual animals. A very similar statement may be made as to the sea-snails, whether the curious winged snails (Pteropods) or the ordinary crawlers (Gastropods). The former come in early, and are represented by PalA|ozoic forms finer than any now extant. The genus _Conularia_ (Fig. 62) presents some Silurian species six inches or more in length, which are giants in comparison with any now living. The forms of more ordinary Gastropods from the Silurian represented in Fig. 63 will suffice to show that their styles are not very dissimilar from those still extant.[17] As in the case of the ordinary bivalves, however, the modern Gastropods much exceed in numbers and magnitude those of the PalA|ozoic.

_a_, _Murchisonia bicincta_ (Hall). _b_, _Pleurotomaria umbilicatula_ (Hall). _c_, _Murchisonia gracilis_ (Hall). _d_, _Bellerophon sulcatinus_ (Billings).]

The highest group of Mollusks, represented in the modern ocean by the Nautili and Cuttle-fishes, has a history so strange and eventful, and so different from what might have been anticipated, that it perhaps deserves a more detailed notice, more especially as Barrande has recently directed marked attention to it in his magnificent work on the PalA|ontology of Bohemia.

The Cuttle-fishes and Squids and their allies are, in the modern seas, a most important group (Fig. 64). The great numbers in which the smaller species appear on many coasts, and the immense size and formidable character of others; their singular apparatus of arms, bearing suckers, their strange forms, and the inky secretion with which they can darken the water, have at all times attracted popular attention. The great complexity of their structures, and the fact that in many points they stand quite at the head of the invertebrates of the sea, and approach most nearly to the elevation of the true fishes, have secured to them the attention of naturalists. Some of these animals have shelly internal supports, and one genus, that of the Argonauts, or Paper Nautili, has an external protective shell. Allied, though more distantly, to the Cuttle-fishes, are the true Nautili, represented in the modern sea principally by the Pearly Nautilus, though there are two other species, both of them very rare. The modern pearly nautilus (Fig. 65) may be regarded as a peculiar kind of cuttle-fish provided with a discoidal shell for protection, and also for floatage. The shell is divided into a number of chambers by partitions. Of these the animal inhabits the last and largest. The others are empty, and are connected with the body of the animal only by a pipe, or siphuncle, with membranous walls and filled with fluid. Thus provided, the nautilus, when in the water, has practically no weight, and can move up or down in the sea with the greatest facility, using its sucker-bearing arms and horny beak to seize and devour the animals on which it preys. The buoyancy of the shell seems exactly adapted to the weight of the animal; and this proportion is kept up by the addition of new air-chambers as the body increases in size. In the modern seas this singular little group stands entirely isolated, and its individuals are so rare that it is difficult to procure perfect specimens for collections, though its mechanical structure and advantages for the struggle for existence seem of the highest order. But in the old world of past geological time the case was altogether different.

_a_, Mantle. _b_, Its dorsal fold. _c_, Hood. _o_, Eye. _t_, Tentacles. _f_, Funnel. _g_, Air chambers. _h_, Siphuncle.]

The Nautiloid shell-fishes burst suddenly upon us in the beginning of the Siluro-Cambrian, or Lower Silurian, BarrandeaEuro(TM)s second fauna; and this applies to all the countries where they have been studied. In this formation alone about 450 species are known, and in the Silurian these increase to 1,200; and here the group culminates. It returns in the Devonian to about the same number with the Lower Silurian, diminishes in the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms are replaced by others of the type of the Ammonites, becomes largely reduced. In the Tertiary there are but nineteen species, and, as already stated, in the modern world _three_. These statements do not, however, represent the whole truth. In the PalA|ozoic, in addition to the genus _Nautilus_, we have a great number of other genera, some with perfectly straight shells, like _Orthoceras_ (Fig. 66), others bent (_Cyrtoceras_), others differing in the style of siphuncle, or aperture, or chambers (_Endoceras_, _Gomphoceras_, _Lituites_, Figs. 67 to 69), or inflated into sac-like forms (_Ascoceras_). There is, besides, the family of the _GoniatidA|_ (Fig. 70), with the chambers thrown into angular folds and the siphuncle at the back. Further, some of the early forms, as the OrthoceratidA|, attain to gigantic dimensions, being six feet or more in length, and nearly a foot in diameter. Thus the idea that we should naturally form from the study of the Nautilus, that it represents a type suited for much more varied and important adaptations than those that we now see, is more than realised in those PalA|ozoic ages when these animals seem to have been the lords of the seas.

_a_, Shell, reduced. _b_, Section, showing siphuncle.]

When we leave the PalA|ozoic and enter the Mesozoic, though the Nautiloid shells still abound, we find them superseded, in great part, by a nobler form, that of the _AmmonitidA|_ (Figs. 71, 72). These are remarkable for the ornate markings on the surfaces of their shells, and for the beautifully waved edges of the partitions (Fig. 72_a_), which, by giving a much more complete support to the sides of the shell, must have contributed greatly to the union of lightness and strength so important to the utility of the shell as a float. This type admits of all the same variety of straight, bent, and curled forms with the simpler Nautiloid type, and some of the species are of great size, Ammonites being known three feet or more in diameter. These animals, unknown in the PalA|ozoic, appear in numerous species in the Early Mesozoic, culminate in hundreds of beautiful species in the middle of that era, and disappear for ever at its close, leaving no modern successors. Many and beautiful species of Ammonites and their allies have been obtained from the Mesozoic rocks of British Columbia and other parts of the west coast of North America, perfectly representing this group as it occurs at the same period in Europe, and closely resembling the Mesozoic Ammonites of India. These animals have all perished, yet the Atlantic and the Pacific roll between, apparently with conditions as favourable for their comfortable existence as those of any previous time. They perished long ago, at the dawn of the Tertiary; yet the genus Nautilus, one of the oldest and least improved of the whole, survived, and still testifies to the wonderful contrivance embodied in these animals.

_a_, _Baculites_. _b_, _Ancyloceras_. _c_, _Crioceras_. _d_, _Turrilites_.]

These are merely general considerations, but Barrande, in his _A%tudes GA(C)nA(C)rales_, goes much farther. He sums up all the known facts in the most elaborate manner, considering first the embryonic characters of the shell in the different genera, then their distribution in space and time, then all the different parts and characters of the shells in the different groupsaEuro"the whole with reference to any possible derivation of the species; and he finds that all leads to the result that in every respect these shells seem to have been so introduced as to make any theory of evolution with respect to them altogether untenable. In his concluding sentence this greatest of PalA|ozoic palA|ontologists affirms that, aEurooeThe theoretical evolution of the Cephalopods is, like that of the Trilobites, a mere figment of imagination, without any foundation in fact.aEuroŁ[18]

I have reserved no space to notice the geological history of the other and higher group of Cephalopods, including the true Cuttles and Squids. This is perhaps less to be regretted, as, from the absence of external shells, they are likely to be much less perfectly known as fossils. So far as known, they are vastly younger than the Nautiloids, for no examples whatever have been found in the PalA|ozoic. They appear abundantly in the Mesozoic, but are there represented principally by an extinct group of squids (Belemnites and their allies, Figs. 74, 74_a_), remarkable for the great and complicated development of their internal support, which has a chambered float as well as a solid sheath. This family becomes extinct at the close of the Mesozoic, though the cuttles as a whole perhaps culminate in the modern.

_a_, _Paradoxides_. _b_, _Dikellocephalus_. _c_, _Conocoryphe_ (head). _d_, _Agnostus_ (head and tail).]

The remarkable group of the Trilobites had precedence in order of time of the Nautiloid shell-fishes. No animal structures can well be more dissimilar than those of the two great groups of aquatic animals which popular speech confounds under the name of aEurooeshell-fishes.aEuroŁ Take a whelk and a crab, for example, and compare their general forms, the structure of their shells, and their organs of motion, and it is scarcely possible to imagine any two animals more unlike; and when we examine their anatomy in detail this difference does not diminish. They have, it is true, corresponding parts, and these parts serve similar uses, but in plan of structure they are wholly different. Yet both animals may live in the same pool, and may subsist on nearly the same food. If we attempt to find some common type which both resemble, we may trace the structure of the crab back to those of some of the marine worms with which it has some affinity, and those of the whelk to such creatures as the _Lingula_, which are supposed to have a resemblance to the worms. But still the two types, that of the Mollusk and the Articulate, are distinct even from their first appearance in the egg, nor have either any close affinities with the Protozoa, the Hydroids, or the Corals.

_a_, Dorsal shell. _b_, Visceral cavity. _c_, Legs. _d_, EpipoditeaEuro"gill-cleaner or palp. _e_, Spiral gills.]

Both types meet us in the Early Cambrian, but while the Mollusk is there represented only by low forms, the Articulate is then not only in the humble guise of the worm, but in the complex and highly organised form of the Trilobite (Figs. 28 and 75). What older phases they may have passed through we know not; but in the Lower Cambrian we have various forms of these animals, including some of the largest known as well as some of the smallest; some of the most complex in number of parts as well as some of the simplest. These animals, in short, seem to have appeared at once all over the world fully formed, and in a variety of generic and specific forms; and nothing short of a very large faith in the imperfection of the geological record can suffice to account for their evolution.

A Trilobite is a creature in whose structure the number three is dominant. Seen from above, it presents three divisions from front to rear:aEuro"first, a cephalic shield or head-piece; secondly, a thorax, divided into several segments movable upon each other; and thirdly, a tail-piece or pygidium, which, when brought against the head by the rolling up of the body segments, effectually covers the lower parts. This lower portion was until lately little known; but the discoveries of Billings and of Wolcott have enabled us to restore the jaws under the head, the jointed legs and spiral gills under the thorax, and thus to complete the structure of the animal, and understand better its relations to modern crabs and shrimps (Fig. 76). Of these it certainly comes nearest to the King-crabs and Horseshoe-crabs, a somewhat limited group at present, and one which reaches back in geological time only to the Upper Silurian, when the Trilobites had perhaps already passed their culmination.

Constructed as above described, the Trilobite could swim, as is supposed, usually on its back or side. It could crawl on the bottom. Using its snout as a shovel, it could burrow like a modern King-crab (Fig. 76_a_); and when pressed by danger some species could roll themselves into balls and defy their enemies.

_a_, _Isotelus_. _b_, _Homalonotus_. _c_, _Calymene_.]

This type of animal, entering on the stage in full force in the Older Cambrian, continues under many forms through the whole PalA|ozoic age, dying out finally in the Carboniferous. Figs. 77 and 78 show a few of the forms of the Silurian, Devonian, and Carboniferous.

Contemporaneously with the dawn of the Trilobite group, appear some small shrimp-like forms (Fig. 28),[19] and others with bivalve shells (Fig. 79), which are closely allied to modern forms,[20] and, like the _LingulA|_, persist through the succeeding formations with little more than specific changeaEuro"presenting in this a strange contrast to the Trilobites. While the latter were still flourishing, about the close of the Lower Silurian, a remarkable group of large and highly-developed creatures, allied to the Trilobites, but suited for rapid swimming rather than creeping, was introduced; and in the Upper Silurian and Devonian these creatures[21] attained to gigantic sizes, exceeding, probably, any modern Crustaceans, and were tyrants of the seas. _Pterygotus anglicus_ (Fig. 80) is supposed to have attained the length of six feet. Yet these noble representatives of the Crustaceans became extinct in the Carboniferous. On the other hand, a few small king-crabs appear in the Upper Silurian, and this type still continues, and seems to culminate as to size in modern times; so diverse have been the fortunes of these various groups.

_a_, _Phaceps latifrons_ (Bronn). _b_, _Philipsia Howi_ (Billings) (tail).]

_a_, _Bairdia_. _b_, _Cytherella inflata_ (Jones). _c_, _Cythere_. Carboniferous. _d_, _Beyrichia Jonesii_ (Dn.). Carboniferous. _e_, _Beyrichia pustulosa_ (Hall). Silurian.]

The higher, or decapod Crustaceans, now familiar to us in the modern crabs and lobsters, are first found in a few small species in the Devonian[22] and Carboniferous, and they are accompanied in the Devonian by at least one species of the allied group of the Stomapods (Figs. 81, 82).

The PalA|ozoic age of geology is thus emphatically an age of invertebrates of the sea. In this period they were dominant in the waters, and until toward its close almost without rivals. We shall find, however, that in the Upper Silurian, fishes made their appearance, and in the Carboniferous amphibian reptiles, and that, before the close of the PalA|ozoic, vertebrate life in these forms had become predominant. We shall also see that just as the leading groups of Mollusks and Crustaceans seem to have had no ancestors, so it is with the groups of Vertebrates which take their places. It is also interesting to observe that already in the PalA|ozoic all the types of invertebrate marine life were as fully represented as at present, and that this swarming marine life breaks upon us in successive waves as we proceed upward from the Cambrian. Thus the progress of life is not gradual, but intermittent, and consists in the sudden and rapid influx of new forms destined to increase and multiply in the place of those which are becoming effete and ready to vanish away or to sink to a lower place. Farther, since the great waves of aquatic life roll in with each great subsidence of the land, a fact which coincides with their appearance in the limestones of the successive periods, it follows that it is not struggle for existence, but expansion under favourable circumstances and the opening up of new fields of migration that is favourable to the introduction of new species. The testimony of palA|ontology on this point, which I have elsewhere adduced at length,[23] in my judgment altogether subverts the prevalent theory of aEurooesurvival of the fittest,aEuroŁ and shows that the struggle for existence, so far from being a cause of development and improvement, has led only to decay and extinction, whereas the advent of new and favourable conditions, and the removal of severe competition, are the circumstances favourable to introduction of new and advanced species. This testimony of the invertebrates of the sea we shall find is confirmed by other groups of living beings, to be noticed in the sequel.[24]

* * * * *

NOTE.aEuro"The term aEurooeSiluro-Cambrian,aEuroŁ as used in this and the next chapter, is synonymous with aEurooeOrdovicianaEuroŁ of Lapworth, which is now coming into somewhat general use.