In regard to the Sea-worms, the burrows, castings, and trails found in the pre-Cambrian beds are scarcely, if at all, different from those now seen on sandy and muddy shores, and would seem to indicate that these highly organized and very sensitive and active creatures swarmed in the muddy bottom of the pre-Cambrian Sea, and lived in the same way as at present. It is impossible, however, to know anything of the internal structures of these creatures, but the marks left by their bristle-bearing feet seem to indicate that some of them at least belong to the higher group of Sea-centipedes, creatures rivalling the Crustaceans in complexity of organization, and near to them in plan of structure, though at present usually widely separated from them in current systems of classification. In the Ordovician system, next above the Cambrian, Hinde has found many curiously formed jaws of animals of this kind, which show at least that their alimentary arrangements were similar to those now in force. If any of the problematical "Conodonts" discovered by Pander in the Cambrian of Russia belonged to marine worms, this inference would be extended back to the Lower Cambrian, so that if the evidence of structure anywhere remains we may hope to find that the pre-Cambrian worms were not inferior to their more modern successors, perhaps even that in this early period, when they probably played a more important part in nature, they were of higher organization than in later times.
The evidence as to pre-Cambrian mollusks, so far as it goes, is even more curious. The little shell called Volborthella, so far as can be judged from its form and internal structure, is a miniature representative of these straight Nautili, the Orthoceratites of the Ordovician and later Palæozoic rocks; and no one doubts that these latter belong to the highest class of the Mollusks, a class approaching in the development of nerve system and sensory organs to the Vertebrates themselves. This tiny member of the great class of Cuttle-fishes may perhaps have been more nearly allied to the modern Spirula than to the Nautilus. In any case, if, as seems altogether probable it was, a mollusk, it must have been one of advanced type, and with a highly complex structure, as well as the singular apparatus for flotation implied in a chambered shell with a siphuncle.
Next to this among these primitive Mollusks are straight and spiral shells representing those delicate and beautiful animals of the modern seas, the Pteropods, or wing-footed Sea-snails, beautiful and graceful creatures, the butterflies of the sea, and moving in the water with the greatest ease and beauty by the aid of membranous fins, or wings, sometimes brightly coloured. These creatures abound in all latitudes in the modern ocean, and their delicate shells sometimes accumulate in beds of "Pteropod sand." They very early entered on the arena of marine life, and have continued to this day.
We miss here the two great Molluscan groups of the creeping Sea-snails like the limpet and whelk, and of the ordinary bivalves like the oyster and cockle. Both are present in the lowest Cambrian, though in small numbers compared with their present abundance. Possibly they had not yet appeared in the Etcheminian Sea, though the muddy and sandy bottoms, evidenced by its slates and sandstones, would seem to have afforded favourable habitats, and warrant the expectation that species may yet be found.
The case was different with the little group of the Lamp-shells, or Brachiopods. These creatures, somewhat resembling the ordinary bivalves in their shelly coverings, were very dissimilar in their internal structure, and once settled on the bottom they were attached for life, not having even the limited means of locomotion possessed by the Sea-snails and common bivalves. They collected their food wholly by means of currents of water produced by cilia, or movable threads, on arms or processes within their shells. In this they resembled the young or embryo stages of some of the more ordinary Mollusks, though they are so remote from these in their adult condition that they have usually been placed in a distinct class, and some naturalists have thought it best to separate them from the Mollusks altogether. Their history is peculiar. Coming into existence at a very early date, they became very abundant in early Palæozoic times, then gradually gave place to the ordinary bivalves, and in the modern seas are represented by very few species. Yet while in the middle period of their history they are represented by very many peculiar specific and generic forms. Some of the earliest types, like Obolus and Lingula, persist very long, and the latter has continued without change from the Early Cambrian to the Modern period.
The great group of the Sea-stars and Sea-urchins appears only in a few of its lower forms, and seems to be the only class represented by embryonic types. The coral animals are absent, so far as known. The Jelly-fishes and their allies cannot be preserved as fossils, but some peculiar markings, at one time regarded as plants, are now supposed to be trails made by the tentacles of creatures of this kind moving over muddy bottoms. A few spicules indicate Sponges, and the ubiquitous groups of the marine Protozoa, the Foraminifera and the Radiolaunus, are represented by shells scarcely distinguishable from those of modern species. The great and peculiar forms represented at this early time by Cryptozoon and its allies seem long ago to have perished, and we shall have to return to them in a later stage of our inquiry.
To sum up the little that we know of this earliest Palæozoic life:—It was perfect of its kind, equally pregnant with evidences of design, and of the nicest and most delicate contrivance as the animal life of any later time, and it presupposed vegetable life and multitudes of minute organic beings altogether unknown to us to nourish the creatures we do know. As an example of this, a little Brachiopod or sponge nourished by the currents produced by its cilia, or a Jelly-fish gathering food by its thread-like tentacles, or a Globigerina selecting its nourishment by its delicate gelatinous pseudopods, required an ocean swarming with minute forms of life, which probably can never be known to us, but every one of which must have been an inscrutable miracle of organization and vital function.
Lastly, with reference to our present subject, the Etcheminian fossils carry life backward one whole great period earlier than the Lower Cambrian, and appear to indicate that we are approaching a beginning of living things in the Palæozoic world. Much no doubt remains to be discovered, but it would seem that any future discoveries must fail to negative this conclusion.
The Huronian.
In whatever way the rocks immediately below the Cambrian may be classified, it is certain that the next system in descending order is that to which Logan long ago gave the name Huronian, from its development on Lake Huron[12]—a name to which it is still entitled, though there may, perhaps, be some grounds for dividing it into an upper and lower member.[13] To this sub-division, however, we need not for the present give any special attention. In the typical area of Lake Huron the Huronian consists of quartzites, which are merely hardened sandstones, of slates which are muddy or volcanic-ash beds, of conglomerates or pebble-rocks, and of coarse earthy limestone. With these rocks are deposits of igneous material which represent contemporary volcanic eruptions. In other districts, as in New Brunswick, Newfoundland, etc., the beds have been considerably altered, and are locally more mixed with igneous products. The physical picture presented to us by the Huronian is that of a shore deposit, formed under circumstances in which beds of pebbles and sand were intermixed with the products of neighbouring volcanoes.