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Updated: June 15, 2025
But many facts of embryology and comparative anatomy point to such a form as a very possible ancestor of all forms higher than flat worms, viz., mollusks, arthropods, and vertebrates. It was probably rather long and cylindrical, resembling a small and short earthworm in shape. The skin may have been much like that of turbellaria.
The molluscs of that time had a shell more universally than those of to-day. The arthropods in general were provided with a carapace; most of them were crustaceans. The more ancient fishes had a bony sheath of extreme hardness.
Thus arose the early Arthropods, on tough-coated, jointed, articulated animals. Some of these remained in the water, breathing by means of gills, and became the Crustacea. Some, however, migrated to the land and developed what we may almost call "lungs" little tubes entering the body at the skin and branching internally, to bring the air into contact with the blood, the tracheae.
AMPHIBIANS. The vertebrates had now followed the arthropods and the mollusks upon the land, and had evolved a higher type adapted to the new environment. Amphibians the class to which frogs and salamanders belong now appear, with lungs for breathing air and with limbs for locomotion on the land.
Our observations upon Lucy and Eliza, however, had the effect of making us take an increased interest thenceforth in spiders in general, which till that time we had treated with scant courtesy, and set us about learning something as to the extraordinary variety of life and habit to be found within the range of this single group of arthropods, at first sight so extremely alike in their shapes, their appearance, their morals, and their manners.
Of invertebrate land animals the same must be said. The land mollusks and the great order of insects and other land arthropods only to a minor extent dwell in the open light. Very many species haunt the semi-obscurity of trees or groves, hide among the grasses, lurk under bark, sticks, and stones, or dwell through most of their lives underground. Hosts of others are nocturnal.
How later Crustacea increase the toughness of the coat by deposits of lime, and lead on to the crab and lobster, and how one early branch invades the land, develops air-breathing apparatus, and culminates in the spiders and insects, will be considered later. We shall see that there is most remarkable evidence connecting the highest of the Arthropods, the insect, with a remote Annelid ancestor.
Then again arises another divergence which gives rise to two paths or tendencies, one along the line of the arthropods, at the end of which come the ants and the bees with their instincts, and the other along the line of the vertebrates, at the end of which is man with his intelligence.
These animals may have resembled some of our worms, but with this difference, however, that the worms living to-day, to which they could be compared, are but the empty and fixed examples of infinitely plastic forms, pregnant with an unlimited future, the common stock of the echinoderms, molluscs, arthropods, and vertebrates.
These shells and armors begat a kind of torpor and immobility which has continued down to our day with the echinoderms and mollusks, but the arthropods and vertebrates escaped it by some lucky stroke. Now you and I are here without imprisoning shells on our backs; but how or why did we escape? Bergson does not say. Was it a matter of luck or chance?
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