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The structural features that distinguish the vertebrates from the invertebrates are so prominent that there was the greatest difficulty in the earlier stages of classification in determining the affinity of these two great groups.
In the scheme, the material must become living by spontaneous generation; some plants must become invertebrate animals; some invertebrates must become vertebrates; some marine animals must become amphibians; some amphibians must become reptiles; some reptiles must become mammals; some mammals must become humans; some senseless, soulless simians must acquire a soul and become spiritual enough to bear the image of God.
Three different layers of cells, arranged in strings, that interpenetrate to form a network directly bathed by blood, that breaks in upon them from open blood vessels, compose the cortex. Most remarkable is this method of blood supply for it is exceedingly common among the invertebrates and rare among the vertebrates.
Adrenalin, that weapon of a gland tracing its ancestry back to the begetter of the brain itself, for brain and adrenal gland both have evolved from the small nerve ganglia of the invertebrates, would have backed up to the hilt his argument, which he had to elaborate on the indirect grounds of analogy and induction.
According to this view one cell accidentally developed the attributes of vegetable life; a further accident leads another cell to initiate the line of invertebrates; another that of fishes, let us say; another of mammals: the number varying according to the views of the theorist on phylogeny.
In the invertebrates, who date further back in the history of the planet than any vertebrate, the nervous system consists of discrete patches of nerve cells, the ganglions composing the ganglionic system of which the vegetative or autonomic nervous system of man is the direct descendant and representative.
The continental areas thus enlarged were peopled during the ensuing Mesozoic time with multitudes of strange reptiles, many of them gigantic in size. The waters, too, still teeming with invertebrates and fishes, had their quota of reptilian monsters; and in the air were flying reptiles, some of which measured twenty-five feet from tip to tip of their batlike wings.
The auscultory organ of most of the Invertebrates has substantially the same composition. It usually consists of a closed vesicle, filled with fluid, and containing otoliths, with the acoustic nerve expanding on its wall. But, while the auditory vesicle is usually of a simple round or oval shape in the Invertebrates, it has in the Vertebrates a special and curious structure, the labyrinth.
He soon learned that the museum curator was not only an authority on his own subject of marine invertebrates, but that he was interested to the utmost in all sorts of affairs, and he admitted confidentially to the boy that he was an inveterate baseball fan.
Whatever the cause, his principle of classification was not fully appreciated; but it opened a new road for study, and gave us the keynote to the natural affinities among animals. Lamarck, his contemporary, not recognizing the truth of this principle, distributed the animal kingdom into two great divisions, which he calls Vertebrates and Invertebrates.
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