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Yet it was precise and hugely expert too; for though there was a strange mist on the brain, the body felt its way with a singular certainty, as might some molluscan dweller of the sea, sensitive like a plant, intuitive like an animal. Yet at times it seemed that this vast body overcoming the mountain must let go its hold and slide away into the darkness of the depths.
This supposition is negatived by the fact that the mollusca on the two coasts, separated by the narrow Isthmus of Darien, are almost entirely distinct, whilst we know that since the glacial period there has been little change in the molluscan fauna, nearly, if not all, the shells found in glacial deposits still existing in neighbouring seas.
There is no a priori improbability in such a belief, for besides other annelids, especially the Arenicola marina, which throws up such a profusion of castings on our tidal sands, and which it is believed thus subsists, there are animals belonging to the most distinct classes, which do not burrow, but habitually swallow large quantities of sand; namely, the molluscan Onchidium and many Echinoderms.
It is well known to the polar traveller as a migratory bird of the American continent. Like the others of the same family, it feeds upon vegetable matter, generally on marine plants, with their adherent molluscan life. It is rarely or never seen in the interior; and from its habits may be regarded as singularly indicative of open water.
The remains which do become embedded, if in sand or gravel, will when the beds are upraised generally be dissolved by the percolation of rain-water. I suspect that but few of the very many animals which live on the beach between high and low watermark are preserved. The molluscan genus Chiton offers a partially analogous case.
Do the facts sustain this assumption? The little animals whose remains compose the great chalk-beds are alive and working. Inarticulate or molluscan life is seen in a sub-fossil condition in the Post Pliocene clays of Canada. They are just as they were in the beginning of their history. Species seem to be immutably fixed.
They have an abundance of food, and "wax fat." The clam is so completely protected by his shell and the mud that he has little to fear from enemies. They have increased and multiplied and filled the mud. "Requiescat in pace." But zoölogy has its tragedies as well as human history. Let us turn to the development of a third molluscan line terminating in the cuttle-fishes.
Its molluscan characteristics were only skin-deep, evidently an adaptation to a mode of life like that of mollusks. The old artificial systems were based too much on merely external characteristics, the results of adaptation. When the internal anatomy had been thoroughly studied their groups had to be rearranged.
The molluscan and articulate lines, though divergent, are of great importance to us as throwing a certain amount of light on vertebrate development; and still more as showing how a certain line of development may seem, and at first really be, advantageous, and still lead to degeneration, or at best to but partial success.
The clams and oysters show even more clearly what we might call the logical results of molluscan structure. They increased the shell until it formed two heavy "valves" hanging down on each side of the body and completely enclosing it.
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