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In the remaining Molluscs the higher or more effective types are displacing the older. It is interesting to note that the oyster is fully developed, and has a very large kindred, in the Mesozoic seas. Among the Brachiopods the higher sloping-shoulder type displaces the square-shoulder shells.
The Elodians, found in the shallow pools of the Amazonian Valley, swim with facility, and move quickly over the ground. They feed not only on vegetables, but prey on living animals river molluscs, and other water creatures. The Potamians, which are found in vast numbers in the larger rivers, grow to a great size, some weighing seventy pounds. They feed much as the last described.
The fact that molluscs do intern foreign and obnoxious substances is testimony to their decency and love of cleanliness, and so may the pearl be still accepted as the embodiment of purity. Though all its little soul be dirt, the pearl is pure, and but for the dirt or the germ of a filthy ailment it would not be pearl.
In the bulk of the Devonian sharks these developed into what are significantly called "pavement teeth." They were solid plates of enamel, an inch or an inch and a half in width, with which the monster ground its enormous meals of Molluscs, Crustacea, sea-weed, etc. A new and stimulating element had come into the life of the invertebrate world.
Experimentally I have placed these molluscs in fresh water, to find it quickly dyed to a rich amber colour while acquiring quite remarkable pungency. Even after the third change the water was impregnated.
And we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part, as we find in the other great classes of the animal and vegetable kingdoms.
A boat indeed! It bears the creature which secretes it without its adhering to it. For nearly an hour the Nautilus floated in the midst of this shoal of molluscs. Then I know not what sudden fright they took. But as if at a signal every sail was furled, the arms folded, the body drawn in, the shells turned over, changing their centre of gravity, and the whole fleet disappeared under the waves.
The bed of the sea was converted into dry land, the strata were flexured and denuded, and a depression was formed in the granitic floor of Central France, which, in the succeeding Miocene period, was converted into an extensive lake peopled by molluscs, fishes, reptiles, and pachyderms of the period. 2nd Stage.
Some of these molluscs do not build shells. But most of them build a shelly house for themselves; they do this to defend their soft bodies from the attacks of a host of enemies. Some build two shells the Oyster and Mussel do, as you know.
Already he meditated a conchology of Corsica, a colossal history of all the molluscs which live upon its soil or in its waters. He analysed, described, classed, and co-ordinated not only the marine species, but the terrestrial and freshwater shells also, extant or fossil.
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