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It is incontestable that the musculature of the Vertebrates has been evolved from that of lower Invertebrates; and among these we have to consider especially the unarticulated Vermalia. They have a simple cutaneous muscular layer, developing from the mesoderm.
In this tail is developed starting from the primitive gut a cylindrical string of cells, the fore end of which pushes into the body of the larva, between the alimentary canal and the neural canal, and is no other than the chorda dorsalis. This important organ had hitherto been found only in the Vertebrates, not a single trace of it being discoverable in the Invertebrates.
The first of the three sections into which this period of life is divided is known as the Silurian age, the age of invertebrates. The word invertebrate is an unscientific but convenient term under which we embrace all the animals below those having backbones.
Nearly forty years ago, in his first great work on fossil fishes, in developing principles of classification, he wrote in quotations, 'An invisible thread in all ages runs through this immense diversity, exhibiting as a general result that there is a continual progress in development ending in man, the four classes of vertebrates presenting the intermediate steps, and the invertebrates the constant accessory accompaniment.
Now, just as plants have been arranged in different classes, so animals are classified according to the various plans upon which they have been formed. So, besides the two great divisions of the Vertebrates and the Invertebrates, the latter have been classed as
The legs of an insect, for instance, are small tubes with the muscles inside. The limbs of vertebrate animals, on the other hand, have the muscle outside the bone. Invertebrates commonly have the main nerve trunk in front, or underneath, instead of at the back, and likewise often have their brains in their abdomens. Some of them, such as the grasshopper, even hear with their abdomens.
It was a fortunate accident that Kowalevsky first examined just those larger specimens of the Ascidiae that show most clearly the relationship of the vertebrates to the invertebrates, and the larvae of which behave exactly like those of the Amphioxus in the first stages of development.
The decomposition of the spores has made the shales highly bituminous, and the oil and gas have accumulated in the reservoirs of overlying porous sandstones. INVERTEBRATES. We must pass over the ever-changing groups of the invertebrates with the briefest notice. Chain corals became extinct at the close of the Silurian, but other corals were extremely common in the Devonian seas.
The break in the supposed chain of evolution between the invertebrates and the highly organized vertebrates of the Lower Silurian is one of the greatest in the whole geological record. The vast gulf between these structures must, I think, remain unbridged except by the imagination." The late Prof.
Thus, all the giant reptiles died out the dinosaurs, the mesosaurs but the reptilian impulse still survives. How many types of invertebrates have perished! but the invertebrate impulse still goes on. How many species of mammals have been cut off! yet the mammal impulse has steadily gone forward. These things suggest the wave that moves on but leaves the water behind.
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