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Moreover, we have seen that the parapodia of annelids naturally point to the development of an external skeleton, for their muscles are already a part of the external body-wall and attached to the already existing horny cuticle. The logical goal of their development was the insect.
But if we exclude the Platodes and the Annelids from this stem, we find a fairly satisfactory unity of organisation in the remaining classes. Among these worms we find some important forms that show considerable advance in organisation from the platode to the chordonia stage.
Thus the filmy and delicately beautiful structures that decorate the pleasant home of the quaint little seahorse in the aquarium structures having more the appearance of miniature palm-trees than of animals are really annelids. One can view Dr.
"Monkey" and "Worm" have been the bywords of reproach among the more supercilious of human beings, whether schoolboys or theologians. And it was precisely through the Anthropoid Apes, and more remotely the Annelids, that Darwin sought to trace the ancestry of Man. The Annelids have been rejected, but the Arachnids have taken their place.
They are numerous on some hills near Turin at from 2000 to 3000 feet above the sea, and at a great altitude on the Nilgiri Mountains in South India and on the Himalaya. Earth-worms must be considered as terrestrial animals, though they are still in one sense semi-aquatic, like the other members of the great class of annelids to which they belong.
But it is conceivable that the now utterly lost branchiae might have been gradually worked in by natural selection for some quite distinct purpose: in the same manner as, on the view entertained by some naturalists that the branchiae and dorsal scales of Annelids are homologous with the wings and wing-covers of insects, it is probable that organs which at a very ancient period served for respiration have been actually converted into organs of flight.
That the mouth of an animal can migrate seems at first impossible, but if we had time to examine the embryology of annelids and insects, it would no longer appear inconceivable or improbable. And its backward migration brought it among the legs which were grasping and chewing the food. And in vertebrates the mouth has changed its position, though not in exactly the same way.
Slender as our knowledge is of the earlier evolution of the Invertebrate animals, we return to our Cambrian population with greater interest. The uncouth Trilobite and its livelier cousins, the sluggish, skulking Brachiopod and Mollusc, the squirming Annelids, and the plant-like Cystids, Corals, and Sponges are the outcome of millions of years of struggle.
Thus in the advance of annelids to insects the muscular system increases in relative bulk, and absolutely in complexity. But a change or increase in the muscle must be accompanied by corresponding changes in the motor-nerve fibrils; and these again would be useless unless accompanied by increased complexity and more or less readjustment of the cells and fibrils of the nerve-centres.
We know as yet almost nothing of their history; when deciphered it will be as strange as any romance. The vertebrates are of course the most important line, as including the ancestors of man. But we must take a little glance at mollusks, including our clams, snails, and cuttle-fishes; and at the articulates, including annelids and culminating in insects.
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