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To form its skeleton the crab had only to thicken the cuticle already present in the annelid. It had to modify the already existing parapodia and their muscles, changing them to legs. The external skeleton gave from the start a double advantage protection and better locomotion. Every grain of thickening aided the animal in the struggle for existence in both these ways.
The annelid had two anterior segments differing from those of the rest of the body; these may, perhaps, be considered as the foreshadowings of a structure not yet realized; they can only by courtesy be called a head. Thirdly, the insect has legs. The annelid had fin-like parapodia, approaching the legs of insects about as closely as the fins of a fish approach the legs of a mammal.
In this part of the body the skeletal ring of each segment is joined to that of the segments before and behind it in the same manner. But in other parts of the body we shall find the skeletal pieces of each segment and the rings of successive segments fused in one plate of mail. The legs are the parapodia of annelids carried to a vastly higher development.
In the annelid they are projections of the parapodia, in the mollusk expansions of the skin, where the foot or creeping sole joins the body. Why did the vertebrate take a new and strange, and, at first sight, disadvantageous mode of breathing? There must have been some good reason for this.
These latter have been grouped in the four quadrants, a much more effective arrangement than the cylindrical layer of the schematic worm. Furthermore, the animal has on each segment a pair of fin-like projections, stiffened with bristles, the parapodia. These are moved by special muscles and form effective organs of creeping.
The surface of the body no longer suffices to gather oxygen, hence we find special feathery gills on the parapodia. But these gills are merely expanded portions of the body wall, arranged so as to offer the greatest possible amount of surface where the capillaries of the blood system can be almost immediately in contact with the surrounding water.
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.
Between these two was the perivisceral cavity, filled with nutritive fluid, lymph, and furnishing a safe lodging-place for the more delicate viscera. It represented fairly the trunk of higher animals. The annelid added segmentation, and thus greater freedom of motion by the parapodia. But the segments were still practically alike.
And it must already have become clear to you that the destiny of these different lines was fixed not so much directly by the skeleton itself as by its reflex effect in moulding the muscular, and ultimately the nervous, system. The insects formed their skeleton by thickening the horny cuticle of the annelid. They transformed the annelid parapodia into legs and developed wings.
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