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d. Mechanical origin of the limbs and of their jointed structure
We have already hinted at the mode of origin of the limbs of arthropods. Like the body or trunk, the limbs are chitinous dermo-muscular tubes, with a dense solid cuticle, and internal muscles, and were it not for their division at more or less regular intervals into segments, forming distinct sets of levers, set up by the strains in these tubular supports, there would be no power of varied motion.
Even certain worms, as already stated, have their tentacles and parapodia, or certain appendages of their parapodia, more or less jointed, but there are no indications of claws or of any other hard chitinous armature at the extremity, and the skin is thin and soft.
In the most simple though not the most primitive arthropods, such as the Tardigrades, whose body is not segmented, there are four pairs of short unjointed legs, ending each in two claws, which have probably arisen in response to the stimulus of pushing or dragging efforts.