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Thus, for instance, an insect, standing at the head of the articulated animals, is, in the larva state, a true annelid, or worm, the annelida being the lowest in the same class. The embryo of a crab resembles the perfect animal of the inferior order myriapoda, and passes through all the forms of transition which characterize the intermediate tribes of crustacea.
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 hit upon a most advantageous line of development, which led ultimately to the insect. The study of the insect will show us clearly the advantages and defects of the annelid plan. First of all, the insect, like the mollusk, has an external skeleton. But the skeleton of the mollusk was purely protective, a hindrance to locomotion.
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