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We have seen that in the Worms the respiratory organs are mere vesicles, while in the Crustacea they are more highly organized gills; but in Centipedes, Spiders, and Winged Insects, the breathing-apparatus is aerial, consisting of air-holes on the sides of the body, connected with a system of tubes and vessels extending into the body and admitting air to all parts of it.
They differ from the Brachiopods chiefly in the higher character of their breathing-apparatus; for they have free gills, instead of the net-work of vessels on the lining skin which serves as the organ of respiration in the Brachiopods.
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