• Bony fishes have the most complex gills, composed of gills filament which are made up of richly vascularized transverse plates.
• A continuous one way circulation of water from outside, through the mouth and over the the gill filaments ventilates the gills. Gaseous exchange is enhanced by blood flowing in the opposite direction to water flow.
• The tracheal system in arthropods consists of a branching system of air tubes (tracheae and tracheoles) which open to the exterior through spiracles.
• Air flow in the tracheal system is regulated by opening and closing the spiracles. In large and active insects, air is pumped in and out of the tracheal system by alternately flattening and relaxing the body.
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