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In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome and still found in many fish. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift.In the end, both buoyancy and breathing may have been important, and some modern physostome fishes do indeed use their bladders for both.As the fish swims, water flows into the forward pair, across the olfactory tissue, and out through the posterior openings.

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To function in gas exchange, lungs require a blood supply.

In cartilaginous fishes and teleosts, the heart lies low in the body and pumps blood forward through the ventral aorta, which splits up in a series of paired aortic arches, each corresponding to a gill arch.

The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period.

The early types resembled their cartilaginous ancestors in many features of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.

This allowed for a movable joint at the base of the fins in the early bony fishes, and would later function in a weight bearing structure in tetrapods.

As part of the overall armour of rhomboid cosmin scales, the skull had a full cover of dermal bone, constituting a skull roof over the otherwise shark-like cartilaginous inner cranium.

In order for the lungs to allow gas exchange, the lungs first need to have gas in them.

In modern tetrapods, three important breathing mechanisms are conserved from early ancestors, the first being a CO The second mechanism for a breath is a surfactant system in the lungs to facilitate gas exchange.

The evolution of the tetrapods' internal nares was hotly debated in the 20th century.

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