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Updated: May 23, 2025
So again, while the group of fishes, termed ganoids, is, at the present time, so distinct from that of the dipnoi, or mudfishes, that they have been reckoned as distinct orders, the Devonian strata present us with forms of which it is impossible to say with certainty whether they are dipnoi or whether they are ganoids.
But the condition of the Dipnoi, which possess lungs but do not walk on land, does not support this supposition, for they possess fins which are either filamentous or fin-like, having a central axis with rays on each side.
The fish lives altogether in the water and breathes by gills, but the dipnoi among fishes breathes by lungs as well as gills. As long as respiration takes place by gills alone, the circulation is simple; the blood flows from the heart to the gills, and thence directly all over the body; the oxygenated blood from the gills does not return directly to the heart.
Some species of fish known as the Dipnoi or "double-breathers," have a remarkable dual system of breathing. That is, they have gills for breathing while in the water, and also have a primitive or elementary "lung" in the shape of an air-bladder, or "sound," which they use for breathing on land.
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