“How do earthworms breathe in the earth?” “

They breathe… air!

There are different types of soils which differ, among other things, by their porosity. While some soils are very compact, such as clay, and do not allow water or gas to pass (and therefore no oxygen to breathe), others are more permeable. The soils in which earthworms live are permeable enough to allow air (and water) to seep between the grains. In addition, earthworms dig galleries by “eating” the earth to feed themselves (they sort out what is edible in their digestive tract before throwing the earth behind them). They therefore actively participate in the aeration and hydration of the soil, which makes them valuable aids for good soil health.

So air circulates over the skin of the earthworm. But it has neither lungs (like us), nor gills (like fish or sea worms). He breathes through the skin! Over a large part of her body, she is extremely thin. This is called skin respiration. However, the skin must remain moist so that the oxygen in the air mixes and can pass (we say to diffuse) through the skin. The earthworm therefore secretes mucus which makes it viscous to the touch. And in addition it helps it to slide in its galleries.

And just under the skin there are tiny little blood vessels, capillaries, with really red blood flowing. Red, because earthworms have hemoglobin in their blood, almost like us. Almost, because they do not have red blood cells: their hemoglobin is a large molecule, 50 times larger than ours, directly in solution in the blood and capable of capturing a hundred molecules of oxygen (compared to 4 with us … ) as it passes through the capillaries of the skin.

The blood in the capillaries once recharged with oxygen (and free of CO₂, carbon dioxide, at the same time) joins the large dorsal vessel of the earthworm where the blood circulates forward. Half a dozen lateral hearts will propel the blood to the ventral vessel which will irrigate the worm’s organs (muscles, digestive tract, etc.) and provide them with oxygen (and also food) by circulating the blood towards it. ‘back. Leaving the capillaries of the organs, the blood, poor in oxygen, joins the dorsal vessel and circulates forwards until it finds the capillaries of the skin and is recharged with oxygen. The circle is complete !

Skin respiration is undoubtedly less efficient than pulmonary respiration, but it is more than enough to meet the oxygen requirements of the earthworm, which are much less than ours. Provided that the earthworm does not grow too large because then the surface of its body would no longer be sufficient to provide it with enough oxygen. It is therefore impossible to find giant worms in the sand of the desert, they do not exist on Earth… only on Dune!

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