How Many Hearts Does an Earthworm Have? Five Pairs Explained

Where are an earthworm’s hearts?

In the common earthworm (Lumbricus terrestris), the hearts sit just behind the head, in body segments 7, 8, 9, 10 and 11. Each segment has one pair, one on the left and one on the right. Every arch is a thick, muscular tube that loops around the gut, joining the main blood vessel above the gut to the main vessel below it.

Because most textbooks count each pair as one unit, you will often read that an earthworm has “five hearts”. Counting each pumping vessel separately gives ten. Both describe the same five pairs of aortic arches.

The number is not the same in all earthworms. It varies between families and species, and some Asian earthworms commonly used in biology classes, such as Pheretima, have four pairs instead of five.

How does an earthworm’s circulation work?

Earthworms have a closed circulatory system, which means the blood stays inside vessels, as it does in people. Two long vessels run the length of the body:

• The dorsal vessel lies along the top of the gut. Its muscular wall contracts in waves from back to front, pushing blood toward the head.
• The ventral vessel lies under the gut and carries blood from the front toward the tail.

The five pairs of aortic arches link the two. They take blood from the dorsal vessel and squeeze it down into the ventral vessel, which acts like an aorta and sends it out through branches to every segment. Valves in the vessels keep the blood flowing in one direction. Smaller vessels then return blood from the skin, gut and nerve cord.

So an earthworm has no single main pump. The dorsal vessel does much of the work, and the aortic arches act as booster pumps that keep up the pressure in the vessel that feeds the body.

Earthworms also have a second transport system: the fluid in their body cavity. This coelomic fluid flows between segments, carries food-storing cells and helps with healing and fighting infection.

Why is earthworm blood red?

Earthworm blood is red because it contains hemoglobin, the same oxygen-carrying pigment as in human blood. The difference is where it is found. Our hemoglobin is packed inside red blood cells, while an earthworm’s floats freely in the blood plasma as enormous molecules, sometimes called erythrocruorin, that are many times larger than human hemoglobin.

The blood picks up oxygen at the skin, carries it to the muscles, gut and nerves, and brings carbon dioxide back to the surface.

How do earthworms breathe without lungs?

Earthworms have no lungs or gills. They breathe through their skin, a process called cutaneous respiration. Oxygen dissolves in the thin film of mucus that covers the body and passes through the skin into a dense web of capillaries just beneath it. Carbon dioxide leaves the same way.

This only works when the skin is moist. Glands in the skin produce mucus, and small pores along the back release body fluid that helps keep the surface wet. A worm that dries out in the sun or wind cannot take in oxygen and soon dies, which is why earthworms come to the surface mainly on damp nights and after rain; see why worms come out when it rains. Skin breathing also works in water, so earthworms do not drown easily.

Earthworm hearts compared with other animals

An earthworm’s arches are simple tubes with no chambers. They do a similar job to a heart but are built quite differently from ours. Octopuses solve the problem another way, with three hearts and copper-based blue blood; see how many hearts an octopus has.

What other organs does an earthworm have?

Despite its simple look, an earthworm has a full set of organ systems:

• Brain: a pair of cerebral ganglia above the throat in segment 3, linked to a nerve cord along the belly with a cluster of nerve cells in each segment.
• Gut: a mouth, a muscular throat that sucks in food, an esophagus with calcium-producing glands, a storage crop, a grinding gizzard that uses sand grains, and a long intestine with an inner fold that increases its surface.
• Excretory organs: most segments contain a pair of nephridia, small kidney-like tubes that filter waste from the body fluid and release it through tiny pores.
• Reproductive organs: each worm has both male and female organs, found in segments 9 to 15 in the common earthworm, plus the clitellum, the swollen band that makes egg cocoons.

Earthworms lack lungs, eyes, ears, bones, teeth and jaws. Instead of eyes, light-sensitive cells in the skin, concentrated at the head end, tell them whether it is light or dark. How these organs are packed into the front of the body also explains why a worm cut in two rarely survives as two worms, as described in can a worm survive being cut in half.