Annelids our own human heart. However, unlike our

Annelids make up an interesting phylum of segmented worms.

While these species look quite bland, they have unique circulatory systems. In this lesson, we’ll discover the incredible adaptations of the annelid circulatory system.

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What Are Annelids?

Have you ever seen an earthworm squirming in the soil and wondered if it has a heart like you do? Well, the worm actually has several hearts as well as flowing blood that helps make up its circulatory system. The earthworm is an annelid, which means it’s a member of the phylum Annelida. Also known as segmented worms, annelids include our friend the earthworm, leeches, and ragworms, among the thousands of different species in the phylum Annelida.

Let’s explore how these species pump blood by looking at their circulatory systems.

Annelid Circulatory Structure

Much like humans, annelids have a closed circulatory system, which means the blood circulates through a closed network of blood vessels. In some cases, such as with polychaete worms, the blood is pumped directly through self-contracting blood vessels. In other cases, blood is carried throughout the body from a central pumping organ, just like our own human heart. However, unlike our single heart, some annelids have five. Can you imagine hearing five heartbeats in your chest?A great example of this five-heart arrangement can be seen in the earthworm.

The heart-like structures in the earthworm are called aortic arches, which pump blood out into the network of vessels that run in a circuit through the body. The aortic arches sit near the head of the worm and encircle the esophagus, the tube through which food passes on its way to the gut.After being pumped out from the aortic arches, the blood moves to the dorsal blood vessel, which sends blood toward the front of the earthworm, as well as the ventral blood vessel, which sends blood to the posterior of the worm. In this way, it circulates blood in a continuous circuit from the aortic arches through the vessels.Along the intestines, or gut tube, of an annelid, there are lateral vessels. These blood vessels help bring nutrients away from the gut and out toward the rest of the body.

The word ‘lateral’ refers to the sides. These vessels run all along the sides of the intestines. This may be helpful when remembering the location of the lateral vessels.

Annelid Circulatory Function

Earthworms can absorb oxygen directly through their moist skin through tiny blood vessels, or capillaries.

The blood then carries this oxygen as well as nutrients through the body. Actually, what carries the oxygen within the blood is the transport protein hemoglobin, which is a transport protein also found in human blood.There are no red blood cells, or erythrocytes, in an earthworm’s body. Red blood cells normally carry hemoglobin in humans. So how do earthworms cart hemoglobin, and therefore oxygen, around their bodies? They’re carried around the bloodstream in a large protein complex that floats in the blood liquid, called blood plasma.

Earthworm blood does, however, have white blood cells (amoebocytes) to provide immune defense and carry metabolic waste. Carbon dioxide is also expelled through the blood stream and out through the skin capillaries.

Lesson Summary

The circulatory system of annelid worms is simple but effective. It’s a closed circulatory system in which blood moves through a closed network of vessels. Some annelids, such as polychaete worms, have no heart, while others have multiple hearts.The earthworm, which is perhaps the most popular of all the annelids, has five heart-like structures called aortic arches.

Along with dorsal and ventral vessels, the aortic arches help blood flow through the closed circulatory system and reach both ends of the body. In addition, other vessels, called lateral vessels, branch out into the gut tube, or intestines, via capillaries.Hemoglobin, the oxygen-carrying protein, is not carried by red blood cells in annelid bodies, as there are no red blood cells at all in annelids. Rather, hemoglobin floats freely in the blood in a protein complex called plasma. Annelids do have white blood cells, which carry waste and provide immune protection.


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