Phylum Annelida Circulatory & Nervous Systems

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In this lesson, we'll learn about the structure and function of the circulatory and nervous systems in the group, or phylum, that includes earthworms - Phylum Annelida. Annelids have simple, but interesting ways of pumping blood through their bodies, and also in sensing their surrounding environment.

Earthworms, Leeches, and Polychaetes - Oh My!

It's warm and raining outside, and when you go out for a walk, you may see the familiar earthworm crawling around on the sidewalk.

Ew, worms!

But wait, don't step on 'em!

Even the leeches!
leeches

There's a lot more to these animals than their simple form projects, and all that writhing and squirming are powered by simple, yet effective, nervous and circulatory systems.

Do they have a heart? Or more than one heart? Is there a brain in there, somewhere, guiding them to seek high ground after it rains?

Let's dive in to the world of the Phylum Annelida - the earthworms, leeches, and polychaete worms, with a look at how they pump blood, and how their nervous system functions.

Annelid Circulatory System Structure and Function

Earthworm circulatory system.
earthworm systems

Earthworms, leeches, and polychaete worms have a closed circulatory system; meaning, the inner space of their bodies are filled with blood vessels that carry blood, nutrients, and oxygen throughout the body. Earthworms can absorb oxygen directly through their skin, but the blood is essential for carting nutrients around. Like our veins and arteries, these vessels move blood from a series of hearts that lie near the head. Each heart has a small aortic arch vessel that takes blood to and from the hearts, similar to our own single aorta.

Important blood vessels within an annelid's body include the dorsal blood vessel, which runs from the hearts at the 'back', or dorsal, side of the body; and the ventral blood vessel, which runs from the hearts at the 'front', 'belly', or ventral, side of the body.

Another important set of blood vessels are the lateral vessels that run along the intestines or gut tube of the annelid body structure. These blood vessels help bring nutrients away from the gut and out toward the rest of the body.

Annelid Nervous System Structure and Function

Think about where our spinal cord runs. Close your eyes and visualize it.

It's along our back, right?

So it must be the same in annelid worms?

Think again! Annelids have their equivalent of a spinal cord running along the ventral, or underside, of the body!

In fact, there are single or double nerve cords that run from the front of the body to the tail end, along what is essentially the annelid's belly.

A simple diagram of the annelid nervous system.
annelid nervous system

The nerve cord originates at the head end of an annelid worm, a region known as the cerebral ganglion - that's the annelid version of a brain. It's a bundle of nerves that coordinates movement towards and away from external stimuli, such as light, temperature or touch.

Some annelids, such as earthworms, have another smaller ganglion beneath what serves as the worm's throat, or pharynx. This ganglion is therefore known as - you guessed it - the subpharyngeal ganglion. Remember, the prefix sub- means 'under' or 'beneath'.

Special Adaptations

Annelids have a variety of special sensory adaptations, each of which are determined by the activity of the nerves in the annelid body.

A plate illustration of a vareity of marine polychaetes, many with spectacular sensory tentacles.
polychaete worms

The marine polychaete worms have special head tentacles and palps that can sense touch and chemical changes.

Polychaetes are also known to have simple eye spots, organs near the head that can detect light, or even more complex, eye-like organs that are on the end of the sensory tentacles.

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