Earthworm: Anatomy & Reproduction

Instructor: Amanda Robb
In this lesson we'll be learning about earthworms. Although these small animals might seem gross, they are actually incredibly important for our ecosystems and have a unique anatomy to help them do their job.

What Are Earthworms?

After a hard rain, you might see birds flocking to open areas of grass. They're not there to drink, but rather to find earthworms that have been flooded out of their underground homes. Although many of us might feel squeamish about these organisms, earthworms are incredibly important to the ecosystem.

You don't have to go far to see earthworms. They live all over the world and have over 1,800 species.

Earthworms are a diverse group. Although we usually see small earthworms in our backyard, some can grow to be 11 feet long!

Earthworms are invertebrates composed of body segments called annuli. Each annulus is covered in small bristles called setae which helps them move and dig. All earthworms have annuli, but the number depends on the length of the worm. Some worms have up to 150 while others have considerably less. Earthworms generally live underground and are decomposers, actually eating the dirt that they live in.

The ingested dirt contains bits of detritus that needs to be broken down before it can be used again by plants. In addition to decomposing and returning nutrients to the soil, earthworms also aerate the soil by creating tunnels. This allows water and nutrients to penetrate deeper into the soil, which makes it more accessible to plants. Many gardeners are happy with earthworms in their soil, as the worms are quite beneficial to plants.

Many parts of the earthworm's anatomy have evolved specifically for its underground, decomposing lifestyle. Today, we're going to look at each of the body systems of the earthworm in depth.

Digestive System

Surprisingly, earthworms have quite a few digestive organs in common with humans. Digestion starts in the mouth, which takes in soil. The food is then transferred to the pharynx, which swallows the dirt into the esophagus. Instead of a stomach, earthworms have a crop which stores food and connects to the gizzard. The gizzard contains small stones the earthworm swallows. The stones act like a grinder, helping to smash up the soil and organic material. The food then moves to the intestine which releases chemicals to help further digest the food. The food is then absorbed through the walls of the intestine into the body. Like humans, any remaining waste is expelled from the anus.

Earthworm waste, called castings, contains important nutrients plants need to grow
earthworm waste

Respiratory System

Take a minute to observe your breath. As humans, we use lungs to bring in oxygen and release carbon dioxide. All organisms need to do this process of gas exchange. However, earthworms don't have the complex lungs that we have. Instead they breathe through their skin. Keeping the skin moist allows them to release carbon dioxide and take in oxygen from the soil. Earthworms don't leave the soil during the day because the sun is harsh and would dry their skin. However, at night when the sun is down and humidity is higher they can come to the surface and still carry out gas exchange.

Cardiovascular System

The cardiovascular system pumps blood around the body. Although earthworms don't have a heart like we do, they have blood vessels and heart-like structures that carry blood filled with oxygen and nutrients to the body. The five aortic arches function as a heart and are located in the anterior portion of the earthworm, around the outside of the esophagus. They connect to blood vessels that carry blood to the rest of the body. The blood vessels that carry blood to the front of the earthworm are called the dorsal vessels and the vessels that carry blood to the back of the earthworm are called the ventral blood vessels.

Excretory System

All animals need to get rid of metabolic waste. In humans our kidneys do this job, producing urine that we excrete. Earthworms have pairs of filtering structures called metanephridia in each body segment. Since earthworms take in liquid through their skin, each metanephridium absorbs liquid directly from the body cavity. Cilia beat inside the tubule, creating currents that draw water in. Anything the earthworm needs, such as water or nutrients, are reabsorbed by cells lining the metanephridium. The waste is sent to a bladder-like structure and is released through an opening called the nephridiopore.

Nervous System

Earthworms don't have a brain like humans, but they do have a central structure in the nervous system called the cerebral ganglion that processes information. This structure connects to the rest of the body via a ventral nerve cord, similar to our spinal cord.

This nervous system is segmented, just like the earthworm. The ventral nerve cord runs along the bottom of the earthworm. In each segment the ventral nerve cord connects with branches called segmental nerves. The segmental nerves run into each body segment. Earthworms are capable of sensing light, touch, chemicals, and vibration, although they are unable to see or hear.

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