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Earthworm Dissection and Earthworm Anatomy

Pictures: Modern Biology, Holt

The following is a classification of a species in the earthworm family Lumbricidae. This common species is Lumbricus terrestris also known as the night crawler or dew worm. In this article, we will cover earthworm anatomy and an introduction on dissecting an earthworm. 

Objectives:

In this earthworm dissection guide, you will learn to:

• Describe the appearance of various organs found in the earthworm.
• Name the organs that make up various systems of the earthworm.

Materials:
Safety goggles, dissecting pins, gloves, forceps, lab safety apron, scissors, paper towel, scalpel, water, dissecting probe, preserved earthworm, hand lens, dissection tray.

Purpose:
In this lab, you will dissect an earthworm in order to observe the external and internal structures of earthworm anatomy, whilst following the all-important lab safety procedures.

This guide is perfect for students in colleges or universities. 

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Background:

Among the most familiar invertebrate animals are the earthworms, members of the phylum Annelida. The word annelida means “ringed” and refers to a series of rings or segments that make up the bodies of the members of this phylum. Internally, septa, or dividing walls, are located between the segments. External segments are called metameres. There may be more than 100 segments in an adult worm. The clitellum is a swelling of the body found in sexually mature worms and is active in the formation of an egg capsule, or cocoon. This is the earthworm reproductive organ. Eggs are produced in the ovaries and pass out of the body through female genital pores. Sperm are produced in the testes and pass out through tiny male genital pores. During mating, sperm from one worm travel along the sperm grooves to the seminal receptacles of another worm. Fertilization of the eggs takes place outside the body as the cocoon moves forward over the body, picking up the eggs of one worm and the sperm of its mate. The pumping organs of the circulatory system are five aortic arches. Circulatory fluids travel from the arches through the ventral blood vessel to capillary beds in the body. The fluids then collect in the dorsal blood vessel and reenter the aortic arches. The earthworm takes in a mixture of soil and organic matter through its mouth, which is the beginning of the digestive tract. The mixture enters the pharynx, which is located in segments 1–6. The esophagus, in segments 6–13, acts as a passageway between the pharynx esophagus and the crop. The crop stores food temporarily. The mixture that the earthworm ingests is ground up in the gizzard. In the intestine, which extends over two-thirds of the body length, digestion and absorption take place. Soil particles and undigested organic matter pass out of the worm through the rectum and anus. The nervous system consists of the ventral nerve cord, which travels the length of the worm on the ventral side, and a series of ganglia, which are masses of tissue containing many nerve cells. The nerve collar surrounds the pharynx esophagus and consists of ganglia above and below the pharynx. Nervous impulses are responsible for movement and responses to stimuli. Each segment contains an enlargement, or ganglion, along the ventral nerve cord. Excretory functions are carried on by nephridia, which are found in pairs in each body segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has no gills or lungs. Gases are exchanged between the circulatory system and the environment through the moist skin.

External Earthworm Anatomy

What is the external anatomy of an earthworm?

The external body of an earthworm is well adapted for living in the soil, similar to the external structure of other insects. The front or head of the worm is called the anterior. The very first section of the anterior contains the mouth and prostomium. The prostomium is a kind of lip which is located on the front of the mouth. Earthworms lose moisture and breathe via their skin. They have light-sensitive cells across their external structure, which are scattered around the skin. These cells give earthworms the ability to detect changes in lighting, and these cells are also sensitive to chemicals and touch. The body is separated in segments which resemble rings. Each segment has a number of bristly hairs attached to it, which helps the earthworm to move around. On mature earthworms, you will find a saddle or glandular ring called a clitellum. When an earthworm has mated, the clitellum will secrete a sack of eggs. The final segment of an earthworm contains the anus which is where waste is secreted. 

Dissection Guide:

1. Put on safety goggles, gloves, and a lab apron.

2. Place earthworm in the dissecting tray & rinse off the excess preservative. Identify the dorsal side, which is the worm’s rounded top, and the ventral side, which is its flattened bottom. Turn the worm ventral side up, as shown in the earthworm anatomy diagram below.

3. Use a hand lens as you observe all parts of the worm, externally and internally. Locate the conspicuous clitellum, a saddle-like swelling on the dorsal surface.  The clitellum produces a mucus sheath used to surround the worms during mating and is responsible for making the cocoon within which fertilized eggs are deposited.  The anterior of the animal is more cylindrical than the flattened posterior and is the closest to the clitellum.  The ventral surface of the earthworm is usually a lighter colour than the dorsal surface.  The mouth is located on the ventral surface of the first segment while the anus is found at the end of the last segment. Find the anterior end by locating the prostomium (lip), which is a fleshy lobe that extends over the mouth. The other end of the worm’s body is the posterior end, where the anus is located.

4. Locate the clitellum (the reproductive organ), which extends from segment 33 to segment 37. Look for the worm’s setae, which are the minute bristle-like spines located on every segment except the first and last one. Run your fingers over the ventral surface of the earthworm’s body.  You should be able to feel bristle-like setae used for locomotion

5. Refer again to the diagram of the ventral view of the worm to locate and identify the external parts of its reproductive system. Find the pair of sperm grooves that extend from the clitellum to about segment 15, where one pair of male genital pores is located. Look also for one pair of female genital pores on segment 14. There is another pair of male genital pores on about segment 26. Try to find the two pairs of openings of the seminal receptacles on segment 10.   Note: These openings are not easy to see.

Internal Earthworm Anatomy

What is the internal anatomy of an earthworm?

At the very front of an earthworm, you will find the pharynx. Earthworms push the pharynx from inside their mouths to grab hold of things. They pull food into their mouths and then soak it in saliva. As earthworms don’t have teeth, they have to use strong muscles called a gizzard, along with sand and soil, to grind up the food they are eating. Once the food has been fully ground up, it travels to the intestines where it is further broken down so that it can be absorbed. The majority of earthworms have five aortic arches which are like hearts and these move around their bodies. A large blood vessel runs across the top of the earthworm and this is called the dorsal blood vessel. This vessel contracts and pumps blood around to the aortic arches. There is a further blood vessel on the lower side of the earthworm which is called the ventral blood vessel. An earthworm has a very simple nervous system. A ventral nerve cord which runs the entire length of its body connects to the cerebral ganglion, which is an earthworm’s brain. Each segment is connected to the cord so that they can sense light and touch, and can move. Each segment is also wrapped in circular muscles, which contract to help the earthworm move. 

Continued: Dissection Procedure

Hint: Position your preserved earthworm dorsal side up and pin it down through the first segment and then again further back behind the clitellum.  Cut a slit in the dorsal surface near the posterior pin.  Using fine scissors extend the cut forward to the first segment.  Be careful not to cut too deep as to affect the internal organs.  Starting at the first segment, cut the septa (thin membranes) that internally divide the segments, so the skin can be laid flat.  Use additional pins to hold the integument open and expose the internal organs.  Continue to lay the skin back until you have uncovered a centimeter or so of the intestine.

6. Turn the worm dorsal side up. Using a scalpel and scissors, make a shallow incision in the dorsal side of the clitellum at segment 33. CAUTION: Scalpels and scissors are very sharp. Report any cuts to your teacher. Using the forceps and scalpel, spread the incision open, little by little. Separate each septum from the central tube using a dissecting needle, and pin down each loosened bit of skin. Continue the incision forward to segment 1.

7. Use the diagram below to locate and identify the five pairs of aortic arches, or hearts. Then find the dorsal blood vessel. Look for smaller blood vessels that branch from the dorsal blood vessel.

Digestive System

The earthworm is an example of a foraging herbivorous annelid, obtaining food by eating its way through the soil and extracting nutrients from the soil as it passes through the digestive tract.

Hint: Starting at the anterior end, locate the muscular pharynx (food ingestion).  This is followed by a tube-like esophagus which terminates in a crop (the wider organ) which serves as a storage stomach.  Posterior to the crop you will find the gizzard.  Gently press on the crop and gizzard to test their firmness.  While the crop is soft and thin, the gizzard is muscular (soil is ground up and churned within the gizzard).  The gizzard is followed by a long intestine in which both digestion and absorption occur.  Undigested material is voided through the anus.

8. Locate the digestive tract, which lies below the dorsal blood vessel. Refer to the diagram above to locate the pharynx, esophagus, crop, gizzard, and intestine.

9. To find organs of the nervous system, push aside the digestive and circulatory system organs. Use the diagram below to locate the ventral nerve cord. Trace the nerve cord forward to the nerve collar, which circles the pharynx. Find one pair of ganglia under the pharynx and another pair of ganglia above the pharynx. The ganglia above the pharynx serve as the brain of the earthworm.

10. The worm’s excretory organs are tiny nephridia. There are two in every segment. Use the preceding diagram to locate some nephridia.

11. Use the diagram below to locate and identify a pair of ovaries in segment 13. Look for two pairs of tiny testes in segments 10 and 11. To find these organs, you will again have to push aside some parts already dissected.

12. Dispose of your materials according to the directions from your teacher in your college or university.

13. Clean up your work area and wash your hands before leaving the lab.

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