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PHYLUM ANNELIDA—Earthworm
This lab will explore an example invertebrate. The earthworm, Lumbricus terrestris, is a member
of Phylum Annelida. An earthworm is quite well-adapted to a life of burrowing through the soil. Its
streamlined shape helps it move through the soil. A coating of mucus secreted by the skin lubricates the
earthworm as it passes through the soil. The mucus coating also helps oxygen pass through the
earthworm’s skin while it is in the air or under the water. The earthworm moves through the soil by
sucking the soil in its path into its mouth with the aid of its muscular pharynx. As material passes
through the tubelike digestive system, sand grains in the gizzard help grind the food, which is then
digested and absorbed in the intestine. In this investigation, you will examine the external and internal
structure of the earthworm.
Materials: preserved earthworm, dissecting tray, scissors, dropper, probe, forceps, dissecting pins
*Make sure you can identify the external and internal anatomy of a preserved earthworm. You are
responsible for the bold structures listed below and will be assessed in a practical exam.*
EXTERNAL Anatomy of the Earthworm
1. Rinse your earthworm to remove excess preservative. Observe the difference between the darker
dorsal side and the lighter ventral side. Feel the shape of both surfaces.
a. Describe the shape of the two surfaces.
2. Locate the slightly pointed anterior end and the blunt posterior end of the earthworm. Notice the
segments of its body. Count the total number of segments from anterior to posterior, including the
clitellum, which is the light brown enlarged band around its body.
a. How many segments are in your earthworm?
b. In which segments is the clitellum located?
c. What is the function of the clitellum?
3. Locate the mouth on the ventral side of the anterior end of the earthworm. The flap of skin
overhanging the mouth is the prostomium. Locate the anus, the opening in the last segment in the
posterior end.
4. Slide your fingers along the ventral surface of the earthworm from anterior to posterior and feel the
bristlelike setae. Now slide your fingers along the dorsal surface of the worm.
a. How are these surfaces different?
b. What is the function of the setae?
5. Label the diagram of the external anatomy of the earthworm below: anterior end, posterior end,
mouth, prostomium, setae, segments, clitellum, anus.
INTERNAL Anatomy of the Earthworm
1. Place your earthworm in the tray with the dorsal surface up and stretch out its body. Pin the first
segment of the earthworm to the bottom of the tray. Locate the dorsal blood vessel, a dark line that
runs along the midline of the dorsal surface.
2. About 10 segments from the posterior end, insert the tip of a pair of scissors just to the right of
the dorsal blood vessel. Cut along the blood vessel to the anterior end, keeping the scissors pointed
upwards and cutting as shallow as possible (the skin is very thin). Be especially careful anterior to
the clitellum because the major internal organs are in this region.
3. When the cut is complete, carefully open the body wall and notice the septa, or thin partitions of
tissue due to segmentation. Use dissecting pins to carefully tear the septa and pin the skin open so
you can see the internal structures. Put the pins in at an angle and add water to the earthworm (as
necessary) to keep it from drying out.
Reproductive System
1. An earthworm has both male and female reproductive organs and can produce both eggs and sperm.
Most of the reproductive organs are small and difficult to see in preserved specimens. However, you
will be able to see creamy, lobed organs lying on top of the worm once it is opened and pinned. These
are the seminal vesicles which secrete fluid for reproduction.
Digestive System
1. Trace the digestive system from mouth to anus. The mouth is located at the anterior end, followed
by the muscular-walled pharynx (a slight swelling). The slender esophagus leads to the crop, followed
by the gizzard. The intestine leads from the gizzard to the anus.
Circulatory System
1. Trace the earthworm’s circulatory system by following the dorsal blood vessel anteriorly along the
midline to the five aortic arches. These structures are dark in color. You may have to tease away
tissue to expose the arches.
2. Use a probe to carefully move aside the intestine. Locate the ventral blood vessel. The aortic
arches connect the dorsal blood vessel with the ventral blood vessel.
Nervous System
1. Try to locate the brain, which is a white mass of tissue found anterior and dorsal to the pharynx.
This is difficult to find and you may have stuck a pin in it! With a probe, gently push aside the
intestine to find the ventral nerve cord. This cord runs along the ventral surface from segment 3 to
the last segment.
Excretory System
1. With a probe, push aside the intestine in a region just posterior to the clitellum. Look for small,
white, coiled nephridia. A pair of nephridia occur in every segment except the first three segments
and the last segment.
LABELING
Label the diagram of the internal anatomy of the earthworm below: seminal vesicles, mouth, pharynx,
esophagus, crop, gizzard, intestine, dorsal blood vessel, aortic arches, ventral blood vessel, brain,
ventral nerve cord, nephridia.
ANALYSIS and CONCLUSION
1. Describe 2 ways in which an earthworm’s body is adapted to life in the soil.
2. How do the walls of the crop and gizzard compare? Why?
3. What is the advantage of a tube-like digestive system over a sac-like digestive system?
4. What type of circulatory system does the earthworm have? How do you know?
5. What is the function of the nephridia?
6. How does an earthworm breathe?
7. Explain how earthworms benefit plants and the soil around them.
Now let’s review the characteristics of life!
8. How does an earthworm illustrate the following characteristics of life? Use specific structures and
examples to support each characteristic.
Characteristic of Life
Structure/Example
Organization and Cells
Response to Stimuli
Homeostasis
Metabolism
Growth and Development
Reproduction
9. What characteristic is missing from the above chart? Describe it and relate it to the earthworm.