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Earthworm Dissection Name

Period 1 2 3 4 5 6 7

Objective

The purpose of this study is to learn about the anatomy of the earthworm and how different

parts of the anatomy support the life of the animal. You can then compare these different

structures to other animals such as frogs, insects, fish, and humans.

Background Information

Many of us in Minnesota have had the fun and excitement of fishing with an earthworm as bait.

What most people know as “earthworms” belong to the phylum Annelida (A-Nell-Ih-Duh) or

animals with body parts “resembling little rings” (taken from Latin). Annelids are so named

because the length of their bodies is divided into segments or somites, both on the inside and the

outside. There are over 12,000 species of segmented worms found all over the world. Some live

in the oceans, others in freshwater ponds or streams, and other species live in moist soil.

Earthworms serve a vital biological role in the environment; they transform dead organic

material into rich nutrients needed and used by plants and other organisms. Their tunnels and

burrows also aerate the soil making in more porous which is beneficial.

Tools

Dissecting pan, dissecting pins, hand lens, and a dissection kit containing scissors, teasing

needle, curved probe, straight probe, and a scalpel. The dissecting kit will be checked out from

your teacher. All tools and the kit must be cleaned and dried before checking it in.

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Anatomy Terms

Dorsal- the back or upper part of the animal

Ventral- the abdominal side or lower part of the

animal

Cephalic- the head region

Cranial- the upper part of the head

Anterior- the forward or front end of the body

Posterior- the hind or rear part of the body

Caudal- the tail end

Transverse- the cross section

Longitudinal- along the length of the body

The compass points of anatomy (anterior,

posterior, dorsal and ventral) appear in all

forms of animal life, from the simple,

spineless creatures to man himself.

Fill in the appropriate anatomical terms for the regions of the earthworm pictured below.

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External Anatomy Examine your earthworm and determine the dorsal and ventral sides. Locate the two openings on the

ventral surface of the earthworm. The openings toward the anterior of the worm are the sperm ducts.

The openings near the clitellum are the genital setae. Locate the dark line that runs down the dorsal

side of the worm, this is the dorsal blood vessel. The ventral blood vessel can be seen on the

underside of the worm, though it is usually not as dark.

1. Place the specimen in a dissecting tray. Notice that the dorsal side is dark colored and the ventral

side is lighter. Make observations about the shape of the earthworm.

a. What is the earthworm’s general shape?

b. Does the skin have an even smooth texture, or is it bumpy and divided into segments?

Find the thick light colored band encircling the body.

c. What is the thick light colored band encircling the body called?

d. Is the band closer to the anterior or the posterior end of the animal?

2. Slide your finger or dissecting needle up and down along the dorsal surface and then the ventral surface.

a. Which side feels rough like fine sandpaper?

b. What microscopic structures are causing this “roughness”?

c. What is the function of these structures?

3. Place the specimen in your dissecting tray, dorsal side (dark colored) down. Observe the first segment at the

anterior end. Find the small opening at the tip.

a) What is this opening called?

b) What is it used for?

c) This opening has a flap of skin that can open and close (a worm “lip”). What is the scientific term for this

tissue flap?

Fill in the blanks:

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Internal Anatomy

Read completely BEFORE you begin

1. Place the specimen in the dissecting pan with the dorsal side up. Using the scissors or

scalpel carefully cut through the skin from the clitellum to the anus (posterior end). Be

sure to press gently with the scissors/scalpel so that you cut only through the skin and do

not pierce any organs. Try to keep the tip of the scissors pointed up, and only cut through

the skin. Make a second incision from the clitellum to the mouth (anterior end). Be very

gentle or you will destroy many internal organs.

2. Gently life the skin on one side of the incision with your forceps. Use a teasing needle

inside the incision to gently tear the septa (little thread like structures that hold the skin to

organs below it). Work towards the clitellum from the posterior end of the worm. Repeat

the same procedure on the other side of the incision.

3. Start at the clitellum; spread the skin back to expose the internal organs. Fasten the skin

flaps back on both sides with dissecting pins. Angle the pins away from the body for a

better view of the internal organs. The internal structures should now be visible.

Digestive System

Look at segment 1 and find the mouth opening and prostomium, the fleshy lobe of tissue covering the mouth. Within the first 5 segments, locate the pharynx. You will see stringy things attached to either side of the pharynx (pharyngeal muscles). Immediately posterior to the pharynx, in the next 10 segments, locate the tube-like esophagus. The esophagus lies underneath the aortic arches (or heart). The esophagus leads into the thin-walled crop. You should now be about 15 segments from the mouth. The crop leads into a thick-walled gizzard, which occupies about 3 segments. Extending posteriorly from the gizzard and running through the remainder of the earthworm is the thin tubular intestine which ends at the anus.

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Questions on the digestive system

Take your probe and gently press on the crop and then the gizzard.

1. Are the walls of the crop the same as the gizzard? If not, how are

they different?

Next, using your scalpel, cut through the wall of the crop and the gizzard.

2. What differences between the crop and the gizzard do you observe?

3. Which of these structures is best adapted to grind food?

4. Which of these structures is best adapted to store food?

5. Estimate how many segments the intestine passes through, from the gizzard to the anus.

6. What is the function of the intestine?

7. Why is it advantageous for the earth worm to have a long intestine?

Label the diagram below that traces the route of food through the digestive tract.

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Circulatory System

Look for the 5 enlarged blood vessels in segments 7-11. These are “primitive” hearts or aortic

arches.

1. Do earthworms have a closed or open circulatory system?

2. The blood pumps many different molecules including oxygen and carbon dioxide. How

does the worm exchange gases with the environment?

Reproductive System

Examine the whitish sacks in segments 9, 11, and 13. These are seminal vesicles used for

storing sperm made before being discharged from the body. In segments 9 and 10 you may see

small circular white structures called seminal receptacles. They are used for storing sperm

received from another worm during copulation. Earthworms have both male and female sexual

organs which is different than many other organisms in the animal kingdom.

1. Reproduction taking place by two individuals joining and exchanging generic material, is

sexual or asexual reproduction?

2. Describe how earthworms reproduce.

Critical Thinking

1. The shape of the earthworm allows it to move easily through the soil. Why?

2. Why might it be advantageous for the earthworm to have setae located on the ventral

surface rather than the dorsal surface?

3. The digestive system of an earthworm is sometimes described by biologists as a “tube

within a tube”. Why is this accurate description?

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4. Why would having a darker colored dorsal side be a beneficial evolutionary adaptation?

(How would the darker color on the top of the worm help it to survive?)

Identify each organ or structure Describe its function

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

1.

2. 3. 4.

5.

6. 7.

8. 9.

10. 11. 12. 13.

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Worm Vocabulary- anus Opening at the tail end of the earthworm.

aortic arches (hearts) Five blood vessel loops in segments 7-11.

buccal cavity Tube from the mouth opening in segments 1-3.

clitellum The glandular swelling from segments 31-37 over the sides and back of the worm.

coelom This cavity formed between the body wall and the food tube. The body wall is lined with a membrane called the peritoneum.

convoluted tubule Provides a bladder-like function to help the worm rid itself of liquid waste. In the image, on page 7, they are located near the clitellum.

crop A thin walled storage organ for food which is part of the food tube in segments 15 and 16.

dorsal blood vessel Vein found just over the intestine to carry blood to the hearts.

esophagus Food tube from segments 6-14.

ganglia Primitive brain found above and below the buccal cavity in segment 3 with connectives around the cavity.

gizzard A thick walled grinding organ which is part of the food tube in segments 17 and 18.

intestine The food tube from segment 19 to the anus. Cut open the intestine along part of its length to see the food material inside.

lip (prostomium) The fleshy lobe covering the mouth at the head end of the worm.

mouth The opening at segment 1.

oviduct opening On segment 14 note the two small pores for release of eggs.

pharynx Muscular part of the food tube in segments 4-5.

segments (somites) The earthworm is formed in segments both externally and internally. The segments are numbered beginning with segment 1 at the mouth in order to locate and study specific structures of the earthworm.

seminal receptacle openings

The openings to receive sperm which are found between segments 9-10 and 10-11.

seminal receptacles Whitish small sacks in segments 9 and 10.

seminal vesicles Whitish sacks in segments 9, 11, and 13.

septa The thin membranes that separate the segments in the coelomic cavity.

setae The 4 pairs of stiff hairs found on each somite except the first one and the last one. Two pairs are on the ventral surface and two pairs are on the sides of each segment. Feel the setae by lightly moving a finger forward and then back along the surface of the worm.

sperm duct opening On segment 15 note two openings for release of sperm.

typhiosole Look at the cross section of the worm previously cut off. Observe the fold of the intestine (typhiosole). Note the internal cavity (lumen) of the intestine.

ventral blood vessel Artery found just under the intestine to carry blood away from the hearts.

ventral nerve cord Main nerve cord found just under the ventral blood vessel from segment 4 to the end.

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Organ systems For the picture below, color code the organ systems for the earthworm using the following key: Circulatory System - Red Reproductive System - Blue Digestive System - Green Nervous System - Yellow