Getting Ready to Dissect Lumbricus terrestris!!

We have studied the organization of animals from cells, to tissues, to organs, to systems, to organisms. To see how all

the organs form systems and how the systems form an organism, you will study the anatomy of a common

invertebrate, the earthworm (also called the dew worm): Lumbricus terrestris. Earthworms are invertebrates. That

is, they do not have a backbone. Insects, sea stars, spiders, jellyfish, and millipedes are other examples of

invertebrate animals.

Earthworms are important helpers in gardens and fields. Their tunneling mixes up the soil and brings rich soil to the surface. You can observe the organs of these tiny creatures by dissecting one. Let’s answer some questions you may have about dissection.

Why dissect?

Dissection is a valuable tool for visualizing the anatomical structure of different animal classes and species. We actually learn anatomy more easily through dissections because we are simultaneously engaging the sense of sight and touch along with analytical thinking at the same time.

What tools do I need?

The basic dissection tools are a dissection tray, pins, and scissors and two probes to examine delicate parts. You will also have a guide to show you how to dissect the specimen. You will also need markers/colored pencils/crayons (orange, light brown, red, green, yellow, pink, purple)

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How will we dispose of dissected specimens?

When you have completed your dissection and have had it checked, wrap all parts in a paper towel and dispose of them in the container provided. DO NOT dump any of the remains in the sink or take them out of the room. Doing so will result in a detention. Wipe out the dissecting tray with a Clorox wipe. Clean and replace equipment.

How many people will be in my group?

We will plan one specimen and one set of tools per two students. This provides the best learning situation. Students working in pairs have ample opportunity to fully participate in the dissection and to carefully examine the specimen. You are also able to share and discuss your observations. The pairs will be decided by your wise, wonderful teacher!

Where will we do our dissection?

It is important to do your dissection on a solid, uncluttered surface with plenty of light. You will need to wipe off the table or counter top you used with a Clorox wipe also.

Important points:As you dissect, proceed slowly and carefully. Dissecting consists of carefully separating organs and removing surrounding tissues to see an organ clearly. Carelessness can damage structures and make them difficult or impossible to find.

Follow directions exactly and do all steps in the order indicated. As you do each step, it is suggested that you check it off. Do not cut out any organs unless specifically directed to do so, but always remove enough of the surrounding tissue to see an organ clearly. A good dissection should reveal all organs clearly enough so that a person could examine the specimen and see the relationships and connections between organs with no difficulty

Following are definitions of directional terms you will encounter when reading dissecting instructions.

Anterior (toward the head) vs Posterior (toward the tail)

Dorsal (back) vs Ventral (front/belly)

Now, let’s look closely at an earthworm and at your lab directions.

“DISSECTING AN EARTHWORM” LAB

PURPOSE: To dissect an earthworm and observe the organs of the digestive system.

MATERIALS: Preserved earthworm, dissecting pan, scissors, dissecting pins, forceps, paper towels, dissecting probe, hand lens, markers or crayons (orange, light brown, red, green, yellow, pink, purple)

PROCEDURES:

_____ 1. Obtain a dissecting kit and check to see that it has a scalpel, scissors, forceps, metric ruler, and 2 dissecting probes.

_____ 2. Obtain an earthworm and place it in a dissecting pan. _____ 3. Note that the body of the earthworm is divided into ringlike units, the segments.

_____ 4. Notice that the earthworm has a rounded dorsal (back) surface and a flatter ventral (belly) surface. Usually the dorsal surface is darker than the ventral surface. The darker dorsal side is marked by a dark line running the length of the animal. This is the dorsal blood vessel seen through the skin.

_____ 5. Find the anterior (front) end of the earthworm by locating the fleshy bump over its mouth, called the prostomium. The opening is in segment number 1. Each succeeding segment is numbered consecutively throughout the length of the worm.

_____ 6. The posterior (back) end has a small hole where solid waste is expelled, called the anus. This is used for the removal of digestive waste and soil which the worm has ingested.

*The length of the worm is made up of many tiny segments, each separated by a thin wall called a septum.

_____ 7. About one-third of the way back from the mouth, surrounding segments 32-35, you should see a thick, smooth belt of the worm. This is called the clitellum, and is involved in reproduction. _____ 8. Lightly rub your finger along the ventral side toward the posterior end of the worm. You should feel a roughness caused by tiny bristles called setae (se`etee). Using a hand lens, try to see the setae. These aid in the locomotion of the worm by gripping the soil as the worm pulls itself along.

_____ 9. Look for the openings on segment 15. These are the openings to the seminal vesicles. Sperm cells leave the body of the worm through these openings.

_____ 10. In segment 14 are the openings of the oviducts. Egg cells pass through these openings on their way to being fertilized. They may be too small to see with your eyes alone. _____ 11. Between segments 9-10 and 10-11 are the openings of the seminal receptacles, the organs which receive sperm cells during mating. The earthworm is a hermaphrodite, that is, it has organs of both sexes. During copulation, or mating, sperm cells are transferred from the seminal vesicles of one worm into the seminal receptacles of another worm.

Let the dissecting begin…

______1. Place the earthworm with its top (darker) side up in the dissection pan.

______ 2. Use dissection pins to secure each end on the tray.

Start your dissection about an inch posterior to the clitellum. Lift up the skin with a pair of forceps and snip an opening with a pair of dissecting scissors. Insert the scissors into the opening and cut in a straight line all the way up through the mouth. Go slowly and make short shallow cuts and be sure to cut just the skin so you do not damage the organs inside.

__________3. Using the forceps and dissection pins, carefully pull apart the two flaps of skin and pin them flat on the tray. You may need to drag a pin along the inside of the skin to sever the septum walls to make it easier to spread the skin.

__________ 4. Look at the labeled picture to help you find the following features. Use the coloring guide to color the internal parts.

Pin at an angle rather than straight up and down. After you have pinned from the clitellum to the mouth, check with your teacher before going on.

Observe the main tube that runs down the center of

the worm. This is the digestive system. __________ 5. Find the pharynx, a wide tube at the beginning of the digestive system. Color it orange in the drawing.

__________ 6. On top of the pharynx is the ganglion, the worm’s equivalent to a brain. It is a very tiny white structure which has two tiny lobes. On the drawing, color it light brown.

__________ 7. Following the pharynx is the esophagus, a tube which runs to about segment 14. Color it red.

__________ 8. The esophagus leads into the crop, an enlarged organ for temporary food storage. Color it green.

__________ 9. The crop is followed by the gizzard, an organ which grinds food. Color it yellow.

__________ 10. The remaining organ of digestion is the intestine which extends to the posterior end of the worm. It is the place where digestion and absorption of the food takes place. Color it purple.

__________ 11. Surrounding the esophagus near the anterior end (segments 3-11), are a series of dark-colored loops. These are the aortic arches. They are sometimes called the earthworm’s hearts, but actually they only assist in pumping the blood. They are fragile and may be broken apart in your specimen, but you can usually find some of them. On the drawing, color the aortic arches pink. If time permits:

Reproductive organs: the light colored tissue above and around the hearts are seminal vesicles. Other reproductive parts appear as small white organs on the ventral side of the hearts.

Ventral nerve cord: with your forceps, gently push aside the intestine to view the long white nerve cord running along the length of the worm beneath it.

Have your teacher grade your dissection (see below)

_____ /25 for a successfully-dissected worm

-____ points deducted for _____________________________

_____ Total Score

Clean up time…

Remove all dissection pins and wrap the earthworm in the paper towel and put it in the designated bucket. Clean up all of the equipment and return all instruments to your CLEAN dissection tray.

Final thoughts…

How many segments does your earthworm have? ______________

Check the organs that you were able to observe. (not graded)

____ crop ___esophagus ___ gizzard ___ganglia ___anus

___ 5 pairs of hearts _____ pharynx _____ intestine

_____ reproductive structures

I knew you could do it!!!!!!!