The Earthworm Belongs to a Group of Animals Called Invertebrates (They Have No Bones)

Earthworm Dissection

Name: ______

Pre-lab:

The earthworm belongs to a group of animals called invertebrates (they have no bones).

The Earthworm's body has to stay moist & slimy because they breathe through skin. Oxygen is diffused from the air directly through the worm’s skin.

The earthworm hunts food at night and therefore has been called a "night crawler." A worm moves through a small tunnel in the earth which it makes by "eating" its way through the soil. Worms eat decaying plant matter and animal debris.

It has been estimated that an acre of good soil contains over 50,000 earthworms.

By their continuous eating and tunnelling, these worms turn over 18 to 20 tons of soil per acre and bring over one inch of rich soil to the surface every four to five years.

Purpose:

To observe the organ systems of earthwormsand compare those structures to those of humans.

Materials:

One earthwormRubber gloves magnifying glass

Dissection Scissors Dissection tray scalpel

PinsTweezersdissecting probe

Safety:

Make sure you handle all specimens with rubber gloves. Handle all dissection equipment carefully. Clean all equipment when finished.

Procedure:

Observation: External Anatomy

1. Find the anterior (front) end of the earthworm by locating the fleshy bump over its mouth, called the prostomium.

The posterior (back) end has a small hole where solid waste is expelled, called the anus.

The length of the worm is made up of many tiny segments (looks like rings), each separated by a thin wall called a septum.

2. About one-third of the way back from the mouth you should see a thicker and smoother section of the worm. This is called the clitellum, and it is involved in reproduction.

3. 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.

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.

Using a magnifying glass, try to see the setae.

4. With your magnifying glass look for tiny pores on each segment. Liquid wastes are expelled through these pores.

Near the front end of the worm you should see some larger pores that can be easily seen without magnification. These are genital pores and are important in reproduction.

Dissection: Internal Anatomy

1. Lay the worm on your dissecting tray with its dorsal (back) side facing up.

Pin the worm to the dissection tray just inside its mouth and past its clitellum.

Start your dissection about a half centimetre from theclitellum. 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 lift up as you cut to be sure that you cut just the skin--if you go too deep you may damage the internal organs.

2. Using the forceps and dissection pins, carefully Spread the skin of the worm out, Gently tear the septa (little thread like structures that hold the skin to organs below it). Place pins in the skin to hold it apart – set them at an angle so they aren’t in the way of your view.

3.

Reproductive System

The first structures you probably see are the seminal vesicles. They are cream coloured and located toward the anterior of the worm. These are used for producing sperm.Use tweezers to remove these white structures to reveal the digestive system that lies underneath it. Check the box when this is complete.

Circulatory system

The dorsal blood vessel appears as a dark brownish-red vessel running along the intestine. The heart (or aortic arches) can be found over the esophagus (just posterior to the pharynx). Carefully tease away the tissues to expose the arches of the heart.
How many aortic arches can you count? ______

The ventral blood vessel is opposite the dorsal blood vessel, and cannot be seen at this time because the digestive system covers it.

Digestive System

The digestive system starts at the mouth. You will trace the organs all the way to the anus and identify each on the worm.

Find the mouth opening, the first part after the mouth is the pharynx, you will see stringy things attached to either side of the pharynx (pharyngeal muscles). The esophagus leads from the pharynx but you probably won’t be able to see it, since it lies underneath the heart. You will find two structures close to the clitellum. First in the order is the crop, followed by the gizzard. The gizzard leads to the intestine, which is as long as the worm and ends at the anus.

Use your scissors to cut open the crop and the gizzard. Which one has the harder exterior? ______

Nervous System

Locate the brain in the far anterior region of the worm. It is very very tiny and whitish colored. If you can’t find it, it is probably because it was destroyed when you cut the worm. You CAN locate the ventral nerve cord by removing the intestines and searching for the white string-like structure that runs the length of the worm.

Remove the intestine and locate the ventral nerve cord. Check the box when this is complete.

4. Draw alabelleddiagram to show all the features listed below in the chart:

Feature / Description / Found it? (your teacher will check ONE when are ready to be tested)
Pharynx: / This is the light-coloured organ just inside the mouth. Its muscular contractions pass food on down to the esophagus.
Esophagus: / Tube extending from the Pharynx to the crop.
Hearts: / Behind the pharynx are five dark loops wrapped around the esophagus. These are the blood vessels that serve as the hearts of the worm.
Dorsal blood vessel: / This is a dark line extending from the hearts over the top of the crop.
Crop: / Food from the esophagus is temporarily stored in the crop.
Gizzard: (“stomach”) / Food comes from the crop into the gizzard, where it is ground up.
Intestine: / The intestine is the long tube extending from the gizzard all the way to the anus. Food is digested and absorbed here.
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.
Reproductive organs: / The light coloured tissue above and around the hearts are seminal vesicles. Other reproductive parts appear as small white organs on the ventral side of the hearts.

Post-Lab Discussion:

Answer the following questions with full sentences!!

1. (answer True or False)

1. ____ The brain attaches to the ventral nerve cord.
2. ____ The dorsal side of the worm is lighter than the ventral side..
3. ____ The clitellum is located toward the anterior end of the worm.
4. ____ The esophagus lies beneath the pharynx
5. ____ Earthworms are hermaphrodites.
6. ____ The ventral nerve cord and the ventral blood vessel are connected.
7. ____ The pale string-like structure running the length of the ventral side of the worm is the blood vessel.
8. ____ An earthworm has fourteen aortic arches.
9. ____ The dorsal blood vessel can be seen from the worm’s exterior.
10. ____ Seminal vesicles are part of the worm’s digestive system.

1. a) What is the function of the worm’s digestive system?

(Hint: it has the same general function as a human’s)

b) Name the organs you identified in your dissection that are part of the worm’s digestive system.

c) Compare a worm’s digestive system to a human’s.

1. a) What is the function of the worm’s respiratory system?

(Hint: it has the same general function as a human’s)

b) How do worms breathe?

c) Compare a worm’s respiratory system to a human’s.

1. Compare at least one other human organ system with an organ system you observed in your worm dissection.