Download Observation of Earthworms Name__________________ Block

Observation of Earthworms
Name__________________ Block ____
Purpose:
In this lab, you will look at live earthworms, observe their external structures, and test their responses to
environmental stimuli.
Background:
Earthworms are classified as annelids, or segmented worms. They have digestive, circulatory, and nervous
systems. Gas exchange is through the skin. They live in rich soil, which they eat, digesting the organic matter in it
and passing the inorganic dirt particles out of the body. Earthworms are not very mobile animals. They spend their
lives in one small area and as a result do not encounter many other earthworms. Their reproductive strategy is well
suited to this type of life. Each earthworm is a hermaphrodite, that is, it has both male and female sex organs.
Thus, any individual earthworm can cross fertilize with any other earthworm it encounters.
Feature
Observations
Body color
Length of worm (cm)
Number of segments from
the anterior end of worm
to the egg sac
Tail region: can you see the
anus
Color of egg sack
Other observations about
your earthworm
Earthworm Anatomy
Complete the chart on the back by referring to the diagram, the Earthworm Anatomy posted
picture, and the living worm. The first one has been done for you.
a. Locate each part of the worm.
b. Describe the location using terms such as anterior, posterior, dorsal, and ventral.
c. Use the Earthworm Anatomy handout to help you summarize the function of each part of
the earthworm.
Name Of Part
Description and
Sketch of the Part
Prostomium
The Prostomium is the
first body segment.
Egg Sac
Saddle or annular?
Flared or Unflared
Setae Pattern
Closely Paired, Widely
Paired, or Separate?
Periproct
Location of Part
Function of Part
Anterior end of the
worm on its dorsal
surface
The Prostomium
contains the mouth.
Procedure:
1. Moisten a paper towel, and place it in a clean dissecting pan. Place the worm on the paper towel. You are
working with a live animal. Handle it gently, and follow all lab instructions carefully. Watch the worm move,
and notice which end leads. The worm’s leading end is its anterior end. Identify the worm’s posterior end, that is,
the end away from the leading end.
2. To differentiate between the worm’s dorsal (back) and ventral (stomach) sides, roll the worm over. Describe
the worm’s response to being put ventral side up.
3. Notice how the body of the earthworm is divided into segments. Starting at about segment number 32 there is
a thickening in the body, called the clitellum, as shown in the diagram. The clitellum produces a mucus sac in which
the earthworm deposits its eggs.
Why do all earthworms have a clitellum?
4. Using bristles called setae, located on its ventral surface and sides, the worm moves through the soil. Setae
give the worm traction. Use a hand lens to examine the worm’s body and locate the setae. You can also use your
fingers to locate setae by touch.
How many pairs of setae are on each segment?
5. To determine if the earthworm is sensitive to light, cover half the tray with the construction paper. Place the
worm so that its head is in the light and half in the dark. Record your observations of how the worm reacts.
6. Repeat number 5 but switch the ends so that the posterior is in the light and the anterior is in the dark.
7.
How might this sensitivity to light be an adaptation to the earthworm’s way of life?
8. To determine whether an earthworm prefers a moist or dry environment, place a dry paper towel next to the
wet one. Place your worm across the two towels, as shown in the diagram below.
Wet towel
Dry towel
Does your worm move toward the wet towel or the dry towel? Why?
10. Look carefully along the dorsal side of the worm. You will see a thick purple line running down the entire
length. This is the dorsal aorta, the major blood vessel of the earthworm. Add cool tap water (very little you do
not want to drown your worm) to a large petri dish until the bottom is barely covered. Place the earthworm in the
dish, and observe it with a hand lens. Look for rhythmic contractions of the dorsal aorta. Each ripple of
contractions represents a single heartbeat. Determine the heart rate by counting the number of ripples that
occur in exactly one minute. Record the heart rate below. Now do the same for warm (not boiling hot you do not
want to fry your worm, unless you are going to eat it!).
11. Check the earthworm’s “robin response.” Gently grab a live earthworm’s anterior end as a robin would. What is
the worm’s response?
12. Return your earthworm to the container in the front. Throw paper towels in the garbage cans. Rinse
dissecting tray and return it to the front of the room.
Answer the following discussion questions
1. Are any areas of the earthworm’s body more sensitive to light than other areas? Which ones?
2. Which kind of surface does the worm prefer? Why do you think it prefers that surface?
3. Describe the relationship between temperature and heart rate in the earthworm.
4. What is the adaptive advantage of an earthworm’s response to light and moisture?
5. What is the adaptive advantage of an earthworm’s variations of heart rate as temperature is varied?