Download Earthworm observation

Live Earthworms
Name _______________
Skills:
- identifying anatomical structures of the earthworm
- organizing and analyzing data
Objectives:
- Infer how physical and behavioral characteristics of the earthworm reflect adaptations
to its environment.
- Describe how an earthworm responds to light and moisture.
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.
Anterior
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 below. 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?
Posterior
5. To determine if the earthworm is sensitive to light, cover the beam of a flashlight with
two pieces of red cellophane. Fasten the cellophane in place with a rubber band. Darken
the room, and shine the beam on the earthworm. Record your observations of how the worm
reacts to red light in the table below
6. Replace the red cellophane with two pieces of blue cellophane. Shine the blue beam on
the earthworm. Record your observations in the table on the preceding page.
7. Remove the cellophane. Shine the light first at the worm’s anterior end, then on its
middle, and then on its posterior end. Note the worm’s responses. Shine the light on one
area of the pan. See if the worm will enter the spot of light. Record your observations in
the table on the preceding page.
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?
9. Place a piece of sandpaper next to the dry towel. Place the worm across the two
surfaces, anterior end on the sandpaper first. Observe the worm’s response. Then place
the worm across the sandpaper and dry towel, this time with the anterior end on the towel.
Record your observations in the table on the preceding page.
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 through
a stereomicroscope. 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. 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.
w
Name ________________________
Use the space below to answer the questions located throughout the procedure.
Heart rate:
Cool water ___________
Hot water _________
Analysis
1. Are any areas of the earthworm’s body more sensitive to light than other areas? Which
ones?
2. Which kind of surface, the rough sandpaper or the dry paper towel, 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?
6. Check the earthworm’s “robin response.” Gently grab a live earthworm’s anterior end as
a robin would. What is the worm’s response?
7. Design an experiment that would test an earthworm’s response to varying levels of
moisture. Describe the experiment in the space below.
Earthworm responses to Environmental Stimuli
Environmental
Stimulus
Response
Red Light
Blue Light
White light to anterior end
of body
White light to midpoint of
body
Approach/avoidance response
to white light
Moist/dry environment
Sandpaper/smooth surface
Label the clitellum, anterior, and posterior parts of the
earthworm.