Lab Exercise
23b
Dissection: The Earthworm
Objectives
Introduction
- To learn some of anatomical structures of the earthworm.
- To be able to make contrasts and comparisons of
structures between different animal phyla as additional
organisms are observed.
- To deduce the adaptive significance of differences in the
structures of animal phyla as additional organisms are
studied.
The following exercise allows you to examine some of the
morphological and anatomical structures of a member
of the phylum Annelida. This phylum includes more than
9,000 species. Members of this phylum are characterized by their segmented bodies, a well developed coelom,
closed circulatory system and a well developed central
nervous system with the sensory organs concentrated at
the anterior end of the animal.
The phylum is divided into three classes; the Polychaeta
(the polychaete worms), the Oligochaeta (which includes
the earthworms), and Hirudinea (the leeches). Here you
will examine the external features and anatomy of the
earthworm, genus Lumbricus. To begin this exercise, go to
the Diversity section of the BiologyOne DVD. Select Dissections and then, after the introduction screen, select the
Earthworm from the list of organisms.
In the dissection exercises, you will be asked to examine
the organisms and learn something of their individual
anatomy. Equally important is a comparison of the anatomical structures of between organisms, noting how they
are similar, how they differ, and how their differences may
be adaptive to the different life styles of these organisms.
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23b 1
Activity 23b.1
External Features
Activity 23b.2
Internal Anatomy
Before examining the internal anatomy of the organism
you should examine its external morphology. From this
you can observe a number of features that show adaptations to the way the organism lives and clues to its internal
structures.
To dissect the earthworm, you would lay it out in a dissecting tray, dorsal side up, and hold it in place by pinning
down its prostomium at the anterior end and also near its
posterior end. You would make a shallow incision behind
the clitellum just to one side of the mid-dorsal line. Cut just
deep enough to cut through the skin but avoid cutting into
the internal organs. Then, insert one tip of a pair of scissors into the incision, lift the skin away from the internal
organs and carefully cut toward the anterior end of the
worm. When you have completed this dissection, reveal
the internal organs by pinning the skin back on either side
of the worm. Click on the forward arrow in the lower right
to complete this dissection of the earthworm.
Examine the external features of the earthworm. Note the
segmented body of the earthworm. The anterior end can
be recognized by noting the location of the clitellum. This
is a lighter colored, swollen region that covers several
segments near the anterior end of the worm. During reproduction, the clitellum slips off the anterior end of the worm
and forms a cocoon for the development of fertilized eggs.
You may also be able to find the prostomium, a projection
that overhangs the mouth. The anus is located at the last
segment of the posterior end.
If you were to rub your finger along the sides of the worm
you would feel short bristles. Except for the first and last,
each segment has a two pair of bristles or setae extending from its side, pointing toward the worm’s back. The
rows of these bristles are located on the ventral side of the
worm. What function could they serve?
If you could closely examine the segments you may also
see pores in the skin of the worm. Each segment has a
pair of excretory pores that remove metabolic wastes from
the worm. In addition, in segments 14 and 15 are pairs
of pores for the male and female reproductive system
respectively (the location of these pores varies some in
different species). The female pores are relatively small,
located on segment 14. The male pores located on segment 15 are surrounded by swollen ‘lips’.
After studying the external features of the earthworm,
label the illustration located in the Results Section.
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As you examine the internal organs of the earthworm, you
should note the lack of organs for respiration. The earthworm relies on diffusion across the skin for gas exchange.
Given the relative size and diameter of these organisms,
how effective do you think this method of respiration is in
the earthworm relative to the other organisms you have/
will study? Do you see any mechanism that increases the
efficiency of gas exchange in the earthworm? What is it?
The earthworm has a closed circulatory system. This
means that the blood is contained within blood vessels.
There is a large dorsal blood vessel and a large ventral
blood vessel that run the length of the worm’s body. In
each segment a pair of vessels extends laterally from
these main blood vessels. In segments 7 through 11, the
lateral extending blood vessels have become enlarged
and are much more muscular. These are the 5 pairs of
‘hearts’ that pump the blood through the worm’s circulatory system.
The earthworm’s digestive system forms a tube extending
the entire length of the body from the mouth located in the
first body segment to the anus located in the body’s last
segment. Near the anterior end this digestive tube has regions that have specialized to perform different functions.
The first part of the digestive system past the mouth is the
small buccal cavity. This leads to an enlarged cavity, the
pharynx located in segments 3 through 5. The pharynx is
attached to the body wall by muscles that create a sucking
23b 2
action when they contract. From the pharynx, food passes
through the esophagus and empties into the thin-walled
crop. Here food is stored until it is ready to pass into the
muscular gizzard that grinds the food into small particles.
Around segment 19 the gizzard joins with the intestine
that runs the rest of the length of the worm’s body until it
reaches the anus at the terminal segment. In the intestine, the chemical digestion and absorption of food takes
place. A fold on the dorsal side of the intestine called the
typhlosole increases the inner surface area of the intestine improving the efficiency of food absorption. What is
another way an earthworm can increase the intestine’s
surface area so that it can absorb more nutrients from the
material passing through its digestive system?
Located in segment three are two large ganglia that wrap
around the digestive tract just anterior to the pharynx.
These make up the brain of the earthworm. A ventral
nerve cord runs the length of the organism.
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Earthworms are monecious. Each individual produces
both sperm and eggs although the eggs must be fertilized by sperm from a different worm. The two ovaries are
located in segment 12. When the eggs are produced they
travel through an oviduct that exits the body in segment
14. Two pairs of testes are located in segments 10 and 11.
After being produced in the testes, the sperm mature and
are stored in the large seminal vesicles. These are located
in segments 9 through 13. When copulation occurs, the
sperm travel through a tube (the vas deferens) that exist
the body at segment 15. During copulation, the sperm
are stored by the receiving worm in two pairs of sacs, the
seminal receptacles. These are located in segments 9
and 10. When the worm’s eggs are mature and it is ready
to release these, the clitellum slips forward, first receiving
eggs as it passes the female pores and then sperm as it
passes the pores to the seminal receptacles.
Study the anatomy of the earthworm and label the illustration in the Results Section.
23b 3
Lab Exercise
23b
Name _______________________
Results Section
Activity 23b.1
External Features
Label the illustration below
3. _____________________
2. _____________________
prostomium
female pore
openings to seminal
receptacles
1. _____________________
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23b 4
4. _____________________
Activity 23b.2
Internal Anatomy
Label the illustration below
5. _____________________
1. _____________________
6. _____________________
7. _____________________
2. _____________________
8. _____________________
3. _____________________
(the black line)
4. _____________________
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23b 5