Biology 521 - Earthworm Dissection
Kingdom
Phylum
Class
Order
Genus
Species
Animalia
Annelida
Oligochaeta
Haplotaxida
Lumbricus
L. terrestris
PRELAB:
The earthworm is an excellent organism to study as an introduction to dissection because of its highly
organized body. Its body segments (each ring is called a somite) are made of rings which can be used to locate
specific organs (see the attached map in Figure 1). The earthworm has a tube within a tube body plan. The
outer tube is the body wall and the inner tube is the digestive tract. It lives in moist, nutrient rich soil all around
the world provided the soil is not too dry or sandy. They are primarily a nocturnal organism and will come out
of their burrows at night. Earthworms exhibit well developed digestive, circulatory, excretory, reproductive, and
nervous systems. Before coming to the lab, the following pages should be read: p. 238-239, p. 222, p. 282-283,
p. 158, p.175-176.
OBJECTIVES:
In this lab, the student will
1. study the internal and external anatomy of the earthworm
2. take care to see and determine the function of each suggested structure
3. be prepared to identify structures and functions of earthworm anatomy.
MATERIALS: preserved specimen, dissecting pan, scissors, probe, teasing needle, dissecting scope, colored
pencils
PROCEDURE:
1. External Anatomy
1. Examine the external features. The thick band near the anterior (segments 32-37) of the animal is
called the clitellum. It separates the anterior end (about 1/3 the body length) from the posterior end. The
clitellum secretes a mucous cocoon around fertilized eggs. The prostomium is the front tip of the worm which
hangs over the earthworm’s mouth. Notice the presence of tiny bristles called setae which are found on the
ventral surface of the earthworm. Another difference between the dorsal and ventral surface is the fact that the
ventral surface is lighter in color.
2. Besides the mouth and anus, there are other openings in the earthworm’s body. Eggs are released on
the ventral surface of segment 14 through two small pores which will require a dissecting scope to view. Sperm
is released through two larger pores on the ventral surface of segment 15. These pores are more easily seen
because of the swollen lips around each pore. Other pores are present but are difficult to observe. They will be
discussed later with the systems to which they belong.
Internal Anatomy
Hold the earthworm ventral side down in your non-cutting hand between your thumb and index finger.
With a fine pair of scissors, cut a small lateral (across its body) incision through the skin of the earthworm just
anterior to the clitellum. Insert one blade of the scissors into the incision and extend the cut one quarter of the
way around the tubular body. Do the same going the other direction. Insert one blade of the scissors to one side
of the center of the dorsal surface and cut all the way to the mouth. Be sure to always pull up on the scissors
as you cut so that you do not damage any of the internal organs. Using a teasing needle, gently tear the
membranes that attach the organs to the body wall and pin the body wall to the tray using the 6-10 “T” pins
provided. Insert the pins at a sharp angle to the dissecting pan so that they do not obstruct your view of the
specimen.
1. Digestive System:
a) Follow the organs of the digestive tract from mouth to anus. Use the map in Figure 1 and the
diagrams from your text to identify the mouth, pharynx, esophagus, crop, gizzard, intestine, and anus.
b) Cut out a small section of the intestine (about 1mm in width). Clean the dirt out of it with a few drops
of water and place it on a petri dish. Examine the cross section to observe the intestinal lumen (the cavity food
goes through), intestinal wall, and the typhlosole. Make a proper scientific drawing of the earthworm
intestinal cross section on a separate sheet of paper.
c) In Figure 2, label the parts of the digestive tract and color them yellow.
d) Complete Table 1 on the digestive system of the earthworm.
2. Circulatory System: Examine the closed circulatory system of the earthworm. Blood is pumped through
vessels by five pairs of aortic arches (similar to hearts) which surround the esophagus. Blood flows from the
arches into the ventral vessel which runs underneath the digestive tract. The vessel branches to form smaller
vessels, then capillaries that serve all cells of the body. The capillaries join to former larger vessels which empty
blood into the dorsal vessel which runs along the top of the digestive system and returns blood to the aortic
arches. The dorsal vessel will contract slightly to move blood back to the aortic arches.
a) Identify the parts of the circulatory system in your specimen. Label these parts in Figure 2 and color
them red.
3. Excretory System: The excretory organs in the earthworms are tiny, white tubes called nephridia. They are
found in pairs in all segments except the first three and the last one. Use a dissecting scope to identify a
nephridium. Each nephridium empties through a pore in the body wall called the nephridiopore.
a) Label the parts of the excretory system in Figure 2 and color them green.
4. Reproductive System: Examine the reproductive structures of the earthworm. The earthworm is
hermaphroditic. It has both male and female reproductive structures however, they rarely fertilize their own
eggs. Earthworms reproduce by exchanging and storing the sperm from another individual. The sperm is
produced in 2 pairs of testes found in segments 10 and 11. However, the testes are quite small and will be
difficult to observe. The immature sperm are stored in the seminal vesicles found in segments 9-14. They are the
3 pairs of lobed structures which wrap around the esophagus. The sperm matures and is stored in the seminal
vesicles until copulation occurs. During copulation, two earthworms lie side by side and exchange sperm. This
sperm from the other individual is stored in two pairs of seminal receptacles found in segments 9 and 10. The
openings to these structures are found between segments 9 and 10 and between segments 10 and 11. These are
quite small and will be hard to observe. It is this sperm which will fertilize the eggs. One pair of ovaries are
found in segment 13 which produce eggs that are discharged through 2 pores in segment 14. The ovaries are
also very difficult to see.
a) Identify the structures of the reproductive system in your specimen. Label these structures in Figure 2
and color them blue.
5. Nervous Systems: The central nervous system of the earthworm consists of a brain and a ventral nerve
cord. The brain actually consists of 2 ganglia fused together and is found at the anterior end of the pharynx on
the dorsal side. Two nerve cords branch off the brain, encircle the pharynx and join to form the ventral nerve
cord which runs along the body wall below the digestive tract. The ventral nerve cord slightly widens within
each segment. Each widened segment is called a ganglion and has lateral nerves extending from it into the
somite. These ganglia and lateral nerves make up the peripheral nervous system.
a) Identify the parts of the nervous systems in your specimen. Label these structures in Figure 2 and
color them black.
Questions:
1.
Why is it important for the skin of the earthworm to remain moist at all times? /1
2.
Why do earthworms come out of their burrows after a heavy rain? /1
3.
Describe the location and position of the setae. How do they aid in locomotion? /2
4.
Describe the function and direction of blood flow of the following structures: aortic arches, ventral
vessel, dorsal vessel. How are the dorsal and ventral blood vessels connected elsewhere besides the
aortic arches? /4
5.
Why is an earthworm’s blood red like a human’s? Explain completely. /2
6.
Why is hermaphroditism an advantage for this species? /2
7.
Describe the major parts of a nephridium and how it works. /3
8.
Describe the structures and functions of the earthworm’s two nervous systems. /3
9.
Why is the earthworm an important component of the ecosystem? /2
10.
Create a proper scientific drawing of the earthworm intestine cross-section; label the lumen and the
typhlosole. /5
11.
Complete Table 1: Digestive System of Lumbricus terrestris /7
12.
Finish coloring and labelling Figure 2: Anatomy of lumbricus terrestris
/12
Hand in the sheets containing Table 1, Figure 2, your lab drawing of the intestine
cross-section, and the TYPED answers to all questions, WITH a cover page.
Figure 1. Segment map of organs for Lumbricus terrestris.
Segment
Digestive
System
Circulatory System
Excretory
System
Segment
Reproductive System
Nervous System
1
Mouth
Dorsal & Ventral Vessels
1
2
Pharynx
Dorsal & Ventral Vessels
2
3
Pharynx
Dorsal & Ventral Vessels
4
Pharynx
Dorsal & Ventral Vessels
Nephridia
4
5
Pharynx
Dorsal & Ventral Vessels
Nephridia
5
Ventral Nerve Cord
6
Pharynx
Dorsal & Ventral Vessels
Nephridia
6
Ventral Nerve Cord
7
Esophagus
Aortic
Dorsal & Ventral Vessels
Nephridia
7
Ventral Nerve Cord
8
Esophagus
Aortic
Dorsal & Ventral Vessels
Nephridia
8
Ventral Nerve Cord
9
Esophagus
Aortic
Dorsal & Ventral Vessels
Nephridia
9
10
Esophagus
Aortic
Dorsal & Ventral Vessels
Nephridia
10
Testes
11
Esophagus
Aortic
Dorsal & Ventral Vessels
Nephridia
11
Testes
12
Esophagus
Dorsal & Ventral Vessels
Nephridia
12
13
Esophagus
Dorsal & Ventral Vessels
Nephridia
13
14
Esophagus
Dorsal & Ventral Vessels
Nephridia
15
Crop
Dorsal & Ventral Vessels
Nephridia
16
Crop
Dorsal & Ventral Vessels
Nephridia
16
17
Crop
Dorsal & Ventral Vessels
Nephridia
17
18
Gizzard
Dorsal & Ventral Vessels
Nephridia
18
Ventral Nerve Cord
19
Gizzard
Dorsal & Ventral Vessels
Nephridia
19
Ventral Nerve Cord
20
Gizzard
Dorsal & Ventral Vessels
Nephridia
20
Ventral Nerve Cord
21
Intestine
Dorsal & Ventral Vessels
Nephridia
21
Ventral Nerve Cord
22
Intestine
Dorsal & Ventral Vessels
Nephridia
22
Ventral Nerve Cord
23
Intestine
Dorsal & Ventral Vessels
Nephridia
23
Ventral Nerve Cord
24
Intestine
Dorsal & Ventral Vessels
Nephridia
24
Ventral Nerve Cord
25
Intestine
Dorsal & Ventral Vessels
Nephridia
25
Ventral Nerve Cord
26
Intestine
Dorsal & Ventral Vessels
Nephridia
26
Ventral Nerve Cord
27
Intestine
Dorsal & Ventral Vessels
Nephridia
27
Ventral Nerve Cord
28
Intestine
Dorsal & Ventral Vessels
Nephridia
28
Ventral Nerve Cord
29
Intestine
Dorsal & Ventral Vessels
Nephridia
29
Ventral Nerve Cord
30
Intestine
Dorsal & Ventral Vessels
Nephridia
30
Ventral Nerve Cord
31
Intestine
Dorsal & Ventral Vessels
Nephridia
31
Ventral Nerve Cord
32
Intestine
Dorsal & Ventral Vessels
Nephridia
32
Clitellum
Ventral Nerve Cord
33
Intestine
Dorsal & Ventral Vessels
Nephridia
33
Clitellum
Ventral Nerve Cord
34
Intestine
Dorsal & Ventral Vessels
Nephridia
34
Clitellum
Ventral Nerve Cord
35
Intestine
Dorsal & Ventral Vessels
Nephridia
35
Clitellum
Ventral Nerve Cord
36
Intestine
Dorsal & Ventral Vessels
Nephridia
36
Clitellum
Ventral Nerve Cord
37
Intestine
Dorsal & Ventral Vessels
Nephridia
37
Clitellum
Ventral Nerve Cord
38
Intestine
Dorsal & Ventral Vessels
Nephridia
38
Ventral Nerve Cord
39
Intestine
Dorsal & Ventral Vessels
Nephridia
39
Ventral Nerve Cord
109
Intestine
Dorsal & Ventral Vessels
Nephridia
109
Ventral Nerve Cord
110
Intestine
Dorsal & Ventral Vessels
Nephridia
110
Ventral Nerve Cord
Last
Anus
Dorsal & Ventral Vessels
Brain
3
Seminal Receptacle
Seminal Vesicle
Ventral Nerve Cord
Seminal Receptacle
Seminal Vesicle
Ventral Nerve Cord
Seminal Vesicle
Ventral Nerve Cord
Seminal Vesicle
Ventral Nerve Cord
Ovaries
Seminal Vesicle
Ventral Nerve Cord
14
Egg Pores
Seminal Vesicle
Ventral Nerve Cord
15
Sperm
Last
Ventral Nerve Cord
Ventral Nerve Cord
Figure 2. Anatomy of lumbricus
terrestris
Labelled and colored
systems of Lumbricus terrestris.
This diagram has been adapted
from “Investigating Living
Systems” Kaskill et. al., 1994.
Table 1. Digestive system of Lumbricus terrestris.
structure
type of digestion
intracellular/extracellular
mechanical/chemical
description of structure and its function