Earthworm Dissection
Pictures: Modern Biology, Holt
The following is a classification of a species in the earthworm family Lumbricidae. This
commonspecies is Lumbricus terrestris also known as the night crawler or dew worm.
Objectives:
• Describe the appearance of various organs found in the earthworm.
• Name the organs that make up various systems of the earthworm.
Materials:
Safety goggles, dissecting pins, gloves, forceps, lab apron, scissors, paper towel,
scalpel, water,dissecting probe, preserved earthworm, hand lens, dissection tray.
Purpose:
In this lab, you will dissect an earthworm in order to observe the external and internal
structures of earthworm anatomy.
Brain
Aortic Arches
Clitellum
Background:
Among the most familiar invertebrate animals are the earthworms, members of the
phylum Annelida. The word annelida means "ringed or segmented" and refers to a
series of rings or segments that make up the bodies of the members of this phylum.
Internally, septa, or dividing walls, are located between the segments. There may be
more than 100 segments in an adult worm. The clitellum is a swelling of the body found
in sexually mature worms and is active in the formation of an egg capsule, or cocoon.
Eggs are produced in the ovaries and pass out of the body through pores. Sperm are
produced in the testes and pass out through a smaller set of pores.
The pumping organs of the circulatory system are five aortic arches. Circulatory
fluids travel from the arches through the ventral blood vessel to capillary beds in the
body. The fluids then collect in the dorsal blood vessel and re-enter the aortic arches.
The earthworm takes in a mixture of soil and organic matter through its mouth, which is
the beginning of the digestive tract. The mixture enters the pharynx, which is located
in segments 1–6. The esophagus, in segments 6–13, acts as a passageway between
the pharynx and the crop. The crop stores food temporarily. The mixture that the
earthworm ingests is ground up in the gizzard. In the intestine, which extends over
two-thirds of the body length, digestion and absorption take place. Soil particles and
undigested organic matter pass out of the worm through the rectum and anus.
The nervous system consists of the ventral nerve cord, which travels the length of
the worm on the ventral side, and a series of ganglia, which are masses of tissue
containing many nerve cells. The nerve collar surrounds the pharynx and consists of
ganglia above and below the pharynx. Nervous impulses are responsible for movement
and responses to stimuli. Each segment contains an enlargement, or ganglion, along
the ventral nerve cord.
Excretory functions are carried on by nephridia, which are found in pairs in each body
segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has
no gills or lungs. Gases are exchanged between the circulatory system and the
environment through the moist skin.
The earthworm hunts food at night and thus has been called a “night crawler.” It usually
extends its body from the surface opening of a small tunnel which it makes by ‘eating’
its way through the soil. The rear end of thw orm’s body remains near the opening while
the head end forages for decaying leaves and animal debris.
It has been estimated that an acre of good soil contains over 50,000 earthworms. By
their continuous foraging and tunneling 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. Thus,
indirectly, the earthworm enriches farmland and provides for more food in a rapidly
expanding population.
Terms to know:
a. anterior - toward the head
b. caudal - tail end
c. cranial - head end
d. dorsal - back
e. lateral - to the side
f. median - to the center
g. posterior- toward the tail
h. ventral - front (belly)
Procedure:
1. Put on safety goggles, gloves, and a lab apron.
2. Place earthworm in the dissecting tray & rinse off the excess preservative. Identify
the dorsal side, which is the worm’s rounded top, and the ventral side, which is its
flattened bottom. Turn the worm ventral side up.
3. Note that the body of the earthworm is divided into ringlike units, the segments.
4. Note that one surface of the worm is slightly flattened and lighter in color than the
other side. This lighter side is the ventral side. The darker dorsal side is marked by a
dark line running the length of the animal. This is the dorsal blood vessel seen through
the skin.
5. At the anterior end of the worm is a small knoblike projection, the prostomium, which
surrounds and covers a small opening, the mouth. The opening is in segment number 1.
Each succeeding segment is numbered consecutively throughout the length of the
worm. Count the number of segments with your group and enter that number here:
__________ Use a hand lens to observe these parts: A. On the dissection drawing
highlight the prostomium.
6. In the most posterior segment is a vertical slit, the anus. This is used for the removal
of digestive waste and soil which the worm has ingested.
7. Surrounding segments 32-37 is a thick belt, the clitellum. This structure secretes a
capsule that becomes a cocoon for developing eggs. B. On the dissection drawing
highlight the clitellum.
8. Look for the worm’s setae, which are the minute bristle-like spines located on every
segment except the first and last one. Run your finger along the ventral surface of the
worm. Notice the many bristles, or setae. These aid in the locomotion of the worm by
gripping the soil as the worm pulls itself along.
External Openings:
9. Refer again to the diagram of the ventral view of the worm to locate and identify the
external parts of its reproductive system. Find the pair of sperm grooves that extend
from the clitellum to about segment 15, where one pair of male genital pores is located.
Look also for one pair of female genital pores on segment 14. These are the openings
of the oviducts. Egg cells pass through these openings on their way to being fertilized.
*SAVE THIS EXERCISE TO THE END OF THE DISSECTION*
INTEGUMENTARY (SKIN) AND MUSCLE SYSTEMS
10. To study the skin and muscular systems, you will prepare a slide of the cross
section of the worm. You will slice 1 segment past the 60 segment and examine it under
the microscope.
13. Locate the epidermis, or skin, the protective outer layer of the earthworm. In addition
to its protective function, the skin is the breathing structure of the worm. The worm's
skin must be kept moist at all times because air will not pass through it unless the air is
dissolved in water.
14. Just below the epidermis are the circular muscles. These contract, narrowing the
diameter of the worm's body.
15. Below the circular muscle layer are the longitudinal muscles. These muscles
increase and decrease the length of the worm's body. The two muscle layers (circular
and longitudinal) function in locomotion by alternately extending and contracting the
body.
16. Clean the slide and return it to the package of slides.
INTERNAL ANATOMY
As you proceed with the dissection, find the parts on the drawing of earthworm internal
anatomy and color them as directed. Turn the worm dorsal side up. CAUTION:
Scalpels and scissors are very sharp. Report any cuts to your teacher.
OPENING THE WORM
17. Insert the point of your scalpel into the segment just anterior (above) to the
prostomium. Using a scalpel and scissors, make a shallow incision. GENTLY make a
posterior cut from the prostomium to 30 segments below the clitellum. Gently, peel back
the two flaps of skin so they can be pinned to the wax. Using this method, you are not
removing any skin and the skin can be folded back onto the worm after you finish
dissecting. The internal organs are usually covered with strips of longitudinal muscles.
These should be removed with the forceps. The worm should now be pinned to the wax
so that it won't move around during dissection. Four to six pins will be sufficient to hold
the worm. Place the pins at angles so they won't interfere with your work. If you place
them straight up & down, they will look like a group of trees and you won't be
able to see anything. Using the forceps and scalpel, spread the incision open, little by
little. Separate each septum from the central tube using a dissecting needle, and pin
down each loosened bit of skin.
CIRCULATORY SYSTEM
18. Use the diagram below to locate and identify the five pairs of aortic arches, or
hearts. Then find the dorsal blood vessel. Look for smaller blood vessels that branch
from the dorsal blood vessel. Surrounding the esophagus near the anterior end
(segments 3-11), are a series of dark-colored loops. These are the aortic arches. They
are sometimes called the earthworm's hearts, but actually they only assist in pumping
the blood. They are fragile and may be broken apart in your specimen, but you can
usually find some of them.
The aortic arches connect the ventral blood vessel, a tube lying under the
digestive tract, with the dorsal blood vessel, a tube lying on top of the digestive tract.
The dorsal blood vessel is the main pumping organ for the blood and it carries blood in
an anterior direction. The ventral blood vessel carries blood to the rear of the worm.
Lateral blood vessels branch off from the two main vessels and carry blood to and from
the internal organs.
DIGESTIVE SYSTEM
19. Locate the digestive tract, which lies below the dorsal blood vessel. Refer to the
diagram below to locate the pharynx, esophagus, crop, gizzard, and intestine.
Find the pharynx, a wide tube at the beginning of the digestive system. Following the
pharynx is the esophagus, a tube which runs to about segment. The esophagus leads
into the crop, an enlarged organ for temporary food storage. The crop is followed by the
gizzard, an organ which grinds food. The remaining organ of digestion is the intestine
which extends to the posterior end of the worm. It is the place where the digestion and
absorption of the food takes place. Digestive waste is eliminated through the anus, the
vertical slit in the last segment.
NERVOUS SYSTEM
20. To find organs of the nervous system, push aside the digestive and circulatory
system organs. Use the diagram below. Locate the ventral nerve cord. Trace the nerve
cord forward to the nerve collar, which circles the pharynx. Find one pair of ganglia
under the pharynx and another pair of ganglia above the pharynx. The ganglia above
the pharynx serve as the brain of the earthworm. On top of the pharynx is the ganglion,
the worm's equivalent to a brain. It is a very tiny white structure which has two tiny
lobes. Connected to the ganglion and under the digestive tract is the ventral nerve cord.
If you cut out a small piece of the intestine, you can find the threadlike white cord
running under it. This is the ventral nerve cord. Branching off from the ventral nerve
cord are lateral nerves that carry impulses to the internal organs.
EXCRETORY SYSTEM
21. The worm’s excretory organs are tiny nephridia. There are two in every segment.
Use the diagram for the nervous system to locate some nephridia. Push the digestive
organs aside slightly on your dissection worm and examine a few segments for the
nephridia. A pair of these should be found in each segment except for a few at each
end. These are very tiny lacy structures consisting of a funnel and a coiled tube. They
can best be seen if you use the stereoscopic microscope.
You have now finished your dissection of the earthworm. Check over the instructions
once again and make sure you have found everything and have the diagrams properly
colored. Dispose of your materials according to the directions from your teacher. Clean
up your work area and wash your hands before leaving the lab.
Post Lab Question
1. Describe two ways in which an earthworm’s body is adapted to live in the soil.