Name ____________________________ Date ___________________ Section_____
EARTHWORM DISSECTION
Anatomy & Physiology
Introduction
In this lab, you will dissect an earthworm in order to observe the external and
internal structures of earthworm anatomy. By the completion of this
dissection, you should be able to describe the appearance of various organs
found in the earthworm and name the organs that make up various systems of
the earthworm. Finally, your assessment will include comparing earthworm
anatomy to human anatomy.
Materials
Dissecting trays
Dissecting kits (scalpel, scissors, forceps, pins)
Earthworms
Procedure
Observation: External Anatomy
1. Find the anterior end of the earthworm by locating the fleshy bump over its
mouth, called the prostomium. The posterior end has a small hole where
solid waste is expelled, called the anus. The length of the worm is made up of
many tiny segments, each separated by a thin wall called a septum.
2. About one-third of the way back from the mouth you should see a thicker
and smoother section of the worm. This is called the clitellum, and it is
involved in reproduction.
3. Notice that the earthworm has a rounded dorsal (back) surface and a flatter
ventral (belly) surface. Usually the dorsal surface is darker than the ventral
surface (though sometimes this is obscured in the preservation process). Along
the ventral side, especially toward the posterior end of the worm, there will tiny
bristles on each segment called setae. Try to see the setae using the dissecting
microscope.
4. With the dissecting microscope, look for tiny excretory pores that can be
found on the anterior edge of most segments. Liquid metabolic wastes are
expelled through these pores. There are also dorsal pores on the dorsal side of
the worm running from segment 12 to the posterior end of the earthworm’s
body. Dorsal pores open from the coelom to the outside.
5. Near the front end of the worm, you should see some larger pores that can
be easily seen without magnification. These are genital pores and are
important in reproduction. On segment 15, there are sperm duct openings
that form slits on the ventral surface. A fine probe may be moved across the
surface of the earthworm until it encounters the opening. The seminal
receptacle openings are the grooves between the 9th and 10th, as well as the
10th and 11th segments. During mating, two earthworms exchange sperm while
the clitellum of one earthworm is aligned with the seminal receptacle openings
of the other earthworm. The oviducts have small openings next to the ventral
pair of setae on the surface of segment 14.
Dissection: Internal Anatomy
1. Lay the worm on your dissecting tray with its dorsal side facing up. Use
dissection pins to secure the worm on the tray. One pin should go through the
first segment, and another further back behind the clitellum.
2. Start your dissection about 3 centimeters posterior to the clitellum. Lift up
the skin with a pair of forceps and snip a shallow opening with a pair of
dissecting scissors. Insert the scissors into the opening and cut in a straight
line all the way up through the mouth. Go slowly and be sure to cut just the
skin--if you go too deep you may damage the internal organs!
3. Using the forceps and dissection pins, carefully pull apart the two flaps of
skin and pin them flat on the tray. (You may need to drag a pin along the
inside of the skin to sever the septum walls to make it easier to spread the
skin.)
4. The first thing to note is the coelom that surrounds the internal organs.
Organs and structures that are visible in this longitudinal view are the
digestive/excretory system, the circulatory/respiratory system, the nervous
system, and the reproductive system.
4. Look at the labeled picture to help you find the following features:
Digestive Tract: Earthworms have a complete digestive tract, running from
the mouth cavity to the anus.
Pharynx: This is the light-colored organ just inside the mouth. Its muscular
contractions pass food on down to the esophagus. In the diagram, the
slender esophagus is somewhat hidden under parts of the circulatory
and reproductive systems.
Crop: Food from the esophagus is temporarily stored in the crop.
Gizzard: Food comes from the crop into the gizzard, where it is ground into
small pieces.
Intestine: The intestine is the long tube extending from the gizzard all the
way to the anus. Food is digested and absorbed here.
Cardiovascular/Circulatory System
Hearts (or aortic arches): Behind the pharynx are five dark loops wrapped
around the esophagus. These are the blood vessels that serve as the
hearts of the worm.
Dorsal blood vessel: This is a dark line extending from the hearts over the
top of the crop.
Reproductive organs: The light yellow-white tissue above and around the
esophagus and hearts are seminal vesicles, in which the sperm mature.
By carefully removing the esophagus and blood vessels, the full extent of
reproductive organs can be seen. The small testes are inside the
seminal vesicles. The testes can be seen by cutting an opening in the
seminal vesicle and turning the vesicle inside-out. The ovaries are
small, white conical-shaped organs, attached to the partition between
segment 12 and 13. The wall between segment 13 and 14 forms an egg
sac, where the eggs are stored. The oviducts start in the egg sac and
open on the surface of segment 14.
Nervous System
Brain: While not a true brain, two masses of nervous tissue coordinate the
nerve fibers of the earthworm.
Ventral Nerve Cord: With your forceps, gently push aside the intestine to
view the long white nerve cord running along the length of the worm
beneath it. This cord consists of a chain of ganglia and has three pairs of
nerves in each segment.
Nephridia: Excretory structures that collect waste products and transport
them out of the body. They lie to the left and right of the intestinal tract
in many of the earthworm’s segments. Each white-colored nephridium
consists of a looped channel. The nephridium begins with a ciliated
funnel in the preceding segments cavity and ends with the excretory
opening on the surface of the worm.
5. Finish cutting the rest of the worm open from the first incision through to
the anus. Observe how the intestine and ventral nerve cord both continue
through the entire length of the worm.
6. For the picture below (your own reference), color code the organ systems for
the earthworm using the following key:
Circulatory System - Red
Reproductive System - Blue
Digestive System – Green
Nervous System – Yellow
Analysis & Discussion
For each of the 5 systems that was observed (nervous, digestive/excretory,
reproductive, integumentary, circulatory), describe at least two similarities and
two differences between earthworms and humans. Since we have not formally
discussed these yet, you should base your answer on your prior knowledge
about human anatomy and physiology. Additionally, include an assessment of
your own performance on dissecting an earthworm.
Grading
This will count as a 50-point lab grade: 40 points for discussion of systems (2
pts per similarity and difference) and 10 points for self-reflection on
performance.