Earthworm Dissection
External Anatomy
Examine your earthworm and determine the
dorsal and ventral sides. Locate the clitellum which is on the anterior end of the worm.
Locate the mouth of the worm on the far anterior end of the worm
The openings toward the anterior of the
worm are the sperm ducts
The openings near the clitellum are the genital setae.
Locate the dark line that runs down the
dorsal side of the worm, this is the dorsal blood vessel. The ventral blood vessel can be seen on the underside of the worm,
though it is usually not as dark.
Locate the worm's anus on the far posterior end of the worm
Note the swelling of the earthworm near its anterior side, this is the clitellum.
Label the earthworm pictured:
A = _________________________________ B = _________________________________
C = _________________________________ D = _________________________________
Internal Anatomy
1. Place the specimen in the dissecting pan DORSAL side up.
2. Locate the clitellum and insert the tip of
the scissors about 3 cm posterior (behind the clitellum).
3. Cut carefully all the way up to the head.
Try to keep the scissors pointed up, and only cut through the skin.
4. Spread the skin of the worm out, use a needle to gently tear the septa (little thread like structures that
hold the skin to organs below it)
5. Place pins in the skin to hold it apart, angle the pins out so that they are not in your way.
Reproductive System
The first structures you probably see are the seminal vesicles. They are cream colored and located toward the anterior
of the worm. These are used for producing sperm. Use tweezers to remove
these white structures from over the top of the digestive system that lies
underneath it.
Circulatory system
The dorsal blood vessel (X) appears as a dark
brownish-red vessel running along the intestine. The heart or aortic
arches (Y) can be found over the esophagus (just posterior to the pharynx). Carefully tease away the tissues to expose the arches of the heart, the run
across the worm. How many aortic arches can you count? ______
The ventral blood vessel (Z) is opposite the dorsal blood vessel, and cannot be seen at this time because the digestive system covers it.
Label the diagram (use the letters next to the bold words above)
Does the earthworm have a closed or open circulatory system? ________________
Digestive System
The digestive system starts at the mouth. You will trace the organs all the way to the anus and identify each on the worm.
Find the mouth opening, the first part after the
mouth is the pharynx, you will see stringy things attached to either side
of the pharynx (pharyngeal muscles) . The esophagus leads from the pharynx
but you probably won’t be able to see it, since it lies underneath the heart .
You will find a two structures close to the clitellum. First in the order
is the crop , followed by the gizzard . The gizzard leads to the intestine which is as long as the worm and ends at the anus .
*Use your scissors to cut open the crop and the
gizzard. Which one has the harder exterior? ___________
Place in the correct order (number)
____ Anus ____ Crop ____ Mouth ____ Gizzard ____ Esophagus ____ Intestine ____ Pharynx
Nervous System
Locate the brain at the far anterior region of the worm. It is very tiny and whitish colored. If you can't find it, it is probably because it was destroyed when you cut the worm. You CAN locate the ventral nerve cord by removing the intestines and searching for a white string-like structure that runs the length of the worm and attaches to the brain. Remove the intestines and locate the ventral nerve cord.
Lab Analysis (Answer True of False; most of the answers can be found in this worksheet)
1. ____ The brain attaches to the ventral nerve cord.
2. ____ The dorsal side of the worm is lighter than the ventral side..
3. ____ The clitellum is located toward the anterior end of the worm.
4. ____ The esophagus lies beneath the pharynx
5. ____ Earthworms are hermaphrodites.
6. ____ The ventral nerve cord and the ventral blood vessel are connected.
7. ____ The pale string-like structure running the length of the ventral side of the worm is the blood vessel.
8. ____ An earthworm has fourteen aortic arches.
9. ____ The dorsal blood vessel can be seen from the worm's exterior.
10. ____ Serminal vesicles are part of the worm's digestive system.
11. Label these structures on the image: pharynx, esophagus, crop, gizzard, aortic arches, brain, dorsal blood vessel, seminal vesicles, clitellum
Other Resources on Invertebrates
Observation of a Live Earthworm - examine an annelid, observe its behavior
Label the Anatomy of an Earthworm - image of an earthworm, label its parts
Squid Dissection - describes procedure for dissecting a squid and identifying structures