Earthworm Dissection

External Anatomy

Examine your earthworm and determine thedorsal 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 theworm are the sperm ductsThe openings near the clitellum are the genital setae.

Locate the dark line that runs down thedorsal 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

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1. Place the specimen in the dissecting pan DORSAL side up. 2. Locate the clitellum and insert the tip ofthe 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 thathold 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 anteriorof the worm. These are used for producing sperm. Use tweezers to removethese white structures from over the top of the digestive system that liesunderneath it.

Circulatory system

The dorsal blood vessel (X) appears as a darkbrownish-red vessel running along the intestine. The heart or aorticarches (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 runacross the worm.How many aortic arches can you count? ______

The ventral blood vessel (Z) is opposite thedorsal blood vessel, and cannot be seen at this time because the digestivesystem covers it.

Label the diagram (use the letters next to the bold words above)


Does the earthworm have a closed or open circulatorysystem? ________________

Digestive System

The digestive system starts at the mouth. Youwill trace the organs all the way to the anus and identify each on the worm.

Find the mouth opening, the first part after themouth is the pharynx, you will see stringy things attached to either sideof the pharynx (pharyngeal muscles) . The esophagus leads from the pharynxbut 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 orderis 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 thegizzard. 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 pharynx5. ____ 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.


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Label these structures on the image: pharynx, esophagus, crop, gizzard, aortic arches, brain, dorsal blood vessel, seminal vesicles, clitellum



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