Lab: Earthworm Dissection

External Anatomy
1) The body of the earth worm is comprised of over 100 segments. Each segment looks like a ring. The segments are numbered in sequence from the anterior end. Numbers in the diagrams refer to the segment number.

2) The anterior is the front and posterior is the back.

**Locate the worm's mouth at the anterior end and anus at the posterior end _______

**Locate the swelling of the earthworm near its anterior side - this is the clitellum ______
The clitellum is a swelling of the body found in sexually mature worms.

3) The openings toward the anterior of the worm, between the mouth and clitellum, are the male genital pores (sperm duct)
This is where the sperm are released.

** male genital pores ______
4) The setae are the bristles that the earthworm uses for movement (locomotion). They are often easier to feel than to see. Rub your fingers along the earthworm's body and feel the setae.

** setae _____
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.
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 teasing 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.

**seminal vesicles ______
Circulatory system
The dorsal blood vessel appears as a dark brownish-red vessel running along the intestine.

** dorsal blood vessel _____
The heart (or aortic arches) 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. If you are careful, you can expose all 5 of them.

** 5 hearts ______
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) .

** pharynx _____

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 which stores food.

** crop _______

Then is the gizzard which grinds the food.

** gizzard ______
The gizzard leads to the intestine which is as long as the worm and ends at the anus.

** intestine ______
Nervous System

Between the hearts and the mouth, locate the brain.

** brain ______