Earthworm Dissection
Materials:
Dissecting pan
Dissecting pins (10)
Apron
Scissors
Lab Sheet
Observation Sheet
Internal Anatomy
1. Place the earthworm in the dissecting pan with the dorsal blood vessel on
top. The dorsal blood vessel is a dark purplish color and will run from
the front to the back of the earthworm. Pin the earthworm in the second
segment and the second to last segment. Make sure you carefully stretch
the earthworm a bit to make it easier to cut.
2. Insert the tip of a pair of scissors just right of the dorsal blood vessel and
about 10 segments from the anus. Remember the clitellum is the smooth
part of the worm and is closer to the mouth of an earthworm. Cut all the
way up and through the mouth. Lift up the skin as you are cutting to
avoid hitting any of the organs. Cut as shallow as possible.
3. When the cut is complete open the worm up to look at the organs. The
worm’s septa will make it difficult to open. Septa holds all of the organs
in place so make sure you are careful. The worm has segments and this
will make it easy to identify the organs of the worm. Place a dissecting
pin on each side of segment 5, 15, 25, 35, and 45 to hold open the body
wall. Start at the mouth when you count segments. Look at figure 2 to
see how to complete this part.
4. The first organs you identify are the reproductive organs. They are easy
to find because they are white and located around the 10th segment from
the mouth.
5. Now, locate the parts of the digestive system. Right behind the mouth in
segments 3-6 will be the pharynx. Next, look at the esophagus located in
segments 6-14 that leads to the crop. The crop is located in 15 and 16
and stores food until the gizzard is ready. The gizzard is in 17 and 18
where the food is ground up into smaller pieces using sand. What is the
difference in the wall of the crop and the wall of the gizzard? The last
part of the digestive system is the intestine, which runs from the gizzard
to the anus.
6. On lab sheet #1, label the following parts of the earthworm’s digestive
system: mouth, pharynx, esophagus, crop, gizzard, intestine, and anus.
7. Next, trace the closed circulatory system of the earthworm. A closed
circulatory system means there is a heart and blood vessels in the
organism. First, in segment 7 to 11 are the five hearts of the worm. The
hearts connect the dorsal blood vessel with the ventral blood vessel.
Dorsal means to run along the back. Ventral means to run along the
bottom or front. To locate the ventral blood vessel remove part of the
intestine from the rear end of the earthworm. It will be located
underneath it. The dorsal blood vessel should be easy to identify because
it is the one that you cut to the right of at the beginning.
8. On lab sheet #2, label the following parts of the circulatory system: 5
hearts, dorsal blood vessel, ventral blood vessel, mouth, and anus.
9. On the back of lab sheet #2, draw what the earthworm’s circulatory
system would look like if you were looking at the worm from the side.
10.On lab sheet #3, label the mouth, segment, clitellum, anus, and setae.
11.Try to locate the nerve cord of the worm, which should be close to the
ventral blood vessel and look like a white train track running from the
brain to the anus of the worm.
12. Now, clean up. Throw your earthworm in the basket. Be careful not to
throw any dissecting pins away. Rinse your dissecting pan, scissors,
and dissecting pins and put them back where you got them. Be sure to
dry your scissors so they don’t rust. Put your goggles and apron back.
Hand in your observation sheets and put the instruction sheets back on
my desk. I need one copy of your observation sheets per group.