Earthworms are important helpers in the garden or field! Their tunneling mixes up the soil and brings rich soil to the surface. You can observe the organs of these tiny creatures by dissecting a preserved earthworm.
Observation: External Anatomy
|Click for full-size pdf|
1. Find the anterior (front) end of the earthworm by locating the fleshy bump over its mouth, called the prostomium. The posterior (back) 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). Lightly rub your finger along the ventral side toward the posterior end of the worm. You should feel a roughness caused by tiny bristles called setae. Using a magnifying glass, try to see the setae.
4. With your magnifying glass look for tiny pores on each segment. Liquid wastes are expelled through these pores. 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.
Dissection: Internal Anatomy
1. Lay the worm on your dissecting tray with its dorsal side facing up. Use dissection pins to secure each end on the tray. Start your dissection about an inch posterior to the clitellum. Lift up the skin with a pair of forceps and snip an 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.
2. 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.)
|Click for full-size pdf|
3. Look at the labeled picture to help you find the following features:
4. Optional: 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.
See the rest of our online dissection guides for pictures of cow eye dissections, sheep brain dissections, and more.
What other users say:
Good first dissection. The earthworms were in good condition, not smelly at all, and large enough to see the anatomy pretty well. My boys are 11 and 12, and neither wanted to dissect anything, but both found the dissection very interesting once they got started.