External Anatomy of the Earthworm (Lumbricus terrestris)
Remove a preserved earthworm from the pail and transfer it to a dissecting pan. Add a little bit of water to your dissecting pan.
a) Locate the conspicuous clitellum, a saddle-like swelling on the dorsal surface. The clitellum produces a mucus sheath used to surround the worms during mating and is responsible for making the cocoon within which fertilized eggs are deposited. The anterior of the animal is more cylindrical than the flattened posterior and is the closest to the clitellum. The ventral surface of the earthworm is usually a lighter colour than the dorsal surface. The mouth is located on the ventral surface of the first segment while the anus is found at the end of the last segment (Figure 1).
Run your fingers over the ventral surface of the earthworm’s body. You should be able to feel bristle-like setae used for locomotion.
Obtain a living earthworm. Place the worm on a piece of moist paper towel and observe its movement. Also, observe the animal’s movement on a smooth surface (eg. The bottom of an enamel pan). After this exercise, continue your observations of the preserved specimen.
Compare the worm’s movements on the smooth and rough surfaces and explain any differences.
Figure 1: External features of the earthworm.
Internal Anatomy of the Earthworm
Digestive System
The earthworm is an example of a foraging herbivorous annelid, obtaining food by eating its way through the soil and extracting nutrients from the soil as it passes through the digestive tract.
a) Position your preserved earthworm dorsal side up and pin it down through the first segment and then again further back behind the clitellum. Cut a slit in the dorsal surface near the posterior pin. Using fine scissors extend the cut forward to the first segment. Be careful not to cut too deep. Starting at the first segment, cut the septa (thin membranes) that internally divide the segments, so the skin can be laid flat. Use additional pins to hold the integument open and expose the organs. Continue to lay the skin back until you have uncovered a centimetre or so of the intestine.
b) Starting at the anterior end, locate the muscular pharynx (food ingestion). This is followed by a tube-like esophagus which terminates in a crop (the wider organ) which serves as a storage stomach. Posterior to the crop you will find the gizzard. Gently press on the crop and gizzard to test their firmness. While the crop is soft and thin, the gizzard is muscular (soil is ground up and churned within the gizzard). The gizzard is followed by a long intestine in which both digestion and absorption occur (Figure 2). Undigested material is voided through the anus.
Figure 2: Pictures of the internal features of the earthworm (top) and a diagram of the internal features of the earthworm (bottom).
