Summary
Highlights
The earthworm is a segmented worm with a clitellum, which aids in reproduction. Identify the anterior (mouth, closest to clitellum) and posterior (anus, farthest from clitellum) ends. The dorsal side is darker than the ventral side. To begin dissection, pin the worm straight, making an incision behind the clitellum on the dorsal side, then cutting towards the anterior end. Cut through the epidermis and cuticle gently to avoid damaging internal organs.
Most organs are in the coelom, a hollow body tube. Septa, transverse tissue walls between segments, hold organs in place. To open the worm, cut the septa from the skin, pinning the skin at a 45-degree angle to expose the coelom without interfering with viewing the organs, primarily located in the anterior region.
The earthworm's digestive system starts with the mouth, leading to the pharynx, then the esophagus (hidden). Food moves to the crop, gizzard, and intestine, which runs the length of the worm. Solid waste (castings) is expelled through the anus. Earthworm castings enrich the soil, making them beneficial for plants and farmers.
Earthworms have a simple nervous system with several ganglia functioning as a brain. The visceral ganglia are located beneath the pharynx. The ventral nerve cord, a white line of tissue, runs from the visceral ganglia to the posterior end and can be seen by lifting the intestine.
Earthworms are hermaphroditic, producing both male and female gametes. Light-colored testicles produce sperm, and seminal receptacles store sperm from other worms. Ovaries and oviducts, located beneath the esophagus and not visible, produce ova (eggs).
Earthworms lack a respiratory system, as they breathe through their skin. They have a simple circulatory system with five pairs of muscle-lined aortic arches in segments 6-10, a ventral blood vessel (hidden), and a dorsal blood vessel. The aortic arches pump blood into the ventral vessel, which then returns via the dorsal vessel.