The earthworm, often specifically the species Lumbricus terrestris, provides an excellent model for understanding segmented invertebrates. Its straightforward, tube-within-a-tube body plan clearly demonstrates fundamental organ systems common to many animals, including digestion, circulation, and nervous control. Dissection offers a hands-on opportunity to visualize how these systems are spatially arranged and function together. This guide provides clear, practical instructions for correctly dissecting an earthworm and identifying its major internal organs.
Necessary Materials and Safety Preparation
Before beginning, gather all necessary equipment to ensure a smooth process. Primary tools include a dissecting tray, T-pins, a sharp scalpel or small dissecting scissors, and a blunt probe. A magnifying glass is also helpful for observing finer anatomical details. The specimen should be preserved or properly anesthetized for examination.
Safety preparation requires protective gear. Always wear safety goggles to shield your eyes from splashing fluid and disposable gloves for hygiene. Handling sharp instruments requires great care; always cut away from your body and fingers. After the procedure, all tools must be thoroughly cleaned and the workspace sanitized to remove biological material or preservative chemicals.
External Observation and Orientation
Correctly orienting the earthworm is necessary before making any incisions. The anterior, or head end, is typically more pointed and features the prostomium, a small fleshy lobe extending over the mouth. Conversely, the posterior, or tail end, is blunter and terminates at the anus.
Identifying the dorsal and ventral sides is equally important. The dorsal side, which will be the top during the dissection, is usually darker and more rounded. The ventral side is typically lighter and flatter. Upon close inspection, you may feel tiny, bristle-like structures called setae that aid in movement. The clitellum, a saddle-like, thickened band, is a prominent external feature located about one-third of the way back from the anterior end, indicating a sexually mature worm.
Step-by-Step Incision and Pinning
The earthworm must be secured to the dissecting tray with the dorsal side facing upward. Pin the anterior and posterior ends to the tray, ensuring the specimen is lightly stretched without tearing. The initial incision should begin just posterior to the clitellum, typically located near segments 32 to 37.
Using fine-tipped scissors or a scalpel, pinch the skin slightly with forceps and make a tiny, shallow opening along the dorsal midline. This incision must penetrate only the body wall, not the organs beneath, which are easily damaged. The goal is to cut just beneath the skin to avoid slicing the dark dorsal blood vessel or the digestive tract below it. Cutting too deep will ruin the view of internal structures.
Insert the tip of the scissors into the opening and carefully slice forward along the dorsal midline toward the anterior end, stopping short of the mouth. Keep the lower blade of the scissors elevated to maintain the shallow depth. The segments are separated internally by thin, membranous walls called septa, which must be carefully severed with the tip of the probe or scissors as you proceed.
After the incision, gently pull the edges of the body wall apart using forceps. Pin the flaps of skin flat against the tray in a T-shape pattern, using a pair of T-pins in each segment flap to keep the body cavity fully exposed. You can rinse the exposed internal cavity gently with water to clear any excess preservative or dark coelomic fluid, allowing for clearer observation of the organs.
Identifying Internal Organ Systems
With the body cavity open, the major organ systems are visible, centered around the straight digestive tube. The dark, prominent dorsal blood vessel runs the length of the worm just beneath the skin.
The first visible structure at the anterior end is the muscular pharynx, which extends from the mouth and is responsible for swallowing food particles. Posterior to the pharynx is the esophagus, a narrow tube often obscured by five pairs of aortic arches. These arches, commonly called the earthworm’s “hearts,” circle the esophagus and pump blood throughout the closed circulatory system.
The alimentary canal then widens into two distinct, successive structures: the crop and the gizzard. The thin-walled crop serves as a temporary storage chamber for ingested soil. Immediately behind the crop is the thick-walled, muscular gizzard. It uses small stones and grit to physically grind the stored food, which is a form of mechanical digestion.
The gizzard leads into the long, straight intestine, which runs most of the remaining length of the body until the anus. The intestine is the main site for chemical digestion and nutrient absorption. Finally, the ventral nerve cord, a white or yellowish thread, can be seen running along the floor of the body cavity, directly beneath the digestive tract.
Post-Dissection Care and Cleanup
Upon completion of the observation phase, the specimen must be disposed of properly. Place the preserved earthworm and any biological waste into a designated biohazard container, following local disposal guidelines. All dissecting instruments, including the tray, scalpel, and pins, must be thoroughly washed with soap and water to remove biological material and residues. The workspace should also be wiped down with a disinfectant solution for sanitization. Finally, thoroughly wash your hands and forearms.