Materials.
(1) Preserved earthworms, as large as you can obtain. (2) Cross sections of earthworms. (3) Longitudinal sections of earthworms.
Definitions.
Body cavity, the space between the body wall and the alimentary canal. Septa (singular, septum), the thin walls between somites, seen when the worm is opened. Pharynx, the hard-walled, rather bulbous, anterior portion of the alimentary canal. Esophagus, the portion of the alimentary canal extending back from the pharynx with thinner walls and smaller diameter. Crop, the short, wide portion of the canal back of the esophagus. Gizzard, the hard-walled, short region, just back of the crop. Stomach-intestine, the portion of the canal reaching from the gizzard to the anus. Ventral nerve cord, a light-colored thread lying against the inner surface of the ventral body wall. Nerve ganglia (singular, ganglion), slight swellings on the ventral nerve cord. Nerve ring or collar, a pair of nerves extending from the ventral nerve cord around the pharynx to a pair of ganglia (often called the "brain") in the dorsal region of the anterior end. Kidney tubes or nephridia, the excretory organs of the earthworm, occurring as slender, paired tubes in nearly every somite.
Directions.
Select a large worm and cut carefully through the body wall along one side, midway between the dorsal and ventral surfaces, from the anterior end to the posterior. Lay the worm on any convenient fairly soft surface (a piece of pine, cork, peat, paraffin), preferably under water, and pin out the walls so that you can see into the interior.
Identify the structures defined above, as well as the dorsal and ventral blood vessels and the "hearts."
The nephridia are not easily distinguished, though they are very numerous. They are long, slender, coiled tubes, two in each somite, lying in the body cavity, one on each side of the alimentary canal. If possible, identify them.
Notice that most of the internal organs are free from the body wall, lying free in the body cavity.