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Biology LibreTexts


This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.

Phylum: Annelida (Annelids)

          Class: Oligochaeta (Earthworms, Leeches)

          Class: Polychaeta (Marine Annelids)

Oligochaeta (Earthworms, Leeches)


Read the following sections in the lecture notes:

Examine the exterior of an earthworm and find the ventral surface. Place the worm in a dissecting pan with the ventral surface down. Identify the clitellum. This structure produces mucous needed for reproduction. Find rows of setae along either side of the ventral surface. These help provide traction as the animal moves through the soil.

Find the dorsal blood vessel. This structure should be visible through the body wall and will appear slightly darker than the rest of the body. It extends the length of the animal. With a scalpel, make a shallow cut along the dorsal surface beginning at a point approximately half way between the clitellum (see photograph below) and the posterior end and ending at the anterior end. Try to avoid cutting the dorsal blood vessel by keeping your cut to one side of the vessel. Try to cut only the body wall but not the internal structures.

The septa between each segment will prevent you from spreading the walls open to view the interior structures. Use a needle or scalpel to carefully cut the septa while spreading the body open. Pins can be used to hold the body open.

Find the structures shown in the photographs below. Be sure that you understand the function of each of these structures.

Below: The anterior end of this earthworm is on the right.

Below: The anterior end of this worm is on the left.

Use a needle to cut the septa in the region of the intestine and roll it aside to expose the nerve cord underneath.

Examine a labeled earthworm composite. This slide shows three different cross sections. The cross-section on the right side of the slide is from the intestinal region. 

Identify the intestine, typhlosole, coelom, nephridium, longitudinal muscles, circular muscles, and dorsal blood vessel. Are any setae present in the cross section?


Read the following section in the lecture notes: Leeches (class: Oligochaeta)

Below: a leech

Marine Annelids (class: Polychaeta)

Read the following section in the lecture notes: Marine Annelids (class: Polychaeta)

  1. Clam worm (Nereis)
  2. Clam worm (Nereis) anterior end