Chitons (Class: Polyplacophora)

Chitons have a dorsal shell composed of 8 plates. A ventral foot is used for locomotion and for attachment to rocks. It pulls itself close to rocks for protection.

Observe the chiton on display.

Below left: chiton, dorsal surface; right: ventral surface

Snails - Class Gastropoda

Read Snails and Relatives (class Gastropoda) in the lecture notes.

Observe the snails and slugs on display.

Below: A slug. Slugs do not have shells.

Bivalves - Clams and relatives (Class: Bivalvia)

Read Bivalves - Clams and Relatives (Class Bivalvia) in the lecture notes.

Obtain a preserved clam for dissection and place it on a dissecting tray. Remove one of the valves (shells) by inserting a scalpel and cutting the adductor muscles on each side of the hinge. See the diagram below for the location of the adductor muscles.

Click on the images to view enlargements.

The photographs below show the anterior and posterior adductor muscles being cut so that the valves can be pulled apart.

The mantle is a membrane that surrounds the internal structures and is characteristic of all mollusks. The portion of the mantle from the exposed surface may have remained attached to the valve that was removed or it may be covering the internal structures. Find the mantle and if necessary, remove it to expose the internal structures of the clam.

Identify the foot, visceral mass, gills, and labial palps. The gills are large because they are used for filter-feeding as well as respiration. Food is trapped by mucus on the gills and moved by cilia to the mouth. Cilia on the labial palps also direct food and mucus to the mouth. 

The heart can be found in the area dorsal to and slightly posterior to the visceral mass. The coelom of bivalves is reduced and limited to the area surrounding the heart. Because the coelom is reduced, it is difficult to see the digestive organs. Cut through the visceral mass and identify the intestine.

In the photograph below, the foot and visceral mass have been lifted so that the mouth can be seen. Notice the labial palps on either side of the mouth.

Squid, Octopus, Nautilus, and Relatives (Class: Cephalopoda)

Read Squid, Octopus, Nautilus, and Relatives (Class: Cephalopoda) in the lecture notes.

Examine representative cephalopods on display.

Annelids

Earthworms (Class Oligochaeta)

Read the following sections in the lecture notes:

Segmentation

Segmented Worms (Phylum Annelida)

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?

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

Leeches (class Hirudinea)

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

1. A Leech