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Biology LibreTexts

Mollusks

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

Phylum: Mollusca (Mollusks)

          Class: Polyplacophora (Chitons)

          Class: Gastropoda (snails)

          Class: Bivalvia (Clams)

          Class: Cephalopoda (Nautilus, Squid, Octopus)

Read Characteristics of Mollusks in the lecture notes.

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.

Below: A Squid