Evolutionarily, the molluscs apparently derive from the platyhelminthes. They are classified as triploblastic eucoelomate animals, as are all phyla remaining to be examined. Most molluscs are bilaterally symmetrical and have well-defined circulatory, respiratory, excretory, and digestive systems.
The molluscs are a large group, second only to arthropods in number of species. The name "mollusc" is derived from the Latin molluscus ("soft"), indicating that the molluscs are soft bodied animals. The group includes the snails, bivalves, chitons, squid, octopuses, and others. In some forms, the soft bodies are protected by a calcareous shell.
The organisms in the phylum Mollusca are characterized by having three main body areas: a head-foot (sensory and locomotion structures), a visceral mass (excretory, digestive, and circulatory structures), and a mantle (which secretes the shell). The gills, which function in respiration, are located between the visceral mass and the mantle.
Representatives of the following classes will be studied in this exercise:
Class Polyplacophora - the chitons
Class Gastropoda - snails, slugs
Class Bivalvia - clams, oysters, etc.
Class Cephalopoda - squid, octopus, etc.
This class is considered to be the most evolutionarily primitive of the phylum. These animals are entirely marine, and have oval bodies with a shell consisting of eight dorsal plates. A broad, flat foot used in locomotion is located ventrally. The mantle cavity is reduced to a groove running on either side of the body between the foot and the margin of the animal.
This class includes snails and slugs. It is by far the largest class of molluscs. Gastropods are primarily marine, but some species also inhabit freshwater and terrestrial habitats. In all forms, the visceral mass is located enclosed in a coiled shell during early developmental stages. In most gastropod species the shell is retained in the adult, but in some, such as the common garden slug, it has been completely lost. Because of this, slugs are restricted to moist areas to prevent desiccation.
Members of this class are characterized by a shell consisting of two valves or halves. Bivalves use a muscular foot for locomotion. Siphons are used to draw in a stream of water which is passed over the gills for feeding and respiratory purposes.
The cephalopods are considered to be the most highly evolved class of molluscs. These organisms have a highly evolved visual system, and tentacles with suction cups. They are all marine, and are active predators. They are, by necessity, fast swimmers which use jet propulsion as a means of locomotion.
1. Examine the chitons on display. Note the characteristic dorsal plates and ventral foot, and the anteriorly located mouth.
2. Several snail shells should be examined. Note the different types of coiling in the various shells, each representing a different species.
3. Examine the demonstration of a preserved squid (Loligo). The following structures should be visible:
- eight arms
- two tentacles
- mantle (enclosing the visceral mass)
- lateral fins
- eyes, located just anterior to the mantle
- siphon, protruding from below the mantle
Water drawn into the mantle cavity can be forcefully expelled through the siphon when muscles of the mantle contract, resulting in jet propulsion. The siphon can direct the jet of water in different directions.
An octopus should also be examined. Compare this organism to the squid, noting the difference in the shape of the mantle and the octopus's lack of fins.
4. Examine a freshwater clam following the procedure described below. Freshwater clams are widely distributed and live on the bottom of lakes, rivers, and streams.
Examine the clam externally. Find the two valves, the hinge ligament that holds them together, the swollen umbo at the anterior end of the hinge, and the lines of growth.
Using the position of the hinge and umbo, determine which side is dorsal, which side is ventral which end is anterior, and which end is posterior.
Locate the position of the anterior adductor muscle and posterior adductor muscle through the narrow opening between the valves. Slip a scalpel BETWEEN THE MANTLE AND THE LEFT VALVE. Use the scalpel to gently pry the adductor muscles away from the valve to which they are attached. Loosen the mantle over the entire area of the left valve and open the valves.
Examine the inner surface of the empty valve. Notice its smooth nacreous surface. Observe the position of the various muscle scars, including anterior adductor muscle scar, posterior adductor muscle scar, anterior protractor muscle scar, anterior foot retractor muscle scar, and posterior foot retractor muscle scar. Also note the pallial line, which is the point where the mantle attaches to the shell.
Now look at the mantle, including the pallial muscle. The mantle of the left and right valves comes together posteriorly to form a ventral incurrent aperture and a dorsal excurrent aperture that allow water to enter and leave the mantle cavity. Water enters through the incurrent aperture and leaves through the excurrent aperture.
The space between the mantle and the body is the mantle cavity. Lift the mantle to expose the visceral mass, foot, gills, and associated structures. The muscular, wedge-shaped foot is at the ventral aspect of the body. The soft tissue making up the bulk of the body is the visceral mass. Between the mantle and the visceral mass lie two gills. At the anterior margin of the visceral mass, note the smaller, flap-like, labial palps. Labial paIps surround, and direct food toward, the mouth. Water coming in from the incurrent aperture reaches the ventral aspect of the gills and passes dorsally through the gills into a suprabrachial chamber. Water is then directed posteriorly and out of the mantle cavity through the excurrent aperture. In the process, suspended food particles are filtered and gas exchange occurs. Food particles are transported by cilia to food grooves along the dorsal margin of the gills. Cilia in the food grooves transport food to the labial palps.
Return the mantle to its original position and locate the pericardium, a thin membrane dorsal to the visceral mass. The circulatory system of bivalves is an open system in which blood leaving the heart flows freely between the organs. Carefully open the pericardium to see the heart. The heart wraps around the intestine where the intestine emerges from the visceral mass. The intestine is running posteriorly to empty at the excurrent aperture. The heart consists of two parts, a thick-walled ventricle surrounding the intestine and two thin-walled auricles attached at either side of the ventricle. If you were careful in removing the pericardium, you should see both. Look for a dark mass of tissue ventral to the pericardial sac. This is an excretory organ known as a nephridium. Nephridia remove metabolic waste products from the blood and release the waste into the mantle cavity near the excurrent aperture.
Finally, use your scalpel to make a sagittal section of the foot and visceral mass. Within the visceral mass note cut sections of intestine. The yellowish tissue surrounding the intestine is gonad. Anteriorly you will have cut through a greenish digestive gland.
Here is a student video of a freshwater mussel dissection.
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