PHYLUM MOLLUSCA – TRIPLOBLASTIC EUCOELOMATES
The molluscs are bilaterally symmetrical and triploblastic eucoelomates with complete digestive tracts. Molluscs show a level of complexity greater than what is seen in cnidarians and platyhelminthes, and most have well-defined circulatory, respiratory, excretory, and digestive systems.
The molluscs are a large and diverse phylum, 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 many forms, the soft bodies are protected by a calcareous shell.
Molluscs 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 in the mantle cavity which is located between the visceral mass and the mantle. Most molluscs also have a radula, a tongue-like structure covered with horny (keratinaceous) teeth used for feeding.
Representatives of the following classes will be studied in this exercise:
Class Polyplacophora - chitons
Class Gastropoda - snails, slugs
Class Bivalvia - clams, oysters, etc.
Class Cephalopoda - squid, octopus, cuttlefish, Nautilus
These animals are entirely marine, and have oval bodies with a shell consisting of 8 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. Chitons live in rocky habitats, where they use their radula to feed upon algal films.
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 have a shell consisting of two valves which are hinged dorsally. Bivalves use a muscular foot for locomotion, and the group, as a whole, is adapted for burrowing in soft substrates. Siphons are used to draw in a stream of water which is passed over the gills for feeding and respiratory purposes. The gills are large and sheetlike. Their ciliated surfaces generate water currents which draw water in via incurrent siphon and expel it through an excurrent siphon. Particles are trapped on the surface of the gills and transported to the mouth. Bivalves lack a radula.
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.
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.
1. Examine the chitons on display. In chitons, the mantle has become the thick tissue, called the girdle, in which the shell plates are partially embedded. Note the characteristic 8 dorsal plates, the ventral foot, and the anteriorly-located mouth.
2. Examine several snail shells, noting that most gastropod shells coil in a right-handed (dextral) manner (but at least one sinistral species will be on display for comparison). When the fingers of your right hand are inserted into a dextrally coiling shell, they will curve with the coiling of the shell and your thumb will point toward its apex.
4. Obtain a fresh squid (Loligo sp.) specimen and examine it externally. Note the 8 arms and the 2 long tentacles. Also note the well-developed mantle enclosing the visceral mass, and the lateral fins. The prominent eyes are found just below the mantle, and the siphon protrudes from the mantle.
Figure 1. External morphology of a squid.
5. On demonstration are shells from a nautilus and also the internal shell of a cuttlefish. The nautiluses are cephalopods with an external shell. The cuttlefish shell, known as cuttlebone, is given to pet birds as a source of calcium and to help the birds trim their beaks.
6. 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 and posterior adductor muscle scars, and the pallial line, which is the point where the mantle attaches to the shell.
Figure 2. Features visible on the inside of the shell of a bivalve mollusc.
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 palps 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 suprabranchial 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.
Finally, use your scalpel to make a sagittal section of the foot and visceral mass. Within the visceral mass note cut sections of intestine and the heart, which wraps around the intestine where the intestine emerges from the visceral mass. The yellowish tissue surrounding the intestine is gonad. Anteriorly you will have cut through a greenish digestive gland.
Figure 3. Internal anatomy of a bivalve mollusc.
Here is a student video of a freshwater mussel dissection.