Biology-Mr.Willets
Lab- Bivalve Dissection
Phylum Mollusca contains over 100,000 species of animals. There are three main types of mollusks. There are great differences among these 3 groups which is an example of divergent evolution.
The gastropods, most of which have coiled shells, are the snails, slugs etc. They are sedentary and grazers. The cephalopods, which have tentacles but generally have no shell, are the octopus and squid etc. They are fast moving and hunt for food. They have good eyes and a well developed nervous system. The third main group is the bivalves. These have a two part shell. The word bivalve means two shells. Clams, oysters, scallops and mussels are members of this group.
Compared to the other two types of mollusks, bivalves move the least and have the greatest amount of protection. Their nervous system is the least developed and they are filter feeders. Even though bivalves move very little (if at all), they have bilateral symmetry as do all mollusks. The specimen you will dissect today is a fresh water mussel.
EXTERNAL ANATOMY
1) The shell consists of two parts (valves)
2) There is a dark outer protective layer on the shell- it is called the horny layer and protects against chemicals in sea water. If a shell has been washing about in the water for awhile, this layer may
be scrubbed off by the sand. If so, the white calcium carbonate (lime) layer beneath can be seen.
3) The two parts of the shell are attached by an elastic hinge. This is the dorsal part of the animal.
4) The ventral side is where the two parts of the shell open.
5) Near the hinge is a hump called the umbo. It is the oldest part of the shell and is located near
or points to the anterior side of the bivalve. Remember which end is anterior.
ANATOMY OF THE MANTLE CAVITY
Inside the shell is a space called the mantle cavity. Water circulates in this area bringing in food and oxygen and removing wastes. There are several structures visible in this area.
Open the shell and look for the following:
1) Notice the inside of the shell. It is made of mother-of-pearl. It is smooth and protects the animal from being irritated by the shell. The other name for mother-of-pearl is nacre. Pearls are made of this.
2) Attached to the shell on either side is a very thin piece of tissue called the mantle. This absorbs chemicals from the sea water and uses them to make the shell. It probably was ripped as a result of opening the shell.
3) Water enters and leaves the mantle cavity through two tubes called the incurrent and excurrent siphons. In bivalves that bury in the sand, these tubes are long enough to reach up through the sand into the water. In steamers, these tubes are called the neck of the clam. Mussels don't bury themselves and so their siphons are only openings and not tubes. Look along the posterior edge of the shell. The dark ridges on either side form these siphon openings when the shell is closed.
4) There are two pairs of gills- one on each side of the main part of the mussel. These remove oxygen from the water and put carbon dioxide into the water. This is called the exchange of gases. The gills are very thin to allow oxygen to easily diffuse through.
5) The main part of the body has two parts to it- the muscular foot and the visceral mass. They appear as one structure. The foot is attached to the visceral mass and is located near where the shell opens. The visceral mass is the large upper section of the main part of the body. It is usually a lighter color. The internal organs (viscera means internal organs) are located in this. The foot is very hard and probably will appear somewhat wrinkled and darker due to the preservative. If you eat clams, the belly of the clam is visceral mass. The chewier part of the clam is the foot.
6) Near the anterior side of the visceral mass is the mouth. You won't see the mouth but there are two pair of "lips" near it. These lips look something like small floppy dog ears. Food is caught by mucus on the gills and is pushed by cilia to the mouth. In some cases, you will have to poke around to see these lips. In other cases, they will be very obvious.
7) Two adductor muscles that held the shell together- one anterior, one posterior. These were cut or ripped when the shell was opened. Scallops have only one large adductor muscle. They use it to open and close the shell rapidly. This pushes water out and creates a jet propulsion that they use to move. It is the part of the scallop that we eat.
INTERNAL ANATOMY
The internal organs are located inside the visceral mass. The upper part of the visceral mass contains the heart and kidneys. These are difficult to find and so we will ignore them. This upper part is where the coelomic cavity is located.
Using the scalpel, make a cut through the foot and into the visceral mass. Continue up as high as possible. The cut should be made so that the foot and visceral mass open into two halves in the same way that the shell does. Check with your friendly teacher about how to make this cut.
The following structures can be found: (note- the colors sometimes
vary due to the preservative)
Notice that the foot is solid muscle.
1) Stomach- dark colored area near where the mouth is. It is surrounded by digestive gland.
2) Intestines- this is a tube that is coiled around and surrounded by a green/yellow material
3) Gonads- the material that surrounds the intestines.
QUESTIONS-
1) Bivalve shells have a dark outer layer whose function is to _____________________________
_________________________________ The layer under this is white and is made of ___________________________ also called ______________. The inner "pretty" layer is made of a material called ___________________ of ______________. It is smooth so that it does not ______________the bivalve.
2) The "bump" on the shell is called the ______________. It is the ____________part of the shell.
3) The space between the two shells is the ___________________ cavity. Water enters and leaves this area through the ___________________ and ____________________ siphons. The ___________ are in this space and get oxygen from the water as it passes over them.
4) What part of a scallop do we eat? _________________________________ It is so large because they use it to __________and______________the shell allowing them to move.
5) The "chewy" part of a clam is its _______________
6) Why do mussels have openings rather than tubes (siphons)leading into the mantle cavity? _________________________________________________________________________