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MOLLUSCA
Lesson 1 - Page 1

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Bivalves have very interesting shells that can help illustrate to your students different living habitats. An important point to emphasize with students is that in shells the living organisms are gone, but on the shell many times information is imprinted. Not in words but in subtle clues that the organism leaves behind. Many of the features that will be described may not be on shells that you purchase from a "shell shop." This is because they polish many of the shells before they sell them, removing some of the "clues."

Let's try to see what key characteristics you can discover and compare on your shells. Have your students compare shell shape, shell weight, structure of the inside of the shell and coloration. Notice the many shapes that your Mollusca samples have in your kit. Most of these shapes are adapted to their living habitat and if you identify which structures give you clues, you can reconstruct where these animals live. The color of many of these organisms depend on how recently they died, for instance the sand dollars are a brilliant purple when alive, but become white after they die and bleached by the Sun or chemicals. The gastropods are colorful in life and maintain this color, but they in time would lose their color.

ABALONE - Only has one shell, the living mantle or fleshy material hugs on rocks. The organisms eats bacteria or other food morsels that may be on the rocks

BIVALVES (ribbed) - The living organism had two mirror image shells. Along the hinge area (see diagram of ideal shell) the teeth that hold the shell together are not large. This gives a clue that the shell lived in the sand or mud of the Florida coast. If the teeth created a tighter hinge this would indicate that it lived on the surface of the sea bottom. If you look on the inside of the shell, you will notice the pallial line (see diagram) has a small lobe. This lobe indicates how large the siphon (brings water and food to mantle) was. The larger the lobe the larger the siphon, the deeper the bivalve dug into the bottom. In this shell, these clues tell us that this organism burrowed only a little.

BIVALVES (smooth) - This is a good shell to illustrate how you can find the age of a clam. Have your students notice the horizontal growth lines on the outside of the shell. This tells you that each growing season, the clam added a new layer. Please note, the word "clam" is just a generic name that can refer to several kinds of shells. Notice on the inside of this shell, the pallial line has a very large indentation. This bivalve was a deep burrower. Also notice the smaller smooth spots. These are where the muscles that controlled the shell's ability to open and close were located. The large ones are to keep the shell closed the smaller ones, just above the larger one, help keep the shell opened.

BIVALVES (mussel) - Mussels are also bivalves, but are unique in that they live on rocks or other hard surfaces. Many times you can see mussels exposed during low tide. Mussels can "sew" themselves to hard surfaces by using black threads that they make. The fibers are so strong that kings in early Europe used the threads to make their robes. The inside of the shell does not have a strong pallial line indentation because these are non-burrowers. Also the muscle scars (smooth surface areas) are missing because they rely on the threads for support. Also the hinge area has no teeth for the very same reason.

BIVALVES (scallop) - Scallops are also bivalves, except they have a reduced pallial indentation and reduced muscle scars. This is because it does not burrow and does not rely on its muscles to secure itself in case someone wants to eat it (like a sea star). It relies on its quickness. Scallops swim in the oceans and are capable of quick movement away from its predators (organisms that want to eat them).

GASTROPODS - (low spiral) - These snails are different from bivalves in that they grow in a spiral pattern and only have one shell. The organism lives throughout the entire shell. Many of the low spiral forms float throughout the water. A high spiral form would not be an ideal shape to float.

GASTROPODS - (high spiral) - This snail shape is more adapted toward living on the bottom of the ocean or along rocks, where they can move around. Some snails tend to be very ornamented while others are smooth. Snails also seem to live anywhere from deserts, to deep oceans, to mudflats, to trees, to even your front yard!

The sea has many areas that organisms can live. A group that has many representatives is within the Phylum Mollusca, which consists of snails, clams, octopus, squid, and abalone.

Mollusks are a diverse group that are have several subgroups including gastropods, cephalopods, bivalves, scaphopods, polyplacophoras, and monoplacophoras. Gastropods include over 35,000 species that live in water or on land. They have one shell that spirals. The entire animal lives inside and moves around with a large food. Bivalves have two equal shells which are hinged by an elastic substance. Bivalves have a foot for burrowing into the sand or mud. Cephalopods include squid, cuttlefish, octopus and nautilus. Most can swim very well, but many do not have shells. Nautiluses’ have a shell that has the geometry called a "whorl" or a coil that wraps around itself. They have tentacles to help them swim around. Scaphopods are burrowing animals that are also called tusk or tooth shells. Polyplacophoras include chitons and have 8 overlapping plates and a foot that helps them attach to a rock. Monoplacophoras include abalone which has one shell with 7 holes. They attach themselves to a hard substance like a rock.

The class Bivalvia also called Pelecypoda or Lamellibranchia includes the clams, oysters, and mussels.  Bivalves are laterally compressed and possess a shell of two valves.  The soft tissue includes the mantle and the foot.  The foot is used for movement, while the mantle is soft tissue that includes the stomach, muscles,  and gills.

            Students should careful dissect the mantle to see if thy can identify the parts shown on the worksheet. 

BACKGROUND:

The marine environment has many different environments where organisms can live.  There are consumers as well as producers, including carnivores, omnivores, and herbivores.  

The marine environment has physical conditions that change like those in the terrestrial environment.  The conditions, however, are different and include water temperature, salinity, ocean currents, depth, and nutrient supply.  Plants can only live in the upper 200 meters of water, because light cannot penetrate any further.  The farther you go down in water depth, the more you will only find consumers.  The ocean environment is very complex. 

Mollusks are a diverse group that are have 6 subgroups including gastropods, cephalopods, bivalves, scaphopods, polyplacophoras, and monoplacophoras. Gastropods include over 35,000 species that live in water or on land. They have one shell that spirals. The entire animal lives inside and moves around with a large food. Bivalves have two equal shells which are hinged by an elastic substance. Bivalves have a foot for burrowing into the sand or mud. Cephalopods include squid, cuttlefish, octopus and nautilus. Most can swim very well, but many do not have shells. Nautiluses’ have a shell that has the geometry called a "whorl" or a coil that wraps around itself. They have tentacles to help them swim around. Scaphopods are burrowing animals that are also called tush or tooth shells. Polyplacophoras include chitons and have 8 overlapping plates and a foots that helps them attach to a rock. Monoplacophoras include abalone which has one shell with 7 holes. They attach themselves to a hard substance like a rock.

PROCEDURE:

 A summary of the mollusk group is given in the sheet labeled "Phylum Mollusca."

1.                  Go over the chart on Mollusca, by showing the students that there are many, many different types of shells that you can find in the Mollusca group. Point out that these are invertebrates (animals without a backbone). Make sure the students know what a backbone is, by having them touch their own spine.
  

2.                  Give each pair of students a bag full of different shells. Have them group the shells into similar shapes and then have them count the number of shells in each group. Discuss why they sorted them into different groups. Talk about sorting like organisms.
  
3. These shells belong to the group called Mollusca which includes snails, clams, and many other shelled animals. Remind students that not all shells are mollusks. The majority of shells in these bags will be gastropods (or snails). They have a spiral symmetry.

4.  Below is more information on the gastropod group.

The gastropods are a very large group of the mollusca family.  The group includes the conches, periwinkles, limpets, garden snails, and slugs.  Most gastropods have shells, generally in the shape of a spiral with numerous turns. Virtually every type of feeding habit is exhibited by gastropods.  Larger bottom dwelling carnivore gastropods burrow into the sand to reach their prey including volutes, bonnets, helmets, olive shells, harp shells, and whelks.  Some species in these groups smother the victims with their feet.  Some may grip the bivalve with the foot, pulling, or wedging the two valves apart with the edge of the shell.  Some are adapted to drill holes in the shells.

The living gastropod has a distinct head with a mouth, eyes and tentacles.  Most have an organ in their mouth area called a radula, a series of rows of minute teeth on a flexible piece of flesh with which they scrape up food, tear the flesh of prey, or bore holes in the shells of clams.  Gastropods may be plant-eaters, carnivores, scavengers, deposit-feeders (obtaining food particles from sediment) or suspension-feeders (straining suspended food particles from the water.  It is very difficult to determine if a snail is a herbivore or carnivore by looking at its shell.  Gastropod shells display an infinite variety of colors, patterns, shapes and sculpturing.  There is one clue that works most of the time when trying to determine if a snail is a herbivore or carnivore, by looking at the siphonal notch area of the shell.

Go over the diagram of a snail with students because the slight modification of the siphonal notch will give students clues on how to determine if a snail is a herbivore or carnivore.  
  
If the notch is indented it is usually a carnivore, if there is no indentation it is usually a herbivore.  Of course, there are exceptions, but that is where books like the Audubon Pocket Guide to Familiar Shells can help.   Have them determine if their shells are from a herbivore or carnivore.

CLASSIFICATION OF MOLLUSCA

ACTIVITY:  Sorting and classifying a bag of shells. 
MATERIALS:  bag of shells

1.  Look at the chart and list the major groups of mollusca that you have in your packet.