In this lesson, students will explore the phylum Mollusca. They will look at both the similarities and difference within the phylum by observing a live snail, dissecting a clam, and viewing videos of several types of other mollusks. Finally, they will look into the development of the coelom and its importance to humans. Throughout the lesson, students will focus on the development of specialized organs and how they have allow mollusks to adapt to the environments in which they live. Here is an overview of what they will learn today.
Give each student group a petri dish with a live land snail and a freshly picked leaf in it. Allow students time to observe the movement and behavior of the snail. Then have students answer the following questions.
Students should record their answers in their lab notebook.
Ask students to respond in their lab notebooks to the following questions.
While students are watching the videos and responding to the questions, collect the snails and transfer them to the large specimen dish for the Student Activity: Observing How Mollusks Eat.
Student groups of two should obtain a preserved clam and dissection kit from the supply table. They should use this student handout with the dissection. First, groups should rinse the clam throughly with tap water to remove any excess preservative. Students should place the clam in a dissecting tray. Next, they should observe the outside of the shell and make a sketch in their lab notebook. They should identify the following structures: hinge ligament, umbo, valve, mantle, and growth lines.
Have students answer the following questions:
To open the clam, students should hold the clam in the dissecting tray firmly with one hand. With a scalpel scrape away some of the rough outer layer of the shell. Students should continue scrape until they see white. (Note: Students should scrape in a direction away from the hand to avoid being cut.) Students should gently place a drop or two of acid on the exposed prismatic layer. Have students write their observations of what occurred once acid was placed on the shell in their lab notebook. Students should carefully rinse the acid off of the shell with tap water before moving onto the next portion of the dissection.
Next, one student should pry open the shell with a screwdriver while the other partners runs a scalpel along the edge of the shell to cut the adductor muscles. Then students should very gently open the shell so as not to damage the internal organs.
Students should notice and describe the following internal structures in their lab notebook: hinge, posterior adductor scar, mantle, foot, gills, incurrent siphon, excurrent siphon, palps, mouth, stomach, intestine, digestive gland, anus, reproductive organ, pericardial cavity, heart, and kidneys. They should provide a written description as well as a sketch. To aid in identification of these structures, students should cut a portion of the mantle away. (Note: This may have already been done if students opened the shell too forcefully.) Once the mantle is removed, students should been able to see the gills and the visceral mass. They should use a probe and a magnifying glass to notice the texture. Students should record their findings in their lab notebook. They should compare the tissue in the lungs with the tissue of the visceral mass.
Using the dissecting scissors, students should cut off the gills and the lower portion of the foot. They should sketch the structures they see in their lab notebook. With the gills and foot removed, students should be able to see the rest of the clam's organs.
To aid in their dissection, students can also refer to the attached student handout.
Since specimens vary at the end of the dissection period, have one student remain with the specimen and have the other student rotate to the nearest lab station for a show and tell period. Have students go through the checklist and identify the parts of the clam on the checklist. Then have the partners switch.
Students will use the attached webquest to compare and contrast the diversity found in the mollusks. This webquest will focus on how mollusks feed and eat. First, students will watch this video to observe how a snail moves.
Next, they will navigate to this webpage and answer the section about gastropod movement.
To understand the movement of bivalves, they will watch this video to observe a cockle flipping with his foot.
They will follow their observation by navigating to this webpage and answering the section about bivalve movement.
Finally, students will watch this video showing cephalopod movement.
Then they will read this webpage and answer the section about cephalopod movement.
For this station, give student groups the following supplies and equipment:
Instruct them to make a model of an open circulatory system. Use this handout with student to guide them through the activity.
One possible solution: Students should attach a ring clamp to a ring stand. Next they should place a nitrile surgical glove on the ring stand and clamp it in place with paper clamps. Using the pipette, students should fill each of the gloves fingers will tap water until all of the fingers are full and part of the palm portion of the gloves is full. Students should gently squeeze one of the fingers and show how the water moves from one area to another freely. They should explain that the entire glove is like the hemocoel in which blood can move easily into open spaces around the organs of mollusks.
Modified from Glencoe Biology. 2012. McGraw-Hill Education.
For this station, one will need the following supplies and equipment for each lab group:
Students should mix 1 part cornstarch to 2 part distilled water to make a thin paste. Students should paint the bottom of a petri dish with a thin layer of the cornstarch mixture. Using a blowdryer, students should dry the cornstarch glaze. Student should place a snail on the glass and replace the lid. They should observe the snail eating the cornstarch. (Note: Students should be able to see the snail's radula as it scrapes food off of the glass.) Using the stereoscope, students should observe the number of strokes of the radula per minute. Have students express this value as a rate in their lab notebook along with a description of their observations or use this student handout.
Modified from Glencoe Biology. 2012. McGraw-Hill Education.
Using fossilized and preserved specimens, students will compare the body plans of brachiopods and other lophophore feeders with the body plan of mollusks. Students will determine why seemingly unrelated animals like snails and squid are put in the same phylum, while brachiopods are in their own phylum. Students will record their findings in their lab notebooks. (Note: If you do not have fossilized or preserved specimens, you can use this powerpoint, Comparing Body Plans of Mollusks.)