The Beginning of Multicellularity
Lesson 1 of 12
Objective: Students will use sponges and cnidarians as model organisms to better understand cell differentiation.
Today students will dissect sponges to better understand tissues and explore some of the diversity seen through the phylum Porifera. Students will also compare fossilized sponges with modern sponge skeletons. Finally, students will compare specialized cells found in sponges with cnidarians to better understand the importance of cell differentiation. Here is an overview of what students will learn today.
For this activity, you will need a small container of modeling clay for each student , a plastic knife or piece of string, and an overheard transparency. Have students construct three shapes (a blob, a sea star, and a person) out of the clay. Encourage students to cut the shapes into symmetric pieces. Have students share how they cut the three shapes and sketch them in their lab notebook.
Have students put their shapes back together. Using the attached powerpoint, students will review the different types of symmetry (asymmetry, radial symmetry, and bilateral symmetry). Have the students cut the blob into four pieces to demonstrate asymmetry. Next, have students cut the sea star into five pie shapes sections to demonstrate radial symmetry. Finally, have students cut the person into two sections done the midline (one right and one left) to demonstrate bilateral symmetry. Students should make sketches in their lab notebooks.
For this activity, divide students into groups of two. Each group needs a preserved sponge (Grantia sp.), dissection tray, scalpel, probe, and dissecting scissors. Using the stereoscope, they view the outside of the sponge and make sketches in their lab notebook. Students should pay particular attention to the pores and spines.
Next, students should bisect the sponge and observe the interior. Students should compare the structures on the interior with the exterior and describe any differences they see.
Finally, each student should remove a small portion of the sponge with dissecting scissors and prepare a wet mount slide of the sponge tissue. They should look at the slide under the microscope at both low and high power. Students should make sketches of what they see in their lab notebook or this worksheet.
Students will compare the structure of modern sponge skeletons and fossilized sponges in this activity. Using the attached handout, students should determine the similarities and differences between the modern and extinct sponges.
(Note: I use modern sponge skeletons that I purchased from Carolina Biological Supply. The sponges fossils I collected at several sites across my state. If you have access to fossils, it makes this portion of the lesson richer. Otherwise, use these images (Fossilized sponge, Calcarea, and Raphidonema faringdonense.)
Using the attached web quest and this playlist, students should explore the similarities and differences between Phylum Porifera and Phylum Cnidaria. As students watch the videos they should consider the following about cnidarians:
- how they eat
- how they transport nutrient throughout their body
- how they structure themselves
- how their cells communicate with each other
- how they reproduce
- how they have adapted to their particular environment
Then, student should create a table of characteristics on the back of their web quest. They should consider what similarities cnidarians have with sponges and indicate those similarities with a (+) sign in the table. Next, they should consider the differences that cnidarians have with sponges and indicate those difference with a (-) sign in the table.
After all students have completed the dissection and most students have completed the web quest, bring them together for a class discussion. Using this powerpoint, discuss how both sponges and cnidarians show the advent of multicellularity in the animal kingdoms. Discuss the importance of multicellularity and the obstacles living things would have to overcome by being multicellular. Finally, discuss the benefits of cell differentiation.
Homework: If they have not completed the web quest, students should finish it outside of class.