It's All Part of the "Body" Plan (Part 2/2)
Lesson 3 of 12
Objective: Student will compare flatworm body plan with roundworm and earthworm body plans.
In today's lesson students are going to explore the different types of animal body plans by observing the body cavities of flatworms, roundworms, and earthworms. Students will begin by using professionally prepared slides of flatworms and roundworms. Next they will explore the diversity in the Phylum Nematoda so that students can realize that not everything matches the generalized model. Then students will dissect two annelids, a leech and an earthworm. By doing these dissections, students will be able to see where these organisms are similar and where they are different. Finally, from their observations, students will development three models of animal body cavities. Here is an overview of what they will learn today. This is day two of a two day lesson. Day one can be found here.
Students should read through the following blog and write a current events summary in their lab notebook.
Then students should answer the following questions in their lab notebook.
- How does the following blog add to what you already knew about planarian regeneration?
Next, student should watch this video and revisit their proposed body plans from yesterday. After viewing the video, students should modify their proposed body plan to show more detail. They should label the three layers of the body plan as ectoderm, mesoderm, and endoderm. They should also show where the digestive cavity, skin, muscles, and nerves are located.
Using prepared slides of cross sections of the flatworm and roundworm body cavities, students will compare the germ layers between flatworms and roundworms. Students should draw a model that best represents a general flatworm body cavity and a general roundworm body cavity.
Students should consider the following questions:
- How many tissue layers (germ layers) do both specimens have?
- List two differences between the flatworm cross section and the roundworm cross section.
(The flatworm has no "empty" fluid filled space in its body. The "tubes" in a flatworm are branching. The roundworm does have an area of fluid in its body cavity. There is one central digestive tube in the roundworm.
- What might be the advantage of having a mesoderm?
(The muscles of the mesoderm can act like a lever in helping the animal move. This is superior to the gel-like body plan of cnidarians.)
- What might be the advantage of having a fluid filled space within the body?
(Organs can grow without being pressed against a hard wall of muscles. The filled filled cavity gives rigid support than just muscles. The skeletal system can work more effectively. Nutrients in the body can be circulated more quickly and wastes can also be eliminated more quickly. The result is a faster moving organism that can grow larger.)
While completing their exercise, students should consider what do all roundworms have in common. They should also consider what types of specialized cells/tissues do certain roundworms have that make them unique.
In this dissection, students will compare two annelids: the earthworm and the leech. This dissection uses bait worms and leeches because they are easy to fix in ethyl alcohol and inexpensive. It can be a safer alternative to using formalin-based specimens.
(Note: To fix the specimens, place them in a 1000 mL beaker filled with 500 mL of ethyl alcohol. Allow several minutes for the specimens to expire. This is best done at the beginning of the day before students arrive.)
Earthworm structures are much easier to see in freshly fixed worm. Students can benefit greatly from viewing fresh tissue.
(Note: Students are asked to answer some reflective questions in this dissection as they compare the two specimens. They are also asked to make several sketches.)
For this dissection, each lab group will need:
- an earthworm
- a leech
- two dissecting trays
- dissecting pins
- two scalpels
- dissecting scissors
- a probe
- safety googles
- disposable gloves
- hand lens
(Note: both the earthworm and the leech can be purchased at any bait store. These will be live and then are fixed in ethyl alcohol before dissection.)
Students should work in groups of two. Have students obtain a freshly fixed earthworm from the class container. They should place the earthworm a dissecting tray and pin its head in place. Using the scalpel, they should gently cut through the skin and pin the skin to either side using dissecting pins. Using the student handout, they should continue to follow the directions and note the specialized organs found in the earthworm. Students pay particular attention to how the earthworm body cavity differs from the roundworm.
Next, students should obtain a freshly fixed leech from the class container. They should lace the leech in a separate dissecting tray and pin its head in place. They should follow the same procedure that they did with the earthworm. Using the student handout, they should continue to following the directions and note the specialized organs found in the earthworm. Students pay particular attention to how the earthworm body cavity differs from the roundworm. In addition, they should compare the body cavities of the leech with the earthworm. Students should also consider how are these two annelids are similar and how are they different.
Sketches should be made in their lab notebook. Specimens should be discarded in the chemical disposable bucket. All equipment should be washed well with soap and water. Equipment should be allowed to air dry. (Note: Because these specimens are not preserved, they should be refrigerated if more than one day is necessary for the dissection.)
Using this powerpoint and these student notes, outline each of the three body cavities found in animals. Students should consider the major differences between the body cavities and how they are formed. (Note: Here is an example of completed student work.)
Using the Frayer method, students should define acoelom, pseudocoelom, and coelom in their lab notebooks.