Nature's Suction Cups?
Lesson 8 of 12
Objective: By observing echinoderm behavior and comparing and contrasting echinoderm anatomy, students will evaluate how the water-vascular system and tube feet are adaptations that enable them to be successful.
In today's lesson, students will observe living echinoderms. They will also dissect a sea star to better understand its internal anatomy. Echinoderm embryonic development will be explained as a model that adds in a better understanding of human development. Students will consider the ethical considerations of using echinoderm embryos, in lieu of using human embryos. Here is an overview of what they will learn today.
Show students the following video (Starfish locomotion)
Ask students to consider the following questions:
- Describe the structure of a sea star's tube feet
- Where are they located on the sea star?
- What do they do for the sea star?
- What happens if the tube foot is touched with a glass probe?
- Why are tube feet important to the sea star?
Students should write their responses in their lab notebooks.
For this activity, students need the following equipment:
- preserved sea star
- dissecting scissors
- dissecting tray
- disposable gloves
- dissecting pins
Students should dissect a preserved sea star to better understand sea star anatomy and the function of the water-vascular system. Students should follow the directions given here.
For this dissection, student will consider how sea stars move using their tube feet. Students should begin by obtaining a preserved sea star and rinsing off the preservative. Students should next determine the symmetry of the organism (which is radial). They should count the number of arms and record it in their lab notebook. Then students should examine the aboral surface of the sea star. They should locate the madreporite on the top of the central disc. They should make a sketch and label the central disc, arms, and madreporite. Students should feel the spines on the top of the sea star and determine their function. They should look at the tip of each arm for the eyespot and make a sketch of the eyespot on the arm in their lab notebook.
Students should turn the sea star on its aboral side so that the oral side is facing upwards. They should locate the mouth of the sea star and find the ring of oral spines that surround the mouth. Next, they should locate the tube feet. Students should compare the tube feet in the preserved specimen with the tube feet from the video that they viewed earlier in the lesson.
On the aboral side of the sea star, students should use a pair of dissecting scissors to clip off the tip of one of the arms. Then they make two parallel incisions along either side of the arm. They should remove the flap of sea star skin and set it aside. Students should locate the digestive glands and make a sketch of them in their lab notebook. Next, students should cut a circular flap of skin from the central disc. Students should take care not to cut into the water-vascular system of the sea star. Student should lift the section of skin carefully with a probe and observe the stomach (which is located directly under the central disc). Students should remove the digestive glands to find the gonads. (Note: these will only be seen if the sea star is in breeding season. Sea stars have different sexes so students will either seen ovaries or testes.) Students will need to remove the gonads to view the water-vascular system. Students should make a sketch of the water-vascular system in their lab notebook and label the tube feet, ampulla, sucker, ossicles, lateral canal, circular canal, and madreporite. Students should then trace the movement of water through the sea star. Finally, students should respond to the post-lab questions in their lab notebook.
Students should use this student handout to complete the webquest. For the first section, they should explore echinoderm locomotion in more details by watching the following videos: (Starfish tube feet, and Sea urchin flipping and sea star creeping).
To assist student in completing the second section of the webquest, they should use this playlist.
Following the video, show students these this image that detail the sea star embryonic development.
Then have them identify the different stages in embryonic development using this image, Sea Star Embryos.
Ask students to consider this following question:
- Why we would study the developmental stages in echinoderms instead of using human embryos?
Students should record their responses in their lab notebook.