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  • Subject:
  • |
  • Grade(s): 6-8
  • |
  • Duration: Two class periods

Lesson Plan Sections


Students will understand the following:
1. An invertebrate is an animal that has no spine, or backbone.
2. Some invertebrates exhibit bilateral symmetry .
3. An animal or plant that exhibits bilateral symmetry produces a mirror image only if a line is drawn through it at one particular place.
4. An animal or plant that exhibits radial symmetry produces a mirror image if any line is drawn dividing it into two equal parts.
5. In the animal kingdom, bilateral symmetry is characteristic of more complex life forms in which different parts of the animal's body perform different functions; radial symmetry is characteristic of simpler life forms, such as jellyfish and starfish, in which the entire body performs all life functions.


For this lesson, you will need:
Research materials about Porifera (sponges), Coelenterata (e.g., jellyfish), and Echinodermata (e.g., starfish)—animals that exhibit radial symmetry
A computer with Internet access


1. Review with your students what they have learned about invertebrates, establishing that an animal that is classed as an invertebrate has no spine, or backbone. See how many invertebrates your class can cite as examples. Add jellyfish and starfish to the list if students do not mention them.
2. Ask a volunteer to draw a picture of a starfish on the chalkboard. Label the drawing with the word invertebrate . Then have another volunteer draw a line dividing the starfish in half, pointing out that the two halves are identical; they are mirror images of each other. Demonstrate that no matter where the animal is divided, the two halves will be the same.
3. Next, draw a simple outline sketch of a human being on the chalkboard, and have a volunteer draw a line dividing the sketch in half so that the two halves are mirror images of each other. Demonstrate that only one line will accomplish this. Any other line will divide the sketch, but the two parts will be different.
4. Introduce the term symmetrical , explaining that both the starfish and the human are symmetrical because each can be divided into two halves that are mirror images of each other. Ask the students to explain the differences between the type of symmetry exhibited by the starfish and the human.
5. Explain that the starfish exhibits radial symmetry , while the human exhibits bilateral symmetry. Ask your students to name animals that exhibit each type of symmetry (radial: starfish, brittle star, sea urchin, sand dollar, jellyfish; bilateral: most other animals).
6. Give students the following homework assignment: Examine your house, yard, and neighborhood for examples of life forms—both animals and plants—that exhibit either radial or bilateral symmetry. Make a sketch of every life form you choose, and be able to explain why you believe it is either radially or bilaterally symmetrical.
7. When students have completed their assignments, have them share their discoveries.
8. Conclude with a discussion about the two kinds of symmetry and what significance they have. Point out that animals that have bodies with many parts that perform different functions exhibit bilateral symmetry, while radial symmetry is found in some simpler animals whose whole bodies perform the same few functions.

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Adaptations for Older Students:
Have each student choose one radially symmetrical animal from each of the phyla Porifera, Coelenterata, and Echinodermata and trace its complete scientific classification. Students should accompany each classification with a labeled sketch of the animal.

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Discussion Questions

1. Describe some of the bodily variations that can be found in jellyfish, and explain how these variations help jellyfish survive in their habitats.
2. Jellyfish alternate between asexual and sexual reproduction. What evolutionary benefits would an organism gain from reproducing this way? How would human society have evolved differently if we had the same form of reproduction?
3. Discuss the senses of a jellyfish. Give some examples of the stimuli they can detect, and explain how these senses help them to survive.
4. Debate the following statement: "Horseshoe crabs are one of the most successful life forms on Earth."
5. For quite a while now, scientists have been using horseshoe crabs in their medical research. Often their experiments do not harm the crabs in any way, but this is not always true of other animal experiments. Debate whether the benefits of using animal research to improve human health outweigh the costs.
6. Horseshoe crabs are often referred to as "living fossils" because they have survived as a species, virtually unchanged for millions of years, without becoming extinct. Compare their evolutionary history to that of human beings. What are the similarities and differences? Which species is more likely to survive indefinitely into the future and why?

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You can evaluate your students on their assignments using the following three-point rubric:
Three points: list includes examples of both types of symmetry; sketches are carefully prepared and labeled; explanations show full understanding of the two types of symmetry
Two points: list includes at least one example of each type of symmetry; sketches are adequately prepared and labeled; explanations show understanding of the two types of symmetry
One point: list includes examples of only one kind of symmetry; some sketches are adequately prepared and labeled; explanations show difficulty understanding the concept
You can ask your students to contribute to the assessment rubric by determining how many examples of each type of symmetry each student's list should include.

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 Guidelines for Horseshoe Crab Experimentation
For several years now, scientists have been studying the immune systems of horseshoe crabs, which are among the most powerful in the animal kingdom. These experiments could potentially lead to ways to help humans heal themselves. Even though not all of the experiments result in harm to the horseshoe crabs, some of them do—and almost all of them involve removing the crabs from their natural habitat. As a result of those facts, many people feel strongly about whether the benefits of this experimentation outweigh the costs to the animals—and some people have gone to great lengths to try to establish explicit guidelines about what is and is not acceptable in animal experimentation. Have your students brainstorm a list of potential conditions that might be included in a set of horseshoe crab experimentation guidelines. Examples might include "Horseshoe crabs may be removed from their natural habitats, but the conditions in which they are kept must reasonably approximate natural conditions" and "All measures must be taken to alleviate any pain in the horseshoe crabs." Then, once the first list is established, invite students to discuss each of the guidelines, refining the wording of each one until a majority of the class either rejects or supports it.

Horseshoe Crab Predators
Horseshoe crabs aren't the only organisms to arrive at Delaware Bay on the East Coast of the United States every spring during horseshoe crab mating season. Horseshoe crab predators come there, too, knowing that there will be plenty for them to feast on. Have your students conduct research to uncover other animals (primarily birds) that make the annual visit. Ask each student to choose one of the animals she or he discovers and study it in greater depth. Students should find out where the animal spends its winters and summers, what it looks like, and how many horseshoe crabs or horseshoe crab eggs it eats.

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Suggested Readings

Drs. Virginia Alvin and Robert Silverstein. Twenty-First Century Books, 1996.
As this introductory volume proves, you don't have to memorize a lot of scientific names to understand how animals are related. You'll enjoy this comparison of animals with no backbone, including the amazing jellyfish, octopus, and horseshoe crab.

The Horseshoe Crab
Nancy Day. Dillon Press, 1992.
Cats may have nine lives, but they still have nothing on horseshoe crabs, which have survived as a species for more than 300 million years! Read about how these "living fossils" are at the forefront of modern medicine because their blood has special protective qualities that are being studied to treat human diseases.

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MASLA: Jellyfish—Marine Aquarium Society of LA
Includes information about life cycle and jellyfish anatomy.

National Aquarium in Baltimore
Includes topics, such as "Jellies 7 Species," "Jellyfish - High Seas Drifters," and "Jellyfish - Drifters."

Jellies (Tennessee Aquarium)
Information about the jellyfish life cycle and anatomy.

Introduction to Scyphozoa -- The True Jellyfish
This site, maintained by University of Berkeley, contains specific and technical information about the jellyfish.

SeaWorld Animal Information Database
Information about the jellyfish and other marine life.

Horseshoe Crab Anatomy Guide
Horseshoe crab anatomy and information. Resources for teachers and students.

National Oceanic and Atmospheric Administration
Teacher resource information. Excellent visuals.

The Horseshoe Crab or King Crab
The site includes good visuals and encyclopedia type articles.

Horseshoe Crab - Living Fossil
Information about the horseshoe crab.

The Assateague Naturalist
The site includes good visuals and information.

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Click on any of the vocabulary words below to hear them pronounced and used in a sentence.

speaker    antibiotic
Definition: A substance derived from a microorganism that is able to inhibit or kill another microorganism.
Context: Horseshoe crabs produce their own antibiotic that fights off invading microorganisms.

speaker    arachnid
Definition: Any of a class of arthropods comprising chiefly terrestrial invertebrates, including the spiders, scorpions, mites, and ticks, and having a segmented body divided into two regions of which the anterior bears four pairs of legs but no antennae.
Context: The class of arthropods most closely related to horseshoe crabs is arachnids.

speaker    compound eye
Definition: An eye made up of many separate visual units.
Context: Horseshoe crabs have a compound eye that is composed of thousands of lenses, each producing a separate image on its brain.

speaker    immune
Definition: Having a high degree of resistance to a disease.
Context: Horseshoe crabs are immune to invading bacteria.

speaker    polyp
Definition: A coelenterate that has a hollow cylindrical body closed and attached at one end and open at the other surrounded by tentacles armed with nematocysts.
Context: The polyp can asexually reproduce many jellyfish.

speaker    tentacle
Definition: Any of various elongate, flexible processes borne by invertebrates chiefly on the head or about the mouth.
Context: Some jellyfish have long, stringy tentacles that contain venomous stinging cells.

speaker    turbulence
Definition: Wild commotion; departure in a fluid from a smooth flow.
Context: Jellyfish are able to detect turbulence in the water.

speaker    zooplankton
Definition: Plankton composed of animals.
Context: Jellyfish eat small, floating, animal-like organisms called zooplankton.

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This lesson plan may be used to address the academic standards listed below. These standards are drawn from Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education: 2nd Edition and have been provided courtesy of theMid-continent Research for Education and Learningin Aurora, Colorado.
Grade level: 6-8, 9-12
Subject area: science
Understands the genetic basis for the transfer of biological characteristics from one generation to the next.
Benchmark 6-8:
Knows that reproduction is a characteristic of all living things and is essential to the continuation of a species.

Benchmark 6-8:
Knows that the characteristics of an organism can be described in terms of a combination of traits; some traits are inherited and others result from interactions with the environment.

Benchmark 9-12:
Knows the chemical and structural properties of DNA and its role in specifying the characteristics of an organism (e.g., DNA is a large polymer formed from subunits of four kinds [A, G, C, and T]; genetic information is encoded in genes as a string of these subunits, and replicated by a templating mechanism; each DNA molecule in a cell forms a single chromosome).

Grade level: 6-8, 9-12
Subject area: science
Understands how species depend on one another and on the environment for survival.
Benchmark 6-8:
Knows relationships that exist among organisms in food chains and food webs.

Benchmark 6-8:
Knows ways in which species interact and depend on one another in an ecosystem (e.g., producer/consumer, predator/prey, parasite/host, relationships that are mutually beneficial or competitive).

Benchmark 6-8:
Knows how an organism's ability to regulate its internal environment enables the organism to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.

Benchmark 9-12:
Knows how the interrelationships and interdependencies among organisms generate stable ecosystems that fluctuate around a state of rough equilibrium for hundreds or thousands of years (e.g., growth of a population is held in check by environmental factors such as depletion of food or nesting sites and increased loss due to larger numbers of predators or parasites).

Grade level: 6-8, 9-12
Subject area: science
Understands the basic concepts of the evolution of species.
Benchmark 6-8:
Knows that the fossil record, through geologic evidence, documents the appearance, diversification, and extinction of many life forms.

Benchmark 6-8:
Knows basic ideas related to biological evolution (e.g., diversity of species is developed through gradual processes over many generations; biological adaptations, such as changes in structure, behavior, or physiology, allow some species to enhance their reproductive success and survival in a particular environment).

Benchmark 9-12:
Knows that heritable characteristics, which can be biochemical and anatomical, largely determine what capabilities an organism will have, how it will behave, and how likely it is to survive and reproduce.

Grade level: 6-8
Subject area: science
Understands the nature of scientific inquiry.
Understands the nature of scientific explanations (e.g., emphasis on evidence; use of logically consistent arguments; use of scientific principles, models, and theories; acceptance or displacement based on new scientific evidence).

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Mary C. Cahill, middle school science coordinator, Potomac School, McLean, Virginia.

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