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Connected Experience: Comparative Physiology of Invertebrates


In this lesson, students will:
  1. learn about invertebrate diversity.
  2. compare marine invertebrates’ respiratory and digestive systems with those of humans.


  • invertebrate trivia cards
  • bag, box, or cap from which to pull the invertebrate trivia cards
  • invertebrate scavenger hunt worksheets
  • internet access
  • poster boards


  • respiratory system: the integrated system of organs involved in the intake and exchange of oxygen and carbon dioxide between an organism and the environment
  • digestive system: an integrated system responsible for the ingestion, digestion, and absorption of food and water
  • invertebrate : an animal that lacks a backbone or spinal column
  • nematocyst: a capsule in the tentacles of cnidarians, such as jellyfish, hydras, or sea anemones, used for stinging
  • cnidarian: an invertebrate animal of the phylum Cnidaria, characterized by a radially symmetrical body with a saclike internal cavity, and including the jellyfishes, hydras, sea anemones, and corals
  • echinoderm: radially symmetrical marine invertebrate of the phylum Echinodermata, which includes the starfishes, sea urchins, and sea cucumbers, having an internal calcareous skeleton and often covered with spines
  • arthropod: an invertebrate animal of the phylum Arthropoda, including the insects, crustaceans, arachnids, and myriapods, that are characterized by a chitinous exoskeleton and a segmented body to which jointed appendages are articulated in pairs
  • mollusk: any of numerous chiefly marine invertebrates of the phylum Mollusca, typically having a soft unsegmented body, a mantle, and a protective calcareous shell and including the edible shellfish and the snails


Before your Visit


  1. Print and cut out the invertebrate trivia cards.
  2. Place the invertebrate trivia cards in a bag, box, or cap.


Quickly review human respiration and digestion. Tell students that all animals need to obtain food and oxygen and dispel waste products and that different types of animals have different adaptations for accomplishing this task. Introduce invertebrates and tell students that they will learn about four phyla of invertebrates. Write the four phyla discussed in the Teacher Background section on the board, add the subphylum Crustacea below Arthropoda since this lesson focuses on marine invertebrates. Ask students to brainstorm types of animals that are categorized into these four phyla.


  1. Have students take turns coming to the front of the class and choosing a trivia card to read aloud.
  2. Have other students in the class try to answer the questions.
  3. Encourage students to take notes during this exercise.
  4. After each trivia question is answered, use the information in the teacher background to explain any pertinent information to the students.
  5. After completing the invertebrate trivia exercise, create a table similar to the one below on the board. Have students help you fill in the information comparing the physiological systems of the four phyla of invertebrates and humans. Use the information in the teacher background section to supplement the information that students recall from the trivia exercise.
  Human Beings Cnidarians Echinodermata Arthropoda: Subphylum Crustacean Mollusca
General Fun Facts Walk on two legs Capture food with stinging structures Water vascular system Phylum has the most described species Most have hard shells
Respiration Lungs Exchange gases directly through the epidermis Exchange gases through tube feet Gills Most have gills
Digestion Complete digestive tract Incomplete digestive tract Most have complete digestive tracts, but some have incomplete Complete digestive tract Complete digestive tract


Have students choose one of the phyla of marine invertebrates discussed in class and write one paragraph about the similarities and differences between its digestive and respiratory systems and those of humans. Then have students write down one thing that surprised them and one question that they still have about invertebrate physiology.

At the Academy


  1. Make copies of the invertebrate scavenger hunt worksheet.


  1. Allow students enough time to complete the scavenger hunt.
  2. Remind them to fill out all parts of the worksheet and to use what they learned in class before the visit to help them answer the questions.

Back at School


  1. Review student discoveries from the scavenger hunt.
  2. Have each student choose one specific invertebrate species from their scavenger hunt list to research in further detail.
  3. In class or as a homework assignment, have students use the resources listed in the “resources” section below as well as other websites and books to research their chosen invertebrate. From the list below, chose a group of topics to have students research.
    • Habitat
    • Physical features
    • Reproduction
    • Respiratory system
    • Digestive system
    • Conservation status
  4. Have students create posters that showcase their invertebrate and give short presentations to explain their posters.


  • Colin, P.L. and Arneson, C. (1995). Tropical Pacific invertebrates: A field guide to the marine invertebrates occurring on tropical Pacific coral reefs, seagrass beds and mangroves. Beverly Hills: Coral Reef Press.
  • Commonwealth Scientific and Industrial Research Organization. Insects and their Allies. Retrieved March 11, 2008 from http://www.ento.csiro.au/education/what_invertebrates.html.
  • Fretter, V. and Graham, A. (1976). A functional anatomy of invertebrates. London: Academic Press.
  • University of Michigan Museum of Zoology. Animal Diversity Web. Retrieved November 30, 2007 from http://animaldiversity.ummz.umich.edu/site/index.html.


California Content Standards

Grades Nine through Twelve

Biology/Life Sciences: Physiology

  • 9a. Students know the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide.



Although many people think of animals as only those that have backbones such as fish, birds, reptiles, and mammals, over 95% of the world’s animals are invertebrates, which lack backbones. Invertebrates were the first animals on the planet, with fossil evidence showing their existence for at least 600 million years. Vertebrates evolved from these animals. Out of the approximately 35 recognized animal phyla, only one, the phylum Chordata, contains vertebrate animals. Even within this phylum, in which human beings are categorized, there are animals, such as tunicates, that are invertebrates as adults. Invertebrates are thus included in every animal phylum and although they all share the characteristic of not having a backbone, there is tremendous invertebrate diversity. They range from single-celled animals to highly intelligent organisms such as octopuses. For the purpose of this activity, we will focus on four invertebrate phyla that can be seen at the California Academy of Sciences: Cnidaria, Mollusca, Echinodermata, and Arthropoda.

As animals, all of these creatures need to obtain food and oxygen and dispel waste materials. Different animals have different adaptations that allow them to accomplish these tasks. Human beings, like all other mammals, have lungs that serve to move oxygen from the air into the blood and to expel carbon dioxide from the body. Human beings and other mammals have digestive systems with one way or complete digestive tracts, taking in food through one opening and eliminating waste through another. Some invertebrates have respiratory and digestive systems that function similarly to those of mammals. For example, spiders have book lungs, organs that enable them to take oxygen from the air. They also have complete digestive tracts with both a mouth and an anus. Other invertebrates, like some of the marine animals discussed below, have respiratory and digestive systems that are quite different from those of humans.

The phylum Cnidaria includes jellyfish, corals, sea anemones, and hydroids. Cnidarians, the majority of which are marine, obtain their food by using stinging structures in their tentacles called nematocysts to capture prey. They then use their tentacles to move the prey into the mouth. The food passes into a hollow, central cavity where it is digested by enzymes and absorbed into the body tissue. The waste from the food is expelled back through the mouth. Cnidarians have what is called an incomplete digestive tract with only one opening to both take in food and expel waste. Cnidarians do not have a specialized respiratory system. Instead, they are able to absorb oxygen from the water around them directly through their epidermis and expel carbon dioxide the same way.

The phylum Echinodermata is comprised solely of marine species and includes seastars, brittle stars, sea cucumbers, and sea urchins. The respiratory and digestive systems of echinoderms are diverse, but all echinoderms have a water vascular system with fluid-filled tube feet, which serve a variety of functions including adhesion, locomotion, feeding, and respiration. In most echinoderms, much of the respiration takes place across these tube feet, which have very thin walls. But this can be supplemented with other methods of gas exchange. Echinoderms feed in a variety of ways including filter feeding, herbivory, and active predation. Most echinoderms have a complete digestive system with two openings, but some echinoderms lack a second opening.

The phylum Arthropoda includes crustaceans, arachnids, centipedes, millipedes, and insects. The subphylum Crustacea is composed primarily of marine invertebrates, which respire with gills that obtain oxygen from the water around them. Crustaceans use many feeding strategies including filter feeding, scavenging, and hunting. They all have a complete digestive tract with two openings.

The phylum Mollusca is extremely diverse and includes clams, oysters, scallops, snails, squids, and octopuses. Despite the diversity, mollusks have a basic body form that includes a head with sensory organs, a large soft mass, and a muscular foot. Most mollusks that live in water have gills to obtain oxygen from the water. Mollusks capture food in a variety of ways including filter feeding like clams, scraping algae off rocks like limpets, and stunning prey with poison like cone snails. Like arthropods, mollusks have a complete digestive system with two openings.


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