} CAS: Teachers - Marine Invertebrate Anatomy

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Connected Experience: Marine Invertebrate Anatomy


In this lesson, students will learn about invertebrate diversity and compare marine invertebrates’ anatomy with those of humans.


In this lesson, students will:

  1. learn about invertebrate diversity.
  2. compare marine invertebrates’ anatomy with those of humans.


  • invertebrate trivia cards
  • pictures of invertebrates
  • invertebrate scavenger hunt worksheets
  • poster boards


  • arthropod /[ahr-thruh-pod]: an invertebrate animal with jointed legs, segmented body and an exoskeleton, when growing it molts (sheds old skeleton); includes the insects, crustaceans and arachnids
  • cnidarians [nahy-dair-ee-uhn]: an invertebrate animal characterized by a radially symmetrical body with a saclike internal cavity, and including the jellyfishes, hydras, sea anemones, and corals
  • echinoderm [ih-kahy-nuh-durm]: radially symmetrical marine invertebrate, which includes the starfishes, sea urchins, and sea cucumbers, having an internal calcareous skeleton and often covered with spines
  • invertebrate: an animal that lacks a backbone or spinal column
  • mollusk [mol-uhsk]: a legless invertebrate animal, typically with a soft body, a mantle, and a protective calcareous shell
  • nematocyst [ni-mat-uh-sist]: a type of stinging cell found in the tentacles of cnidarians
  • phyla [fahy-luh]: the primary subdivision of the animal kingdom that groups together organisms with the same body plan



Begin by reviewing human body parts and functions. Remind 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 marine invertebrates.


  1. Let the students know that they will be going to the California Academy of Sciences where scientists study animals. When they go, they will have a fun ocean scavenger hunt.
  2. Break up your class into small groups. Give each group a copy of the ID cards and have them sort the animals into groups that they think are related to one another. You can tell them that these animals are usually split up into 4 separate groups.
  3. On the board create a table similar to the one below. Then begin to give clues as to which animals fit in specific categories. As you give clues they can rearrange their guesses if needs be. Encourage students to take notes during this exercise.
  4. Have students help you fill in the information comparing the anatomy of the four phyla of invertebrates. Have them fill in the left column on their scavenger hunt.

Invertebrate Table

At the Academy

  1. The scavenger hunt can be completed in many parts of the aquarium exhibits, especially in the California Coast tanks and the Coral Reef tanks. Questions may be answered by a docent (identified by orange lab coats), in the Naturalist Center on Level 3, which is open from 11am to 4pm on weekdays (and 10 to 5 on weekends), or by an aquarium staff member/ diver.
  2. Allow students at least 45 minutes to complete the scavenger hunt.
  3. Remind your students 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. Have students choose one of the marine invertebrates they found on their scavenger hunt and write a paragraph about the similarities and differences between its body parts and those of humans. Then have students write down one thing that surprised them and one question that they still have about invertebrate anatomy.
  2. You may have the students read the trivia cards to help the students learn more about marine invertebrates. You can use the information in the teacher background to explain any pertinent information to the students.
  3. In class or as a homework assignment, have students research answers to their questions using the resources listed on the following page. If students have a hard time coming up with a question, choose a topic from the list below. 
    • Habitat
    • Special Adaptations
    • Physical features
    • Diet
    • Predators
    • 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

Grade 3

Life Science

  • 3b. Students know examples of diverse life forms in different environments, such as oceans, deserts, tundra, forests, grasslands, and wetlands.

Grade 5

Investigation and Experimentation

  • 6a. Classify objects (e.g., rocks, plants, leaves) in accordance with appropriate criteria.



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. 

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 skin 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.

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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