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17.2: Overview of Invertebrates

Difficulty Level: At Grade Created by: CK-12

Key Concept

The majority of living animals are invertebrates. Most invertebrates reproduce sexually, and many have one or more larval stages. Several important animal traits evolved in invertebrates, including multicellularity, tissues and organs, radial and bilateral symmetry, cephalization, mesoderm, complete digestive system, coelom, and segmented body. Most invertebrates belong to one of eight major phyla.


  • CA.9–12. IE.1.d
  • NSES.9–12.A.1.6; NSES.9–12.C.5.4; NSES.9–12.C.6.1
  • McREL.9–, 6

Lesson Objectives

  • Describe general characteristics of invertebrates.
  • Outline major events in invertebrate evolution.
  • Give an overview of invertebrate classification.

Lesson Vocabulary

  • bilateral symmetry: symmetry of a body plan in which there are distinct head and tail ends, so the body can be divided into two identical right and left halves
  • cephalization: concentration of nerve tissue in one end of an animal, forming a head region
  • coelom: fluid-filled body cavity
  • complete digestive system: digestive system consisting of a digestive tract and two body openings (mouth and anus)
  • ectoderm: outer embryonic cell layer in animals
  • endoderm: inner embryonic cell layer in animals
  • hydrostatic skeleton: type of internal support in an animal body that results from the pressure of fluid within the body cavity known as the coelom
  • incomplete digestive system: digestive system that consists of a digestive cavity and a single opening that serves as both mouth and anus
  • larva: (plural, larvae) juvenile stage that occurs in the life cycle of many invertebrates, fish, and amphibians and that differs in form and function from the adult stage
  • mesoderm: embryonic cell layer in many animals that is located between the endoderm (inner cell layer) and ectoderm (outer cell layer)
  • pseudocoelom: partial, fluid-filled cavity inside the body of some invertebrates
  • radial symmetry: symmetry of a body plan in which there is a distinct top and bottom but not distinct head and tail ends, so the body can be divided into two halves like a pie
  • segmentation: division of an animal body into multiple segments

Teaching Strategies

Introducing the Lesson

Ask students to recall from the previous lesson important trends in animal evolution (e.g., evolution of cephalization, true tissues, symmetry). Tell students that these traits evolved in invertebrates, which they will read about in this lesson.

Building Science Skills

Students can build basic science skills by doing the flatworm activity at the URL below. It will lead them through several measurements and observations of living freshwater planaria. (You can obtain low-cost live planaria for the activity at www.wardsci.com.)

Differentiated Instruction

Work with students to make a word wall for lesson vocabulary terms they will need for later chapters (e.g., larva, ectoderm, endoderm, mesoderm, symmetry, segmentation). Each term should be defined and illustrated with an image. Refer students to the word wall whenever they encounter these terms in subsequent chapters.


Challenge one or more advanced students to bring to class objects that demonstrate asymmetry (e.g., natural sponge), radial symmetry (e.g., wheel), and bilateral symmetry (e.g., doll). Ask the students to use the objects to teach the rest of the class about symmetry in animals.

Science Inquiry

Have students use a dichotomous key to classify invertebrate phyla. See the URL below as an example.

Common Misconceptions

If students hold misconceptions about the importance, variety, or intriguing nature of invertebrates, suggest that they read the engaging article How I Learned to Love Spineless Wonders at the URL below. They may also want to read the author’s book-length work of the same title. It explores the fascinating and sometimes bizarre world of invertebrates.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.

Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.

Points to Consider

This chapter presents an overview of invertebrate phyla. The next chapter describes invertebrate phyla in greater detail.

  • What questions do you have about invertebrate phyla now? For example, do you wonder where organisms in the different phyla live or what they eat?
  • Invertebrates evolved hundreds of millions of years ago. Which invertebrate phylum do you think has the greatest number of species today?

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