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17.1: Overview of Animals

Difficulty Level: At Grade Created by: CK-12
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Key Concept

Animals are multicellular eukaryotes that lack cell walls. All animals are heterotrophs, and they reproduce sexually. They also have sensory organs, the ability to move, and internal digestion. Except for the chordates, all animal phyla consist only of invertebrates, or animals without a backbone. Chordates include both vertebrates and invertebrates. The earliest animals evolved from protists more than 600 million years ago. The first animals to live on land were invertebrates. Amphibians were the first vertebrates to live on land, and amniotes were the first animals that could reproduce on land.


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

Lesson Objectives

  • Identify characteristics that all animals share.
  • Give an overview of animal classification.
  • Outline major trends in animal evolution.

Lesson Vocabulary

  • amniote: animal that produces eggs with internal membranes that allow gases but not water to pass through, so the embryo can breathe without drying out (reptile, bird, or mammal)
  • animal: heterotrophic, multicellular eukaryote with cells that lack cell walls; member of the animal kingdom
  • exoskeleton: non-bony skeleton that forms on the outside of the body of some invertebrates and provides protection and support
  • invertebrate: animal that lacks a vertebral column, or backbone
  • notochord: stiff support rod that runs from one end of the body to the other in animals called chordates
  • vertebral column: bony support structure that runs down the back of a vertebrate animal; also called a backbone
  • vertebrate: animal with a vertebral column, or backbone

Teaching Strategies

Introducing the Lesson

Call on students to list familiar animals on the board. Lead the class in brainstorming characteristics the listed animals share. Tell students they will read in this lesson about characteristics that define the animal kingdom.


Direct students to the URL below to interact with the model animal cell. The activity will allow them to review the structures and functions of animal cell organelles. In the Cell Gallery at the same Web site, students can get a virtual microscopic view of different types of animal cells.

Differentiated Instruction

Helpless proficient readers focus their reading by having them make a KWL chart about animals. Before students read the lesson, they should list what they already know about animals as well as what they want to know. After they read the lesson, they should list what they learned. LPR


Ask a group of students who need extra challenges to create a Web site about one or more of the major trends in animal evolution (e.g., body symmetry, complete digestive system, segmented body). Their Web site should illustrate the trends with specific examples and also address how and why animals evolved in these ways. Encourage other students to visit the Web site to learn more about animal evolution.

Science Inquiry

Have students relate the traits of lobe-finned fish to the evolution of amphibians. They can compare a lobe-finned fish and early amphibian (see below and in FlexBook® textbook).

  • Ask: What abilities of lobe-finned fish allowed them to spend brief periods of time on land? (They could breathe air for brief periods and use their fins to walk on land for short distances.)

Common Misconceptions

Students commonly have the misconception that most animals are vertebrates. Be sure to point out that vertebrates make up only 5 percent of animal species. Suggest that students go online to find a complete listing of known invertebrate species and images of a diversity 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

Vertebrates are the animals with which we are most familiar. But there are far more invertebrates than vertebrates on the planet. The next lesson provides an overview of invertebrate animals.

  • Before reading the next lesson, think about what you now know about invertebrates. Can you identify some invertebrate traits?
  • Invertebrates are sometimes referred to as “lower” animals. This is because they evolved earlier and are simpler than vertebrates. Do you think invertebrates are also less adapted to their environments than vertebrates? Why or why not?

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