17.1 Overview of Animals

Lesson Objectives

  • Identify characteristics that all animals share.
  • Give an overview of animal classification.


Chapter 17.1 workbook pages

Get the workbook here: https://guesthollow.com/store/free-high-school-biology-workbook/


  • 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


There is great variation among species that make up the animal kingdom. Some of this variation is shown in Figure below. Despite the variation, there are a number of traits that are shared by all animals.  What traits do all animals share? Read on to find out.

Diversity of Animals. These photos give just an inkling of the diversity of organisms that belong to the animal kingdom. (A) Sponge (B) Flatworm (C) Flying Insect (D) Frog (E) Tiger (F) Gorilla.

Characteristics of Animals

Animals are a kingdom of multicellular eukaryotes. They cannot make their own food. Instead, they get nutrients by eating other living things. Therefore, animals are heterotrophs.

Animal Cells

Like the cells of all eukaryotes, animal cells have a nucleus and other membrane-bound organelles (see Figure below). Unlike the cells of plants and fungi, animal cells lack a cell wall. This gives animal cells flexibility. It lets them take on different shapes so they can become specialized to do particular jobs. The human nerve cell shown in Figure below is a good example. Its shape suits it for its function of transmitting nerve impulses over long distances. A nerve cell would be unable to take this shape if it were surrounded by a rigid cell wall.

Animal Cell. The shape of an animal cell is not constrained by a rigid cell wall. A bacterial cell is shown above for comparison.
Human Nerve Cell. A human nerve cell is specialized to transmit nerve impulses. How do you think the cell’s shape helps it perform this function?

Crash Course Biology: Eukaryopolis – Animal Cells –
Note: There is a brief mention of evolution in this video.

Animal Structure and Function

Animals not only have specialized cells. Most animals also have tissues and organs. In many animals, organs form organ systems, such as a nervous system. Higher levels of organization allow animals to perform many complex functions. What can animals do that most other living things cannot? Here are some examples. All of them are illustrated in Figure below.

  • Animals can detect environmental stimuli, such as light, sound, and touch. Stimuli are detected by sensory nerve cells. The information is transmitted and processed by the nervous system. The nervous system, in turn, may direct the body to respond.
  • All animals can move, at least during some stage of their life cycle. Muscles and nerves work together to allow movement. Being able to move lets animals actively search for food and mates. It also helps them escape from predators.
  • Virtually all animals have internal digestion of food. Animals consume other organisms and may use special tissues and organs to digest them. (Many other organisms absorb nutrients directly from the environment.)
Characteristics of Animals. Most animals share these characteristics: sensory organs, movement, and internal digestion.

Animal Life Cycle and Reproduction

Many animals have a relatively simple life cycle. A general animal life cycle is shown in Figure below. Most animals spend the majority of their life as diploid organisms. Just about all animals reproduce sexually. Diploid adults undergo meiosis to produce sperm or eggs. Fertilization occurs when a sperm and an egg fuse. The zygote that forms develops into an embryo. The embryo eventually develops into an adult.

Animal Life Cycle. An animal life cycle that includes only sexual reproduction is shown here. Some animals also reproduce asexually. How does the animal life cycle compare with the life cycle of a plant?

Classification of Animals

All animals share basic traits. But animals also show a lot of diversity. They range from simple sponges to complex humans.

Major Animal Phyla

Members of the animal kingdom are divided into more than 30 phyla. Table below lists the 9 phyla with the greatest number of species. Each of the animal phyla listed in the table have at least 10,000 species.

Phylum Animals It Includes
17.1h Porifera sponges
17.1i Cnidaria jellyfish, corals
17.1j Platyhelminthes flatworms, tapeworms, flukes
17.1k Nematoda roundworms
17.1l Mollusca snails, clams, squids
17.1m Annelida earthworms, leeches, marine worms
17.1n Arthropoda insects, spiders, crustaceans, centipedes
17.1o Echinodermata sea stars, sea urchins, sand dollars, sea cucumbers
17.1p Chordata tunicates, lancelets, fish, amphibians, reptiles, birds, mammals

Here are a couple videos of the types of animals mentioned above:

TED Ed: Why are there so many insects?

TED Ed: Why the octopus brain is so extraordinary

The first eight phyla listed in Table above include only invertebrate animals. Invertebrates are animals that lack a vertebral column, or backbone. The last phylum in the table, the Chordata, also includes many invertebrate species. Tunicates and lancelets are both invertebrates. Altogether, invertebrates make up at least 95 percent of all animal species. The remaining animals are vertebrates.Vertebrates are animals that have a backbone. All vertebrates belong to the phylum Chordata. They include fish, amphibians, reptiles, birds, and mammals.

Lesson Summary

  • Animals are multicellular eukaryotes that lack cell walls. All animals are heterotrophs. They have sensory organs, the ability to move, and internal digestion. They also have sexual reproduction.
  • Vertebrates have a backbone, but invertebrates do not. Except for the chordates, all animal phyla consist only of invertebrates. Chordates include both vertebrates and invertebrates.

Review Questions


1. Identify traits that characterize all animals.

2. State one way that animal cells differ from the cells of plants and fungi. What is the significance of this difference?

3. Describe a general animal life cycle.

4. State how the phylum Chordata differs from other animal phyla.

Apply Concepts

5. Assume that a new species of animal has been discovered. It is an egg-laying animal that lives and reproduces on land. Explain what you know about its eggs without ever seeing them.

Think Critically

6. Compare and contrast invertebrates and vertebrates.

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?

Previous: Plant Features and Responses

Next: Overview of Invertebrates

2 thoughts on “17.1 Overview of Animals

  1. This is so helpful! So far this is my favorite lesson in this biology curriculum. I loved all the videos used here. Thank you so much!

    1. We’re so glad you enjoyed the lesson and the videos! 😀 Thanks for taking the time to post your feedback!!

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