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17.1: Overview of Animals

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

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


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)
heterotrophic, multicellular eukaryote with cells that lack cell walls; member of the animal kingdom
non-bony skeleton that forms on the outside of the body of some invertebrates and provides protection and support
animal that lacks a vertebral column, or backbone
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
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. The fact that all animals have certain traits in common shows that they share a common ancestor. How did such a diverse group of organisms evolve? 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?

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
Porifera sponges
Cnidaria jellyfish, corals
Platyhelminthes flatworms, tapeworms, flukes
Nematoda roundworms
Mollusca snails, clams, squids
Annelida earthworms, leeches, marine worms
Arthropoda insects, spiders, crustaceans, centipedes
Echinodermata sea stars, sea urchins, sand dollars, sea cucumbers
Chordata tunicates, lancelets, fish, amphibians, reptiles, birds, mammals

Invertebrate vs. Vertebrate

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.

Major Trends in Animal Evolution

The oldest animal fossils are about 630 million years old. By 500 million years ago, most modern phyla of animals had evolved. Figure below shows when some of the major events in animal evolution took place.

Partial Geologic Time Scale. This portion of the geologic time scale shows major events in animal evolution.

Animal Origins

Who were the ancestors of the earliest animals? They may have been marine protists that lived in colonies. Scientists think that cells of some protist colonies became specialized for different jobs. After a while, the specialized cells came to need each other for survival. Thus, the first multicellular animal evolved. Look at the cells in Figure below. One type of sponge cell, the choanocyte, looks a lot like the protist cell. How does this support the hypothesis that animals evolved from protists?

Choanoflagellate Protist and Choanocyte Cells in Sponges. Sponge choanocytes look a lot like choanoflagellate protists.

Evolution of Invertebrates

Many important animal adaptations evolved in invertebrates. Without these adaptations, vertebrates would not have been able to evolve. They include:

  • Tissues, organs, and organ systems.
  • A symmetrical body.
  • A brain and sensory organs.
  • A fluid-filled body cavity.
  • A complete digestive system.
  • A body divided into segments.

You can read about all of these adaptations in the next lesson.

Moving from Water to Land

When you think of the first animals to colonize the land, you may think of amphibians. It’s true that ancestors of amphibians were the first vertebrates to move to land. However, the very first animals to go ashore were invertebrates, most likely arthropods.

The move to land required new adaptations. For example, animals needed a way to keep their body from drying out. They also needed a way to support their body on dry land without the buoyancy of water. One way early arthropods solved these problems was by evolving an exoskeleton. This is a non-bony skeleton that forms on the outside of the body. It supports the body and helps retain water. The video Walking with Monsters is a depiction of the evolution of life from water onto land: http://www.youtube.com/watch?v=gytrNU3iwvM (4:43).

Evolution of Chordates

Another major step in animal evolution was the evolution of a notochord. A notochord is a rigid rod that runs the length of the body. It supports the body and gives it shape (see Figure below). It also provides a place for muscles to anchor, and counterbalances them when they contract. Animals with a notochord are called chordates. They also have a hollow nerve cord that runs along the top of the body. Gill slits and a tail are two other chordate features. Many modern chordates have some of these structures only as embryos.

Primitive Chordate: Tunicate. This tunicate is a primitive, deep-sea chordate. It is using its notochord to support its head, while it waits to snatch up prey in its big mouth.

Evolution of Vertebrates

Vertebrates evolved from primitive chordates. This occurred about 550 million years ago. The earliest vertebrates may have been jawless fish, like the hagfish in Figure below. Vertebrates evolved a backbone to replace the notochord after the embryo stage. They also evolved a cranium, or bony skull, to enclose and protect the brain.

Primitive Vertebrate: Hagfish. Hagfish are very simple vertebrates.

As early vertebrates evolved, they became more complex. Around 365 million years ago, they finally made the transition from water to land. The first vertebrates to live on land were amphibians. They evolved from lobe-finned fish. You can compare a lobe-finned fish and an amphibian in Figure below.

From Lobe-Finned Fish to Early Amphibian. Lobe-finned fish evolved into the earliest amphibians. A lobe-finned fish could breathe air for brief periods of time. It could also use its fins to walk on land for short distances. What similarities do you see between the lobe-finned fish and the amphibian?

Evolution of Amniotes

Amphibians were the first animals to have true lungs and limbs for life on land. However, they still had to return to water to reproduce. That’s because their eggs lacked a waterproof covering and would dry out on land. The first fully terrestrial vertebrates were amniotes. Amniotes are animals that produce eggs with internal membranes. The membranes let gases but not water pass through. Therefore, in an amniotic egg, an embryo can breathe without drying out. Amniotic eggs were the first eggs that could be laid on land. The earliest amniotes evolved about 350 million years ago. They may have looked like the animal in Figure below. Within a few million years, two important amniote groups evolved: synapsids and sauropsids. Synapsids evolved into mammals. The sauropsids gave rise to reptiles, dinosaurs, and birds.

Early Amniote. The earliest amniotes probably looked something like this. They were reptile-like, but not actually reptiles. Reptiles evolved somewhat later.

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.
  • The earliest animals evolved from colonial protists more than 600 million years ago. Many important animal adaptations evolved in invertebrates, including tissues and a brain. The first animals to live on land were invertebrates. Amphibians were the first vertebrates to live on land. Amniotes were the first animals that could reproduce on land.

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.

5. List three traits that evolved in invertebrate animals.

Apply Concepts

6. 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

7. Compare and contrast invertebrates and vertebrates.

8. Relate similarities between choanoflagellates and choanocytes to animal origins.

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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