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  • Invertebrates What You’ll Learn Chapter 25

    What is an animal?

    Chapter 26 Sponges, Cnidarians, Flatworms, and Roundworms

    Chapter 27 Mollusks and Segmented Worms

    Chapter 28 Arthropods

    Chapter 29 Echinoderms and Invertebrate Chordates

    Unit 8 Review BioDigest & Standardized Test Practice

    Why It’s Important About 95 percent of all animals are invertebrates— animals without backbones. These animals exhibit variations, tolerances, and adaptations to nearly all of Earth’s biomes. Understanding how these organisms develop and function helps humans to better understand themselves.

    350 B.C. Aristotle classifies all known animals into eight groups.

    1564 William Shakespeare is born.

    Understanding the Photo This reef was built over many centuries as corals completed their life cycles. Today, it is home to a great diversity of organisms. The corals, crinoids, and sponges shown here are three types of the countless invertebrate animals on Earth.

    1551 The first of five volumes titled Historia Animalium is published—the beginning of the science of zoology.

    1669 First description of invertebrate anatomy is published in Malpighi’s Silkworms.


    1452–1455 Gutenberg prints about 180 copies of the Bible.

    Elephant from Historia Animalium

    ca.bdol.glencoe.com/webquest (tl)Konrad Gessner, (tr)Hulton/Archive, (crossover)Franklin J. Viola/Earth Scenes

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  • 1899 A scientist raises unfertilized sea urchin eggs to maturity by altering their environment.

    1977 New species of giant clams, marine worms, and other organisms are dis- covered living around deep-sea vents near the Galápagos Islands.

    1997 A new species of marine worms is found living 450 m deep in the Gulf of Mexico.

    1822 The first book in which vertebrate and invertebrate animals are dis- tinguished is published.

    1925 The quick-freeze machine is invented—the beginning of the frozen food industry.

    1711 Corals are reclassified as animals instead of plants.

    1769 Patent for the steam engine issued.

    Marine worms


    D. Foster, Woods Hole Oceanographic Institution/Visuals Unlimited

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  • 672

    What You’ll Learn ■ You will identify animal char-

    acteristics and distinguish them from those of other life forms.

    ■ You will identify cell differen- tiation in the developmental stages of animals.

    ■ You will identify and interpret body plans of animals.

    Why It’s Important The animal kingdom includes diverse organisms, such as sponges, earthworms, clams, crickets, birds, and humans. An understanding of other animals will provide a better under- standing of ourselves.

    What is an animal?What is an animal?

    Although they are different in appearance, these fishes and this jellyfish have common character- istics. They are multicellular organisms whose cells do not have cell walls. They also repro- duce, respond, and must take in energy in the form of food. Scientists classify organisms with these characteristics as animals.

    Understanding the Photo

    Visit to • study the entire chapter online • access Web Links for more

    information and activities on animals

    • review content with the Interactive Tutor and self- check quizzes


    Fred Bavendam/Minden Pictures

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  • 25.1 SECTION PREVIEW Objectives Identify the characteris- tics of animals. Identify cell differentia- tion in the development of a typical animal. Sequence the develop- ment of a typical animal.

    Review Vocabulary autotroph: an organism

    that uses light energy or energy stored in chemi- cal compounds to make energy-rich compounds (p. 46)

    New Vocabulary sessile blastula gastrula ectoderm endoderm mesoderm protostome deuterostome

    Fold a sheet of paper in half lengthwise twice.

    Animals Make the following Foldable to help you under- stand what characteristics are common to all animals.

    Fold down 2.5 cm of paper from the top. (Hint: From the tip of your index finger to your middle knuckle is about 2.5 cm.)

    STEP 1 STEP 2

    Anima l


    Animal 2

    Anima l


    Animal 4 Open and draw

    lines along all folds. Label the columns with the names of four different types of animals.

    STEP 3

    Identifying Before reading Chapter 25, identify characteristics of each animal and list them in the corresponding column. After reading about the characteristics of animals, add any missing characteristics to your lists.

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    Characteristics of Animals All animals have several characteristics in common. Animals are

    eukaryotic, multicellular organisms with ways of moving that help them reproduce, obtain food, and protect themselves. Most animals have spe- cialized cells that form tissues and organs—such as nerves and muscles. Unlike plants, animals are composed of cells that do not have cell walls.

    Animals obtain food Examine the animals shown in Figure 25.1. One characteristic

    common to all animals is that they are heterotrophic, meaning they must consume food to obtain energy and nutrients. All ani- mals depend either directly or indirectly on autotrophs for food.


    Typical Animal Characteristics

    Barnacles filter small organisms out of the water.


    A lizard consumes insects.


    Figure 25.1 Animals consume other organisms.

    0673-0679 C25S1 BDOL-829900 8/4/04 12:24 PM Page 673

  • Scientists hypothesize that animals first evolved in water. Water is denser and contains less oxygen than air, but water usually contains more food. In water, some animals, such as barnacles and oysters, do not move from place to place and have adaptations that allow them to capture food from their water environment. Organisms that are permanently attached to a surface are called sessile (SE sul). They don’t expend much energy to obtain food.

    Some aquatic animals, such as the corals shown in Figure 25.2A, and sponges move about only during the early stages of their lives. They hatch from fertilized eggs into free- swimming larval forms. Most adults are sessile and attach themselves to rocks or other objects.

    There is little suspended food in the air. Land animals use more oxy- gen and expend more energy to find food. The sidewinder snake and osprey shown in Figure 25.2B and C, can move about in their environment in an active search for food.

    Animals digest food Animals are heterotrophs that ingest

    their food; after ingestion, they must digest it. In some animals, digestion is carried out within individual cells; in other animals, digestion takes place in an internal cavity. Some of the food

    Marine Biologist

    Would you enjoy spendingyour days studying the organisms found in the oceans? Perhaps you should become a marine biologist.

    Skills for the Job Many marine biologists go

    SCUBA diving in the oceans to find specimens, but they also spend time examining those organisms in labs and doing library research. They focus on topics such as the effects of temperature changes and pollution on ocean inhabitants. Many marine biologists work for government agencies, such as the National Oceanic and Atmospheric Admini- stration (NOAA), and the Environmental Protection Agency (EPA). Some work for private industries, such as fisheries and environmental consulting firms. Other marine biolo- gists teach and/or do research at colleges and universities. Most marine biologists have a master’s degree or a doctor- ate, plus skill in analyzing data and solving problems.

    For more careers in related fields, visit


    Figure 25.2 Animals capture food in a variety of ways.


    A sidewinder rattlesnake barely touches the ground as it follows the trail of its prey.


    Corals capture their food from the water as it moves over them. Infer What types of organisms might be part of a coral’s diet?


    The osprey can dive and snatch a fish from the waters of a lake or stream.


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  • Extended pharynx

    Digestive tract



    that an animal consumes and digests is stored as fat or glycogen, a polysac- charide, and used when other food is not available.

    Examine the digestive tracts of a flatworm and an earthworm in Figure 25.3. Notice that there is only one opening to the flatworm’s digestiv