InvertebratesWhat You’ll LearnChapter 25
What is an animal?
Chapter 26Sponges, Cnidarians, Flatworms, and Roundworms
Chapter 27Mollusks and Segmented Worms
Chapter 28Arthropods
Chapter 29Echinoderms and Invertebrate Chordates
Unit 8 ReviewBioDigest & Standardized Test Practice
Why It’s ImportantAbout 95 percent of all animals are invertebrates—animals without backbones. These animals exhibit variations, tolerances, and adaptations to nearly all ofEarth’s biomes. Understanding how these organismsdevelop and function helps humans to better understand themselves.
350 B.C.Aristotle classifies all knownanimals into eight groups.
1564WilliamShakespeareis born.
Understanding the PhotoThis reef was built over many centuries as coralscompleted their life cycles. Today, it is home to agreat diversity of organisms. The corals, crinoids, andsponges shown here are three types of the countlessinvertebrate animals on Earth.
1551The first of five volumestitled Historia Animalium ispublished—the beginningof the science of zoology.
1669First description ofinvertebrate anatomy ispublished in Malpighi’sSilkworms.
670
1452–1455Gutenberg printsabout 180 copiesof the Bible.
Elephant fromHistoriaAnimalium
ca.bdol.glencoe.com/webquest (tl)Konrad Gessner, (tr)Hulton/Archive, (crossover)Franklin J. Viola/Earth Scenes
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1899A scientist raises unfertilized seaurchin eggs to maturity by alteringtheir environment.
1977New species of giant clams, marineworms, and other organisms are dis-covered living around deep-seavents near the Galápagos Islands.
1997A new species ofmarine worms isfound living 450 mdeep in the Gulf of Mexico.
1822The first book inwhich vertebrateand invertebrateanimals are dis-tinguished ispublished.
1925The quick-freeze machine isinvented—the beginning of thefrozen food industry.
1711Corals arereclassified asanimals insteadof plants.
1769Patent for the steamengine issued.
Marine worms
671
D. Foster, Woods Hole Oceanographic Institution/Visuals Unlimited
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672
What You’ll Learn■ You will identify animal char-
acteristics and distinguishthem from those of other lifeforms.
■ You will identify cell differen-tiation in the developmentalstages of animals.
■ You will identify and interpretbody plans of animals.
Why It’s ImportantThe animal kingdom includesdiverse organisms, such assponges, earthworms, clams,crickets, birds, and humans. Anunderstanding of other animalswill provide a better under-standing of ourselves.
What is an animal?What is an animal?
Although they are different inappearance, these fishes and thisjellyfish have common character-istics. They are multicellularorganisms whose cells do nothave cell walls. They also repro-duce, respond, and must take inenergy in the form of food.Scientists classify organisms withthese characteristics as animals.
Understandingthe Photo
Visit to• study the entire chapter online• access Web Links for more
information and activities onanimals
• review content with theInteractive Tutor and self-check quizzes
ca.bdol.glencoe.com
Fred Bavendam/Minden Pictures
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25.1SECTION PREVIEWObjectivesIdentify the characteris-tics of animals.Identify cell differentia-tion in the development ofa typical animal.Sequence the develop-ment of a typical animal.
Review Vocabularyautotroph: an organism
that uses light energy orenergy stored in chemi-cal compounds to makeenergy-rich compounds (p. 46)
New Vocabularysessileblastulagastrulaectodermendodermmesodermprotostomedeuterostome
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
Animal
1
Animal2
Animal
3
Animal4 Open and draw
lines along all folds. Label the columns with thenames of four differenttypes 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 AnimalsAll animals have several characteristics in common. Animals are
eukaryotic, multicellular organisms with ways of moving that help themreproduce, 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 foodExamine the animals shown in Figure 25.1. One characteristic
common to all animals is that they are heterotrophic, meaningthey must consume food to obtain energy and nutrients. All ani-mals depend either directly or indirectly on autotrophs for food.
673
Typical AnimalCharacteristics
Barnacles filter smallorganisms out of the water.
A
A lizard consumesinsects.
B
Figure 25.1Animals consume otherorganisms.
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Scientists hypothesize that animalsfirst evolved in water. Water is denserand contains less oxygen than air, butwater usually contains more food. Inwater, some animals, such as barnaclesand oysters, do not move from placeto place and have adaptations thatallow them to capture food from theirwater environment. Organisms thatare permanently attached to a surfaceare called sessile (SE sul). They don’texpend much energy to obtain food.
Some aquatic animals, such as thecorals shown in Figure 25.2A, andsponges move about only during theearly stages of their lives. Theyhatch from fertilized eggs into free-swimming larval forms. Most adultsare sessile and attach themselves torocks or other objects.
There is little suspended food inthe air. Land animals use more oxy-gen and expend more energy to findfood. The sidewinder snake andosprey shown in Figure 25.2B and C,can move about in their environmentin an active search for food.
Animals digest foodAnimals are heterotrophs that ingest
their food; after ingestion, they mustdigest it. In some animals, digestion is carried out within individual cells;in other animals, digestion takes placein 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 amarine biologist.
Skills for the JobMany marine biologists go
SCUBA diving in the oceans tofind specimens, but they alsospend time examining those organisms in labs and doinglibrary research. They focus on topics such as the effects oftemperature 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 fisheriesand 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
674 WHAT IS AN ANIMAL?
Figure 25.2Animals capture food in a variety of ways.
ca.bdol.glencoe.com/careers
A sidewinder rattlesnakebarely touches theground as it follows the trail of its prey.
B
Corals capture theirfood from the water as it moves overthem. Infer Whattypes of organismsmight be part of acoral’s diet?
A
The osprey candive and snatcha fish from thewaters of a lakeor stream.
C
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Extendedpharynx
Digestivetract
Mouth
25.1 TYPICAL ANIMAL CHARACTERISTICS 675
that an animal consumes and digestsis stored as fat or glycogen, a polysac-charide, and used when other food isnot available.
Examine the digestive tracts of a flatworm and an earthworm inFigure 25.3. Notice that there isonly one opening to the flatworm’sdigestive tract, a pharynx. An earth-worm has a digestive tract with twoopenings, a mouth at one end and ananus at the other.
Animal cell adaptationsMost animal cells are differentiated
and carry out different functions.Animals have specialized cells thatenable them to sense and seek outfood and mates, and allow them toidentify and protect themselves frompredators. Observe the animals in theMiniLab on this page. Can you iden-tify any specialized cells in these ani-mals? Find out about other specializedanimal cells in the Biotechnology at theend of this chapter.
Identify three char-acteristics of the animal kingdom.
(t)John D. Cunningham/Visuals Unlimited, (b)R.J. Erwin/DRK Photo
Figure 25.3In animals such as planariansand earthworms, food isdigested in a digestive tract.
Planarians feed on small, liveorganisms or on the remains oflarger animals. The planarian’sdigestive tract has only one opening,the pharynx, through which foodenters and wastes exit.
A
Earthworms ingest soil and digest theorganic matter contained in it. Soil entersthe mouth and travels along the digestivetract in one direction. Indigestible waste iseliminated at the anus.
B
Data Table
Animal Observed?Characteristic (Yes or No) Evidence
Multicellular
Feeding
Movement
Size in mm
Digestive tract
Anus
Observe and InferObserving Animal Characteristics Animals differ in sizeand shape, and can be found living in different habitats.
ProcedureCAUTION: Use caution when handling a microscope, glass slides, and coverslips.! Copy the data table.@ Add a few bristles from an old toothbrush to a glass
slide. Add a drop of water containing rotifers to your slide. The drop should cover the bristles. Add a coverslip.
# Observe the rotifers under low-power magnification.$ Use the data table to record the characteristics that you
were able to see. Describe the evidence for each trait.
Analysis1. Describe Are rotifers multicellular? Explain.2. Observe Were you able to observe evidence of rotifers
feeding? Explain.3. Infer Are rotifers autotrophs or heterotrophs? Explain.
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Development of Animals
Most animals develop from a fertil-ized egg cell called a zygote. But howdoes a zygote develop into many differ-ent kinds of cells that make up a snail, afish, or a human? After fertilization, the
zygote of different animal species allhave similar, genetically determinedstages of development.
FertilizationMost animals reproduce sexually.
Male animals produce sperm cells andfemale animals produce egg cells.Fertilization occurs when a sperm cellpenetrates the egg cell, forming a newcell called a zygote. In animals, fertil-ization may be internal or external.
Cell divisionThe zygote divides by mitosis and
cell division to form two cells in aprocess called cleavage. Find out howimportant this first cell division is infrog development by studying theProblem-Solving Lab. Once cell divi-sion has begun, the organism isknown as an embryo. Recall that anembryo is an organism at an earlystage of growth and development.The two cells that result from cleav-age then divide to form four cells andso on, until a cell-covered, fluid-filledball called a blastula (BLAS chuh luh)is formed. In some animals, such as alancelet, the blastula is a single layerof cells surrounding a fluid-filledspace. In other animals, such as frogs,there may be several layers of cellssurrounding the space. The blastula isformed early in the development ofan animal embryo. In sea urchindevelopment, for example, the forma-tion of a blastula is complete aboutten hours after fertilization. Inhumans, the blastula forms about fivedays after fertilization.
GastrulationAfter blastula formation, cell divi-
sion continues. The cells on one sideof the blastula then move inward toform a gastrula (GAS truh luh)—astructure made up of two layers ofcells with an opening at one end.
676 WHAT IS AN ANIMAL?
Interpret Scientific DiagramsHow important is thefirst cell division infrog development?The first division some-times results in two cellswith unequal amounts ofcytoplasm. Does this haveany impact on the devel-opment of an organism?It does in frogs.
Solve the ProblemIn a frog cell, a small, specialized area forms inthe cytoplasm just afterfertilization. This area iscalled the gray crescent.Note its appearance inthe diagram. Follow thechanges in developmentas the first division of cytoplasm occurs equally through the gray crescent and unequally through the gray crescent.
Thinking Critically 1. Explain How does each set of diagrams illustrate the role
of the gray crescent in early frog development? 2. Infer Answer the question posed at the beginning of this
problem-solving lab.3. Predict What would happen to a frog’s development if
the first cell division occurred on the horizontal planerather than on the vertical plane?
Graycrescent
1stcleavage
Dies
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Gastrula formation can be comparedto the way a potter creates a cup orbowl from a lump of clay, as shown inFigure 25.4. First, the clay is formedinto a ball. Then, the potter presses inon the top of the ball to form a cavitythat becomes the interior of the bowl.In a similar way, the cells at one endof the blastula move inward, forminga cavity lined with a second layer ofcells. The layer of cells on the outersurface of the gastrula is called theectoderm. The layer of cells lining theinner surface is called the endoderm.The ectoderm cells of the gastrula con-tinue to grow and divide, and eventu-ally they develop into the skin andnervous tissue of the animal. Theendoderm cells develop into the liningof the animal’s digestive tract and intoorgans associated with digestion.
Formation of mesodermIn some animals, the development
of the gastrula progresses until a layerof cells called the mesoderm isformed. Mesoderm is found in themiddle of the embryo; the term mesomeans “middle.” The mesoderm(MEZ uh durm) is the third cell layerfound in the developing embryobetween the ectoderm and the endo-derm. The mesoderm cells developinto the muscles, circulatory system,excretory system, and, in some ani-mals, the respiratory system. Identifyand review cell differentiation in thedevelopment of an animal as shown inFigure 25.5 on the next page.
When the opening in the gastruladevelops into the mouth, the ani-mal is called a protostome (PROH
tuh stohm). Snails, earthworms, andinsects are examples of protostomes.
In other animals, such as sea stars,fishes, toads, snakes, birds, andhumans, the mouth does not developfrom the gastrula’s opening. An ani-mal whose mouth developed not
from the opening, but from cellselsewhere on the gastrula is called adeuterostome (DEW tihr uh stohm).
Scientists hypothesize that proto-stome animals were the first to appearin evolutionary history, and thatdeuterostomes followed at a latertime. Biologists today often classifyan unknown organism by identifyingits phylogeny. Recall that phylogenyis the evolutionary history of anorganism. Determining whether ananimal is a protostome or deuteros-tome can help biologists identify itsgroup. Even though sea urchins, forexample, are invertebrates and fishesare vertebrates, both are deuteros-tomes and are, therefore, moreclosely related than you might con-clude from comparing their adultbody structures.
25.1 TYPICAL ANIMAL CHARACTERISTICS 677
protostome fromthe Greek wordsprotos, meaning“first,” and stoma,meaning “mouth”;deuterostomefrom the Greekwords deutero,meaning “second-ary,” and stoma,meaning “mouth” A protostome and a deuterostome differ in the loca-tion of the cellsthat become theorganism’s mouth.
Matt Meadows
Figure 25.4You can think ofa blastula as acell-covered,fluid-filled ball.By pushing in onone side of theclay ball, thepotter modelsgastrulation.
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678 WHAT IS AN ANIMAL?
Ectoderm Mesoderm
Endoderm
Sperm cells
Egg cell
Fertilization A zygote is formed when an egg cellis fertilized by a sperm cell.
AA
First cell divisionThe zygote divides by mitosis and celldivision to form twocells. From this point,the developingorganism is called anembryo.
BB
Formation of a blastulaContinuous cell divisions result in a cell-covered, fluid-filled ball, the blastula. During these earlydevelopmental stages, the totalamount of cytoplasm has notincreased from the original cell.
DD
Gastrulation As the embryocontinues to grow, some of thecells of the blastula move inward,forming the gastrula. All animalembryos except sponges passthrough this gastrula stage.
EE
Additionalcell divisionsCell divisioncontinues. Theeight-cell stage is shown here.
CC
Formation of mesoderm In protostomes, themesoderm forms from cells that break away fromthe endoderm near the opening of the gastrula. Indeuterostomes, the mesoderm forms from pouches
of endoderm cells on theinside of the gastrula.
After the formationof mesoderm, devel-opment continueswith each celllayer differentiat-ing into special-ized tissues.
FF
LM Magnification: 80�
Sea urchin blastula
Cell Differentiation in AnimalDevelopmentFigure 25.5The fertilized eggs of most animals follow a similar pattern of development.From one fertilized egg cell, many divisions occur until a fluid-filled ball of cellsforms. The ball folds inward and continues to develop. Critical Thinking Howdo cells differentiate as an embryo develops?
Carolina Biological Supply/Visuals Unlimited
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Understanding Main Ideas1. Identify and list the characteristics of a mouse that
make it a member of the animal kingdom.
2. Explain why movement is an important characteristic of animals.
3. Compare and contrast a protostome and adeuterostome.
4. Identify cell differentiation in the development ofan animal.
5. Describe gastrulation.
Thinking Critically6. Name a land animal that is sessile. Why would this
adaptation be a disadvantage to an animal in aland biome?
7. Sequence Make a concept map of animal devel-opment using the following stages, beginningwith the earliest stage: gastrula, larva, adult, fer-tilized egg, blastula. For more help, refer toSequence in the Skill Handbook.
SKILL REVIEWSKILL REVIEW
25.1 TYPICAL ANIMAL CHARACTERISTICS 679
LM Magnification: 1200�
AA
Growth and developmentCells in developing embryos con-
tinue to differentiate and become specialized to perform different func-tions. Most animal embryos continueto develop over time, becoming juve-niles that look like smaller versions of the adult animal. In some animals,such as insects and echinoderms, theembryo develops inside an egg into an intermediate stage called alarva (plural larvae). A larva oftenbears little resemblance to the adultanimal. Inside the egg, the larva issurrounded by a membrane formedright after fertilization. When the egghatches, the larva breaks through thisfertilization membrane. Animals that
are generallysessile as adults,such as sea urchins,often have a free-swimming larvalstage, as shown in Figure 25.6. Youcan observe development in fishes inthe BioLab at the end of this chapter.
Adult animalsOnce the juvenile or larval stage
has passed, most animals continue togrow and develop into adults. Thisgrowth and development may takejust a few days in some insects, or upto fourteen years in some mammals.Eventually the adult animals reachsexual maturity, mate, and the cyclebegins again.
BB
Figure 25.6Free-swimming larvae (A) developfrom fertilized sea urchin eggs in about 48 hours. A larva willdevelop into an adult sea urchin(B) over the next few months.
ca.bdol.glencoe.com/self_check_quiz(l)Peter Parks/O.S.F./Animals Animals, (r)Jeff Foott/DRK Photo
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25.2SECTION PREVIEWObjectivesCompare and contrastradial and bilateral sym-metry with asymmetry.Trace the phylogeny ofanimal body plans.Distinguish among thebody plans of acoelomate,pseudocoelomate, andcoelomate animals.
Review Vocabularygastrula: an embryonic
structure made up oftwo layers of cells withan opening at one end(p. 676)
New Vocabularysymmetryradial symmetrybilateral symmetryanteriorposteriordorsalventralacoelomatepseudocoelomcoelomexoskeletoninvertebrateendoskeleton vertebrate
680 WHAT IS AN ANIMAL?
Body Plans and Adaptations
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Figure 25.7A sponge (A), an Asian leopard (B), and a jellyfish(C) all exhibit differentkinds of symmetry. AA CC
Form and FunctionUsing an Analogy Objects made by a potter canbe many different shapes and sizes. There is aplan for making each piece of potteryaccording to its function. One planresults in a bowl, another in a vase,and still another in a plate. Animals’bodies also have plans—body shapesthat are suited to a particular way oflife. In this section, you will study animal body plans and see how a specificbody plan is an adaptation to a particular environment. Make and Use Tables After you read about the different types ofanimal symmetry, make a table to categorize 25 animals according to theirsymmetry. Include animals that you are familiar with or have read about inthis book. Compare your table to those of your classmates.
What is symmetry?Look at the animals shown in Figure 25.7. You know that all animals
share certain characteristics, but these animals don’t look like they havemuch in common. The sponge seems to have no particular shape,whereas the leopard has a head, body, tail, and two pairs of legs. The jelly-fish doesn’t have a head or tail, and is circular in form. Each animal canbe described in terms of symmetry (SIH muh tree)—a term that describesthe arrangement of body structures. Different kinds of symmetry enableanimals to move about in different ways.
AsymmetryMany sponges have an irregularly shaped body, as seen in Figure 25.8A.
An animal that is irregular in shape has no symmetry or an asymmetricalbody plan. Animals with no symmetry often are sessile organisms that donot move from place to place. Most adult sponges do not move about.
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The bodies of most sponges consistof two layers of cells. Unlike all otheranimals, a sponge’s embryonic devel-opment does not include the forma-tion of an endoderm and mesoderm,or a gastrula stage. Fossil sponges firstappeared in rocks dating back to morethan 650 million years ago. They rep-resent one of the oldest groups of animals on Earth—evidence thattheir two-layer body plan makes them well adapted for life in aquaticenvironments.
Radial symmetryA hydra feeds on small animals it
snares with its tentacles. A hydra hasradial symmetry. Its tentacles radiateout from around its mouth. As shownin Figure 25.8B, animals with radial(RAY dee uhl) symmetry can bedivided along any plane, through acentral axis, into roughly equal halves.
Radial symmetry is an adaptation thatenables an animal to detect and cap-ture prey coming toward it from anydirection.
Have you ever had your groceriesdouble bagged at the store? The bodyplan of a hydra can be compared to asack within a sack. These sacks arecell layers organized into tissues withdistinct functions. A hydra developsfrom just two embryonic cell layers—ectoderm and endoderm.
Bilateral symmetryThe butterfly in Figure 25.8C has
bilateral symmetry. An organism withbilateral (bi LA tuh rul) symmetrycan be divided down its length intosimilar right and left halves. Bilaterallysymmetrical animals can be divided in half only along one plane. In contrast,radially symmetrical animals can be divided along any vertical plane.
25.2 BODY PLANS AND ADAPTATIONS 681(l)Nancy Sefton/Photo Researchers, (c)G.I. Bernard/O.S.F./Animals Animals, (r)Jane McAlonan/Visuals Unlimited
Figure 25.8All animals have body plans that enablethem to survive in their surroundings.Identify What symmetry does a fishhave? Explain.
These irregularly shapedsponges are examples ofanimals with asymmetricalbody plans.
A
Bilaterallysymmetricalanimals, such asbutterflies, havesimilar halves.
C
A hydra is an example of an animal withradial symmetry. It feeds on tiny animalsby immobilizing them with venom fromstinging cells that are along its tentacles.
B
Anterior
Dorsal
VentralPosterior
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In bilateral animals, the anterior, orhead end, often has sensory organs.The posterior of these animals is thetail end. The dorsal (DOR sul), orupper surface, also looks differentfrom the ventral (VEN trul), or lowersurface. In animals that are upright ornearly so, the back is on the dorsalsurface and the belly is on the ventralsurface. Animals with bilateral sym-metry can find food and mates andavoid predators because they have
sensory organs and good muscularcontrol. Test your ability to identifyanimal symmetry in Problem-SolvingLab 25.2 on this page.
Bilateral Symmetry and Body Plans
Animals that are bilaterally symmet-rical also share other important char-acteristics. All bilaterally symmetricalanimals developed from three embry-onic cell layers—ectoderm, endoderm,and mesoderm. Some bilaterally sym-metrical animals also have fluid-filledspaces inside their bodies called bodycavities in which internal organs arefound. The development of fluid-filledbody cavities made it possible for ani-mals to grow larger because it allowedfor the efficient circulation and trans-port of fluids, and support for organsand organ systems.
AcoelomatesAnimals that develop from three
cell layers—ectoderm, endoderm, andmesoderm—but have no body cavitiesare called acoelomate (ay SEE lumate) animals. They have a digestivetract that extends throughout thebody. Acoelomate animals may havebeen the first group of animals inwhich organs evolved.
Flatworms are bilaterally symmet-rical animals with solid, compact bod-ies, as shown in Figure 25.9. Likeother acoelomate animals, the organsof flatworms are embedded in thesolid tissues of their bodies. A flat-tened body and branched digestivetract allow for the diffusion of nutri-ents, water, and oxygen to supply allbody cells and to eliminate wastes.
Pseudocoelomates A roundworm is an animal with
bilateral symmetry. However, unlike aflatworm, the body of a roundworm
682 WHAT IS AN ANIMAL?
ClassifyIs symmetry associated with other animal traits? Animalsshow different patterns in their symmetry. Symmetry patternsare often associated with certain other characteristics or traitsfound in the animal.
Solve the ProblemStudy these three animal diagrams. Determine the type ofsymmetry being shown.
Thinking Critically1. Identify Animal A shows what type of symmetry? Explain
your answer. Describe other traits associated with animal A.2. List Name some objects other than animals that show the
pattern of symmetry in A.3. Identify Animal B shows what type of symmetry? Explain
your answer. Describe other traits associated with animal B.4. List Name some objects other than animals that show the
pattern of symmetry in B.5. Identify Animal C shows what type of symmetry? Explain
your answer. Describe other traits associated with animal C.6. List Name some objects other than animals that show the
pattern of symmetry in C.
AA BB CC
0680-0691 C25S2 BDOL-829900 8/4/04 12:32 PM Page 682
has a space that develops between theendoderm and mesoderm. It is calleda pseudocoelom (soo duh SEE
lum)—a fluid-filled body cavity partlylined with mesoderm.
Pseudocoelomates can movequickly. How? Think about the wayyour muscles work. The muscles inyour arm lift your hand by pullingagainst your arm bones. If there wereno rigid bones in your arms, yourmuscles would not be able to work.Although the roundworm has nobones, it does have a rigid, fluid-filledspace, the pseudocoelom. Its musclesattach to the mesoderm and braceagainst the pseudocoelom. You canobserve this movement in theMiniLab on this page.
Pseudocoelomates have a one-waydigestive tract that has regions withspecific functions. The mouth takesin food, the breakdown and absorp-tion of food occurs in the middle sec-tion, and the anus expels wastes.
Eric V. Grave/Photo Researchers
Figure 25.9Animals with acoelomate bodies usually havea thin, flattened shape (A). Pseudocoelomateanimals are larger and thicker than theiracoelomate ancestors (B). Coelomates havecomplex internal organs (C).
AcoelomateFlatworm
PseudocoelomateRoundworm
CoelomateSegmented Worm
Coelom
EndodermEctoderm Mesoderm Body cavity Digestive tract
Pseudocoelom
BB CCAA
683
Observe and InferCheck Out a Vinegar Eel Vinegareels are roundworms with pseudo-coeloms. They exhibit an interestingpattern of locomotion because theyhave only longitudinal (lengthwise)muscles.
ProcedureCAUTION: Use caution when handling a microscope and glassware.! Prepare a wet mount of vinegar eels. @ Observe them under low-power magnification. # Note their pattern of locomotion. Draw a series of
diagrams that illustrate their pattern of movement. $ Time, in seconds, how long it takes for one roundworm to
move across the center of your field of view. Determinethe diameter of your low-power field in mm. For help,refer to Calculate Field of View in the Skill Handbook.Time several animals and average their times, then calcu-late vinegar eel speed in mm/s.
Analysis1. Name What type of symmetry is present in vinegar eels? 2. Describe What is the general pattern of locomotion for
vinegar eels?3. Explain How does the pseudocoelom aid vinegar eels in
locomotion?4. Predict Based on the speed of your vinegar eel, estimate
the speed in mm/s for a flatworm. Explain your answer.
Vinegar eel
LM Magnification: 50�
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CoelomatesThe body cavity of an earthworm
develops from a coelom (SEE lum), afluid-filled space that is completelysurrounded by mesoderm. Humans,insects, fishes, and many other ani-mals have a coelomate body plan. Thegreatest diversity of animals is foundamong the coelomates.
Specialized organs and organ sys-tems develop in the coelom. In coelo-mate animals, the digestive tract andother internal organs are attached bydouble layers of mesoderm and aresuspended within the coelom. Likethe pseudocoelom, the coelom cush-ions and protects the internal organs.It provides room for them to growand move independently within ananimal’s body.
Animal Protection and Support
Over time, the development ofbody cavities resulted in a greaterdiversity of animal species. These
diverse animal species becameadapted to life in different environ-ments. Some animals, such as mol-lusks, evolved hard shells thatprotected their soft bodies. Otheranimals, such as sponges, evolvedhardened spicules between their cellsthat provided support.
Some animals developed exoskele-tons. An exoskeleton is a hard cover-ing on the outside of the body thatprovides a framework for support.Exoskeletons also protect soft bodytissues, prevent water loss, and pro-vide protection from predators. Anexoskeleton is secreted by the epider-mis and extends into the body, where itprovides a place for muscle attach-ment. As an animal grows, it secretes anew exoskeleton and sheds the oldone, as shown in Figure 25.10.
Exoskeletons are often found ininvertebrates. An invertebrate is ananimal that does not have a backbone.Many invertebrates, such as crabs,spiders, grasshoppers, dragonflies,and beetles, have exoskeletons.
Other animals have evolved differ-ent structures for support and protec-tion. Invertebrates, such as seaurchins and sea stars, have an internalskeleton called an endoskeleton. It iscovered by layers of cells and pro-vides support for an animal’s body.The endoskeleton protects internalorgans and provides an internal bracefor muscles to pull against. Anendoskeleton may be made of cal-cium carbonate, as in sea stars; carti-lage, as in sharks; or bone. Bonyfishes, amphibians, reptiles, birds,and mammals all have endoskeletonsmade of bone.
684 WHAT IS AN ANIMAL?
coelom from theGreek wordkoiloma, meaning“cavity”; A coelomis a body cavitycompletely sur-rounded by mesoderm.
Tony Florio/Photo Researchers
Figure 25.10A new exoskeleton forms before acrab sheds its old one. Until the newexoskeleton expands and hardens,the crab is vulnerable to predators.
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Figure 25.11Invertebrate animals such as anoctopus (A) and asea slug (B) haveno backbones.Vertebrates withbackbones includea monkey (C) andflamingos (D).
Understanding Main Ideas1. Compare and contrast radial and bilateral symme-
try in animals. Give an example of each type.2. Distinguish between the body plan of an acoelo-
mate and a coelomate. Give an example of an animal with each type of body plan.
3. Explain how an adaptation such as an exoskeletoncould be an advantage to animals in land biomes.
4. Compare movement in acoelomate and coelomate animals.
Thinking Critically5. Explain the relationship between having a coelom
and the development of complex organ systems.
6. Get the Big Picture Construct a table that com-pares the body plans of the sponge, hydra, flat-worm, roundworm, and earthworm. For morehelp, refer to Get the Big Picture in the SkillHandbook.
SKILL REVIEWSKILL REVIEW
25.2 BODY PLANS AND ADAPTATIONS 685(tl)Jeffrey L. Rotman/CORBIS, (tr)Stephen J. Krasemann/DRK Photo, (bl)Mark Boulton/Photo Researchers, (br)R. Van Nostrand/Photo Researchers
CC
DD
AA
BB
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A vertebrate is an animal with anendoskeleton and a backbone. All ver-tebrates are bilaterally symmetrical.Examples of vertebrates include, fishes,amphibians, reptiles, birds, and mam-mals. Figure 25.11 shows examples ofinvertebrate and vertebrate animals.
Origin of AnimalsMost biologists agree that animals
probably evolved from aquatic, colonialprotists. Scientists trace this evolution
back in time to late in the Precam-brian. Although evidence suggeststhat bilaterally symmetrical animalsmight have appeared much later,many scientists agree that all themajor animal body plans that existtoday were already in existence at thebeginning of the Cambrian Period,543 million years ago. Since then,many new species have evolved but allknown species have variations of theanimal body plans developed duringthe Cambrian Period.
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686 WHAT IS AN ANIMAL?
Before YouBegin
The zebra fish (Daniorerio) is a common fresh-water fish sold in petshops. They are ideal animals for study becausethey undergo embryonicdevelopmental changesquickly and major stagescan be observed withinhours after fertilization.
Zebra Fish Development
Problem What do the developmental stages of the zebra fish look like?
ObjectivesIn this BioLab, you will: ■ Observe stages of zebra fish development. ■ Record all observations in a data table.■ Use the Internet to collect and compare data from other
students.
Materialszebra fishes petri dish beaker
(males and females) wax pencil dropperprepared aquarium bulb basterbinocular microscope
Safety PrecautionsCAUTION: Always wear safety goggles in the lab. Use cautionwhen handling a binocular microscope and glassware.
Skill HandbookIf you need help with this lab, refer to the Skill Handbook.
1. Copy the data table.2. Use the bulb baster to transfer water and fish embryos
from the aquarium to a beaker. Allow the embryos to settle to the bottom.
3. Use a wax pencil to write your name and class period onthe edge of the lid of your petri dish. Use the dropper tohalf fill the bottom of your petri dish with aquarium water,and then to transfer several embryos from the beaker toyour petri dish. Place the lid on your petri dish.
4. Your teacher will tell you the approximate time that fer-tilization occurred. Record the age of the embryos inyour data table as hpf (hours past fertilization).
5. Observe the embryos under the microscope. In your datatable, diagram what you observe.
6. Go to to post yourdata.
PROCEDUREPROCEDURE
PREPARATIONPREPARATION
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Matt Meadows
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25.2 BODY PLANS AND ADAPTATIONS 687
Date Age (hpf) Diagram Observations
Data Table
ANALYZE AND CONCLUDEANALYZE AND CONCLUDE
1. Explain Why are zebra fishes ideal animals for studying embryonic development?
2. Think Critically Explain why you may not have been able to see stagessuch as a blastula or gastrula.
3. Collect and Organize Data Visit
for links to internet sites that will help youcomplete sequences of the major changesduring development of zebra fishes:a. between 1 and 10 hpf. Include labeled
diagrams of these changes.b. between 10 and 28 hpf. Include labeled
diagrams.c. between 28 and 72 hpf. Include labeled
diagrams.4. Suggest how you could
change the experiment’s design to allow forobserving blastula and gastrula stages.
ERROR ANALYSIS
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7. Continue to observe your embryos daily for one week.Note when new organs appear and when movement isfirst seen. If you want to continue observing develop-mental changes, ask your teacher for instructions. CAUTION: Wash your hands with soap and waterimmediately after each observation.
8. Clean all equipment asinstructed by your teacher, and return everything toits proper place. Dispose of the water and embryosproperly. Wash your hands thoroughly.
CLEANUP AND DISPOSAL
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Find this BioLab using the link below andpost your data in the data table provided forthis activity. Using the additional data fromother students on the Internet, answer thequestions for this lab. Were there large varia-tions in data posted by other students? Whatmight have caused these differences?
Matt Meadows
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Mighty MouseCells
A round the world, researchers are beginningto understand the enormous potential of
stem cells. It is hoped that better treatments orcures for diseases such as Parkinson’s disease,leukemia, Alzheimer’s disease, and diabetes willcome from stem cell research. What makes stemcells so powerful and unique?
Putting stem cells to work Stem cells areundifferentiated cells that have the ability to pro-duce more stem cells or produce specialized cells.Stem cells are found in embryos and in youngand adult animals where they play key roles. Forexample, if you ever have damaged a muscle,your muscle stem cells helped with the repair.Blood stem cells work throughout your life tomaintain the supply of specialized cell typesfound in your blood.
Somatic Cell Nuclear Transfer (SCNT)Biotechnologies are used to isolate and growstem cells. In a process called somatic cell nucleartransfer, as illustrated at the right, the nucleus isremoved from a normal animal egg cell. Asomatic cell—any body cell other than an egg orsperm cell—is placed next to the egg cell withouta nucleus and the two cells are made to fuse. Thenew cell undergoes many cell divisions and formsa blastocyst from which stem cells are taken.
Mouse stem cells to the rescue Because ofethical concerns, SCNT has only been done withmouse cells and recent studies have yielded inter-esting results. In 2001, researchers associated withthe National Institutes of Health successfully usedmouse stem cells to create insulin-producing cells.For people suffering from Type-1 diabetes, a con-dition in which the immune system mistakenlydestroys cells that produce insulin, these resultsoffer hope. In a study by scientists in England,mouse stem cells were used to create bone cells.This type of research could lead to new treat-ments for bone diseases, as well as improved bone grafts for treating serious bone injuries.
Future research Continued research promisesgreater understanding of stem cells. Once,researchers believed human stem cells could befound only in bone marrow, brain tissue, andfetal tissue. However, in a study conducted byresearchers at the University of California, LosAngeles and the University of Pittsburgh, stemcells were found in human fat. Experiments withthese fat cells produced types of muscle, bone,and cartilage cells. If these cells prove to be asversatile as many scientists expect them to be,their use in treating diseases could be unlimited.Some experts suggest stem cells might one daybe used to grow new organs for transplant, andeffectively treat many disorders by replacing dis-eased cells with healthy ones.
688 WHAT IS AN ANIMAL?
Egg Nucleus removed
Somatic cell
Somatic cellnuclear transfer
Fusion
Blastocyst
Cultured stem cells
Think Critically Some diseases such asAlzheimer’s disease and leukemia result in thegradual loss of healthy cells in specific parts of thebody. Research a human disease that is caused by agradual loss of healthy cells. Prepare a brief reportabout the disease and include how SCNT might beused to treat that disease.
To learn more about stem cells, visitca.bdol.glencoe.com/biotechnology
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Section 25.1
Section 25.2
Vocabularyblastula (p. 676)deuterostome (p. 677)ectoderm (p. 677)endoderm (p. 677)gastrula (p. 676)mesoderm (p. 677)protostome (p. 677)sessile (p. 674)
Typical AnimalCharacteristics
Vocabularyacoelomate (p. 682)anterior (p. 682)bilateral symmetry
(p. 681)coelom (p. 684)dorsal (p. 682)endoskeleton (p. 684)exoskeleton (p. 684)invertebrate (p. 684)posterior (p. 682)pseudocoelom (p. 683)radial symmetry (p. 681)symmetry (p. 680)ventral (p. 682)vertebrate (p. 685)
Body Plans andAdaptations
STUDY GUIDESTUDY GUIDE
CHAPTER 25 ASSESSMENT 689
Key Concepts■ Animal adaptations include asymmetry,
radial symmetry, or bilateral symmetry.■ Flatworms and other acoelomates have flat-
tened, solid bodies with no body cavities.■ Animals such as roundworms have a
pseudocoelom, a body cavity that developsbetween the endoderm and mesoderm.
■ A coelom is a fluid-filled body cavity thatsupports internal organs. Coelomate ani-mals have internal organs suspended in abody cavity that is completely surroundedby mesoderm.
■ Exoskeletons provide a framework of support on the outside of the body.Endoskeletons provide internal support.
25
LM Magnification: 80�
To help you review and iden-tify characteristics of the animal kingdom, usethe Organizational Study Fold on page 673.
ca.bdol.glencoe.com/vocabulary_puzzlemaker
Key Concepts■ Animals are multicellular eukaryotes whose
cells lack cell walls. Their cells are special-ized to perform different functions.
■ All animals are heterotrophs that obtainand digest food.
■ At some point during its life, an animal can move from place to place. Most ani-mals retain this ability.
■ Embryonic development of a fertilized eggcell by cell division and differentiation issimilar among animal phyla. The sequenceof developmental stages is:1. formation of a blastula—a cell-covered,
fluid-filled ball;2. gastrulation—the inward movement of
cells to form two cell layers, the endo-derm and ectoderm;
3. formation of the mesoderm—the devel-opment of a cell layer between theendoderm and ectoderm.
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Review the Chapter 25 vocabulary words listed inthe Study Guide on page 689. Distinguishbetween the vocabulary words in each pair.
1. mesoderm—ectoderm2. coelom—pseudocoelom 3. blastula—gastrula4. radial symmetry—bilateral symmetry5. protostome—deuterostome
6. Which of these organs develops from theectoderm?A. stomach C. intestinesB. skin D. liver
7. Animals that cannot make their own foodare called ________.A. autotrophs C. producersB. heterotrophs D. photosynthetic
8. Coral larvae are ________ but adult formsare ________.A. haploid—diploidB. free-swimming—sessileC. acoelomates—coelomatesD. protostomes—deuterostomes
9. Which of the following sentences doesNOT describe an animal?A. It has cells with cell walls.B. It is a multicellular organism.C. It is a consumer.D. It has a digestive system that breaks
down food.10. Which animal shown below has radial
symmetry?
A. C.
B. D.
11. A fish has a fin on its upper surface. Becausea fish has ________ symmetry, this fin iscalled the ________ fin.A. no—pectoralB. bilateral—anteriorC. radial—posteriorD. bilateral—dorsal
12. Open Ended Look at Figure 25.4. Evaluatethese models as to their adequacy in repre-senting a blastula and gastrulation.
13. Open Ended Examine Figure 25.5. Predictwhat might happen if, at the 4-cell stage, theembryo cells separated.
14. Open Ended If the opening in the gastrulaeventually develops into a mouth, could thisanimal be a bird? Explain.
15. Differentiate Which of these animals—seastar, insect, leech, or clam—shares the mostcharacteristics with an earthworm? Explain.
16. Homeotic orHox genes regulate embryonic developmentin organisms. The Hox genes of organisms,such as zebra fishes, fruit flies, and round-worms, have been studied extensively. Visit
to learn more aboutthese genes. Present your research results asa poster or a multimedia presentation.
17. Concept Map Use the following terms tocomplete this concept map: blastula, ecto-derm, gastrula, endoderm, mesoderm.
REAL WORLD BIOCHALLENGE
690 CHAPTER 25 ASSESSMENT
during the stage.5.
to a
Animals develop from a
zygote
stage in which these tissues form
1.
3.
which forms the
4.
2.
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CHAPTER 25 ASSESSMENT 691
Constructed Response/Grid InRecord your answers on your answer document.
24. Open Ended Describe the relationship between an animal’s body plan and the environment in which it lives. Give examples.
25. Open Ended Explain why the development of a body cavity enabled animals to move and feed more efficiently.
ca.bdol.glencoe.com/standardized_test
Multiple ChoiceWhen vertebrate eggs are developing, they gothrough the first meiotic division and then pause atmetaphase of the second meiotic division until fer-tilization occurs before completing the second mei-otic division. MPF, a protein, regulates the pauseand continuation of meiosis after fertilization.Analyze the graph and answer questions 18–20.
18. MPF is at high levels ________.A. at the beginning of the first meiotic
division and at the beginning of the second meiotic division
B. at the beginning of the first meiotic division and at metaphase of the secondmeiotic division
C. only at the beginning of the first meiotic division
D. only at metaphase of the second meiotic division
19. MPF also slows the beginning of the firstmeiotic division to allow the egg to grow.Therefore, at this time MPF levels are________.A. unchangedB. lowC. highD. high and low
20. Which of the following statements bestdescribes the relationship between MPF andthe slowing of meiosis in the developing egg?A. When MPF is high, meiosis progresses.B. MPF has no effect on slowing meiosis.C. MPF is necessary for mitosis.D. When MPF is high, meiosis slows down.
Study the diagram and answer questions 21–23.
21. The cell layer from which the heart of thezebra fish develops is the ________.A. endoderm C. mesodermB. ectoderm D. yolk sac
22. One day after fertilization the zebra fish has________. A. heart, tail, eyes, headB. heart, tail, fins, headC. no internal organsD. two cell layers in a gastrula
23. Twenty hours after fertilization the heart is________.A. two chamberedB. not divided into two chambers C. four chamberedD. ectoderm
Gastrula (5 hours
afterfertilization)
18 hoursafter
fertilization
20 hoursafter
fertilization
1-day-old
embryo
Yolk cell Heart or heartprimordia
MesodermEctoderm
First meioticdivision
Second meioticdivision
MetaphaseII
arrest
Fertilization
MPF Levels During Meiosis
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