Invertebrates Unit 7 PDF

UNIT 7 Invertebrates THEMES Scientific Inquiry Biologists continue to study invertebrates with observations and technology. Diversity Special adaptations enable invertebrates to live in a wide variety of...

UNIT 7 Invertebrates THEMES Scientific Inquiry Biologists continue to study invertebrates with observations and technology. Diversity Special adaptations enable invertebrates to live in a wide variety of ecosystems. Energy Food supplies the energy needed to carry out life functions. Homeostasis Excretory cells and organs maintain homeostasis in invertebrates. Change Natural selection over a period of time results in newly evolved species of invertebrates. Chapter 24 Introduction to Animals Chapter 25 Worms and Mollusks Chapter 26 Arthropods Chapter 27 Echinoderms and Invertebrate Chordates WebQuest Careers in Biology Entomologists are scientists who study insects and their behavioral patterns. Entomologists who conduct research, such as this field entomologist is doing, contribute to a better understanding of the ecosystem’s function as a whole. 688 688_689_U07_UO_894586.indd 688 5/18/10 4:45 PM  Unit 7 Invertebrates 689 688_689_U07_UO_894586.indd 689 5/18/10 4:45 PM  CHAPTER 24 Introduction to Animals Your one-stop online resource connectED.mcgraw-hill.com Video WebQuest Audio Assessment Review Concepts in Motion ? Inquiry g Multilingual eGlossary Launch Lab What is an animal? Although animals share some characteristics with all other living organisms, they also have unique characteristics. In this lab, you will compare and contrast two organisms and deter- mine which one is an animal. For a lab worksheet, use your StudentWorks™ Plus Online. ? Inquiry Launch Lab Make a layered-look book using the titles shown. Use it to organize your notes on body plans. Body Plans Coelomate Pseudocoelomate Acoelomate 690 Chapter 24 Introduction to Animals 690_691_C24_CO_894586.indd 690 5/18/10 4:47 PM  Sea anemone Sea anemone tentacles Nematocysts LM Magnification: 500× THEME FOCUS Diversity Section 1 Animal Characteristics Animals have a wide variety of characteristics, including body plans and adaptations. Section 2 Animal Body Plans Section 3 Sponges and Cnidarians BIG Idea Animal phylogeny is determined in part by animal body plans and adaptations. Chapter 24 Introduction to Animals 691 690_691_C24_CO_894586.indd 691 5/18/10 4:47 PM  Section 1 Reading Preview Essential Questions Animal Characteristics ◗ How do adaptations enable animals to live in different habitats? MAIN Idea Animals are multicellular, eukaryotic heterotrophs that ◗ How is structure and function related have evolved to live in many different habitats. in animals? Real-World Reading Link When you think of animals, you might think of ◗ What are the stages of embryonic creatures that are furry and fuzzy. However, animals can have other outer development in animals? coverings, such as feathers on birds and scales on fishes. Some animals even Review Vocabulary might be mistaken for plants. protist: diverse group of unicellular or multicellular eukaryotes that lack General Animal Features complex organ systems and live in Recall that biologists have created an evolutionary tree to organize the moist environments great diversity of living things. The ancestral animals at the beginning New Vocabulary of the evolutionary tree are eukaryotic and multicellular—they are made up of many cells. The tiger in Figure 1 and all other present-day invertebrate exoskeleton animals might have evolved from choanoflagellates (KOH uh noh FLA endoskeleton juh layts), which are protists that formed colonies in the sea 570 million vertebrate years ago. Choanoflagellates, such as the ones shown in Figure 1, hermaphrodite might have been the earliest true animals. As animals evolved from this zygote multicellular ancestor, they developed adaptations in structure that internal fertilization enabled them to function in numerous habitats. These features mark external fertilization the branching points of the evolutionary tree and are discussed in the blastula next section. In this section, you will learn about the characteristics gastrula that all animals have in common. endoderm ectoderm mesoderm Feeding and Digestion Animals are heterotrophic, so they must feed on other organisms to g Multilingual eGlossary obtain nutrients. A sea star obtains its food from a clam it has pried open, and a butterfly feeds on nectar from a flower. The structure or form of an animal’s mouth parts determines how its mouth functions. You can investigate how some animals obtain food by performing MiniLab 1. Figure 1 Present-day animals, such After obtaining their food, animals must digest it. Some animals, such as as this Bengal tiger, might have evolved from sponges, digest their food inside specific cells. Others, such as earthworms choanoflagellates such as this colony of and humans, digest their food in internal body cavities or organs. Zoothamnium. LM Magnification: 50× Bengal tiger Colony of Zoothamnium 692 Chapter 24 Introduction to Animals 692_697_C24_S1_894586.indd 692 5/18/10 4:48 PM  Support Just as animals digest their food in different ways, they support their bodies in different ways. Between 95 and 99 percent of ani- mal species are invertebrates—animals without backbones. The bodies of many invertebrates are covered with exoskeletons, which are hard or tough outer coverings that provide a frame- work of support. Exoskeletons also protect soft body tissues, pre- vent water loss, and provide protection from predators. As the animal grows, like the cicada in Figure 2, it must shed the old exoskeleton and make a new one. This process is called molting. Some invertebrates, such as sea urchins and sea stars, have internal skeletons called endoskeletons. If an animal has an endoskeleton and a backbone, it is called a vertebrate. An endo- skeleton grows with the animal, like the squirrel in Figure 2. The material making up the endoskeleton varies. Sea urchins and sea stars have endoskeletons made of calcium carbonate, sharks have Cicada endoskeletons made of cartilage, and fishes, amphibians, reptiles, birds, and mammals have endoskeletons made of bone. An endo- skeleton protects internal organs, provides support for the body, and can provide an internal brace for muscles to pull against. Reading Check Distinguish between vertebrates and invertebrates. Habitats Animal bodies have a variety of adaptations, such as those for feed- ing, digestion, and support. These body variations enable animals Squirrel to live in numerous habitats. Vertebrates and invertebrates live in Figure 2 A cicada must shed its old exoskel- oceans, in freshwater, and on land. They can be found in deserts, eton (outlined in white) in order to grow. grasslands, rain forests, polar regions, and all other land biomes A squirrel has an endoskeleton that grows as the and aquatic ecosystems. squirrel grows. Infer how an exoskeleton might be a disadvantage for animals. 1 Investigate Feeding in Animals ? Inquiry MiniLab How do animals obtain food? Small aquatic animals called hydras consume brine shrimp as their food source. Procedure 1. Read and complete the lab safety form. 2. Obtain several hydras in a plastic Petri dish containing water. 3. Add several brine shrimp to the dish. Using a hand lens or stereomicroscope, observe the activity of the hydras. 4. Record your observations. Analysis 1. Draw Conclusions Based on your observations, how do the hydras react to the food? 2. Infer What factors in their environment might influence how the hydras find food? Section 1 Animal Characteristics 693 692_697_C24_S1_894586.indd 693 5/18/10 4:48 PM  Animal Cell Structure No matter where an animal lives or what adaptations it has, its cells do not have cell walls. Recall that plants also are multicellular organisms, but their cells have cell walls. The cells of all animals, except sponges, are organized into structural and functional units called tissues. A tissue is a group of cells that is specialized to perform a specific function. For example, nerve tissue is involved in the trans- mission of nerve impulses throughout the body and muscle tissue enables the body to move. Careers In biology Connection to History Beginning with Aristotle in the fourth century Systematist Using observation, b.c. and continuing into the nineteenth century, living organisms were inference, and the latest technology, classified into two kingdoms—Animalia (animals) and Plantae (plants). a systematist classifies new species In 1866, Ernst Haeckel, a German scientist, proposed adding a third based on evolutionary relationships. kingdom called Protista. The organisms in this kingdom are mainly unicellular eukaryotes. Some protists have cell walls, while others do not, making them neither plant nor animal. During the 1960s, as more was learned about cell structure, bacteria and fungi were placed into their own kingdoms. Figure 3 illustrates how the classification of living things continues to develop. Movement The evolution of nerve and muscle tissues enables animals to move in ways that are more complex and faster than organisms in other king- doms. This is one notable characteristic of the animal kingdom. A gecko running across a ceiling, a mosquito buzzing around your ear, and a school of minnows swimming against the current are all exhibit- ing movements unique to animals. Some animals are stationary as adults, yet most have a body form that can move during some stage of development. Figure 3 History of Classification The process of scientifically classifying organ- isms began in 350 B.C. when Aristotle, a Greek philosopher, placed organisms into two large groups—plant and animal. Advances in scien- tific knowledge and technology helped develop the classification system we use today. ▼ 1735 Biologist Carolus Concepts in Motion Linnaeus devises a classifi- cation system for all organ- The Interactive Timeline isms using Latin binomial nomenclature. ▼ 1555 The book L’Histoire 1682 Naturalist John Ray 1859 Naturalist Charles de la Nature des Oyseaux establishes the use of spe- Darwin proposes the classifi- (Natural History of Birds) cies as the basic unit of clas- cation of organisms based on uses body form and struc- sification. their shared ancestry. tures to classify species. 694 Chapter 24 Introduction to Animals 692_697_C24_S1_894586.indd 694 5/11/10 3:25 PM  Reproduction Most animals reproduce sexually, although some species can reproduce asexually. Most commonly in sexual reproduction, male animals produce sperm and female animals produce eggs. Some animals, such as earth- worms, are hermaphrodites (hur MAF ruh dites), which produce both eggs and sperm in the same animal body. In general, hermaphrodites pro- duce eggs and sperm at different times, so another individual of the same species still is needed for sexual reproduction. Fertilization occurs when the sperm penetrates the egg to form a fertilized egg cell called the zygote (ZI goht). Fertilization can be inter- nal or external. Internal fertilization occurs when the sperm and egg combine inside the animal’s body. For example, male turtles fertilize the eggs of the female internally. External fertilization occurs when Figure 4 Fertilization is external in some egg and sperm combine outside the animal’s body. This process fishes. In the photo, strands of sperm are being requires an aquatic environment for the sperm to swim to the egg. In shed over eggs laid in the water. many fishes, the female lays eggs in the water and the male sheds sperm Infer why animals lay a large number over the eggs, as shown in Figure 4. of eggs when fertilization is external. Recall that asexual reproduction means that a single parent produces offspring that are genetically identical to itself. Although few animal spe- cies reproduce asexually, when they do, they use one or more methods to do so. Some of the common methods of asexual reproduction follow. budding—an offspring develops as a growth on the body of the parent fragmentation—the parent breaks into pieces and each piece can develop into an adult animal regeneration—a new organism can regenerate, or regrow, from the lost body part if the part contains enough genetic information parthenogenesis (par thuh noh JE nuh sus)—a female animal pro- duces eggs that develop without being fertilized Reading Check Infer the advantages and disadvantages of asexual reproduction in animals. 1977 Microbiologist Carl ▼ 2003 Paleontologists find Woese uses ribosomal RNA to feathered dinosaur fossils that show the evolutionary relation- might alter the classification of ships among organisms. some species. 1891 Marine zoologist 1982 Biologist Lynn Margulis Mary Jane Rathbun begins is instrumental in reorganizing establishing the basic and improving the classification taxonomic information on of organisms into the current 2009 A partial skull fossil of Homo crustaceans. five kingdoms. floresiensis causes debate among scien- tists as they work to determine if it is an early Homo sapiens or a new species. Section 1 Animal Characteristics 695 692_697_C24_S1_894586.indd 695 5/18/10 4:49 PM  Early development In most animals, the zygote undergoes mitosis and a series of cell divisions to form new cells. After the first cell divi- sion, in which the zygote forms two cells, the developing animal is called an embryo. The embryo continues to undergo mitosis and cell division, forming a solid ball of cells. These cells continue to divide, VOCABULARY forming a fluid-filled ball of cells called the blastula (BLAS chuh luh), WORD ORIGIN as shown in Figure 5. During these early stages of development, the Gastrula number of cells increases, but the total amount of cytoplasm in the gastr– prefix; from Greek; meaning embryo remains the same as that in the original cell. Therefore, the stomach or belly total size of the embryo does not increase during early development. –ula suffix; from Latin; meaning In animals such as lancelets, the outer blastula is a single layer of resembling cells, while in animals such as frogs, there might be several layers of cells surrounding the fluid. The blastula continues to undergo cell divi- sion. Some cells move inward to form a gastrula (GAS truh luh), a two-cell-layer sac with an opening at one end. A gastrula looks like a double bubble, one bubble inside another bubble. Look again at Figure 5. Notice how the diagrams of the two-cell Figure 5 The fertilized eggs of most stage, the 16-cell stage, and the blastula differ from the photographs of animals follow a similar pattern of develop- these same stages. The diagrams illustrate early development in ment. Beginning with one fertilized egg cell, cell embryos that develop inside the adult animal. The photographs illus- division occurs and a gastrula is formed. trate early development in embryos that develop outside of the adult animal. The large ball that does not divide is the yolk sac. It provides Concepts in Motion food for the developing embryo. Animation Reading Check Explain the differences between the blastula and the gastrula. Sperm Egg Fertilization Gastrula 2-cell stage 16-cell stage Blastula Color-Enhanced SEM Magnification: 160× Color-Enhanced SEM Magnification: 130× Color-Enhanced SEM Magnification: 160× Yolk Yolk sac sac 2-cell stage 16-cell stage Blastula 696 Chapter 24 Introduction to Animals 692_697_C24_S1_894586.indd 696 5/18/10 4:49 PM  Figure 6 As development continues, each Ectoderm becomes cell layer differentiates into specialized tissues. Endoderm becomes nervous tissue and digestive organs and skin. digestive tract lining. Mesoderm becomes muscle tissue and the circulatory, Opening of excretory, and gastrula respiratory systems. Tissue development Notice in Figure 6 that the inner layer of cells in the gastrula is called the endoderm. The endoderm cells develop into the digestive organs and the lining of the digestive tract. The outer layer of cells in the gastrula is called the ectoderm. The ectoderm cells in the gastrula continue to grow and become the nervous tissue and skin. Cell division in some animals continues in the gastrula until another layer of cells, called the mesoderm, forms between the endoderm and the ectoderm. In some animals, the mesoderm forms from cells that ? Inquiry Launch Lab break away from the endoderm near the opening of the gastrula. In more Review Based on what you have read highly evolved animals, the mesoderm forms from pouches of endoderm about animal characteristics, how would you now answer the analysis questions? cells on the inside of the gastrula. As development continues, mesoderm cells become muscle tissue, the circulatory system, the excretory system, and, in some species of animals, the respiratory system. Remember that Hox genes might be expressed in ways that give pro- teins new properties that cause variations in animals. Much of the varia- tion in animal bodies is the result of changes in location, number, or time of expression of Hox developmental genes during the course of tissue development. Section 1 Assessment Section Summary Understand Main Ideas ◗ Animals are heterotrophs and must get their 1. MAIN Idea Infer why colonial organisms that lived grouped together nutrients from other organisms. might have been one of the first steps toward multicellular organisms in the course of evolution. ◗ Animals have diverse means of support and live in diverse habitats. 2. Infer how an exoskeleton enables invertebrates to live in a variety of habitats. ◗ Animal cells do not have cell walls, and 3. Describe how the evolution of nerve and muscle tissue is related to one most have cells that are organized into of the main characteristics of animals. tissues. 4. Diagram how an animal zygote becomes a gastrula. ◗ Most animals undergo sexual reproduction, Think Critically and most can move. 5. Model the stages of cell differentiation in embryonic development by ◗ During embryonic development, animal comparing them to pushing in the end of a balloon. Draw a diagram of this cells become tissue layers, which become process and label it with the stages of cell differentiation. organs and systems. MATH in Biology 6. Biologists have observed that it is common for an animal that doubles its mass to increase its length 1.26 times. Suppose an animal has a mass of 2.5 kg and is 30 cm long. If this animal grows to a mass of 5 kg, how long will it be? Assessment Online Quiz Section 1 Animal Characteristics 697 692_697_C24_S1_894586.indd 697 5/18/10 4:49 PM  Section 2 Reading Preview Essential Questions Animal Body Plans ◗ How are animal body plans related to phylogeny? MAIN Idea Animal phylogeny can be determined, in part, by body ◗ How are body cavities related to plans and the ways animals develop. animal phylogeny? ◗ What are the two types of coelomate Real-World Reading Link People often classify or group things based on development? what they have in common. If you want to rent an action movie, you would look in the action movie section at the store. You would not find comedies or dramas Review Vocabulary in this section. In biology, animals generally are classified into groups because phylogeny: evolutionary history they have some of the same features. of a species based on comparative relationships of structures and comparisons of modern life-forms Evolution of Animal Body Plans with fossils Recall that the evolutionary tree is organized like a family tree, and the phylogeny of animals is represented by the branches. For example, all New Vocabulary of the mammals in Figure 7 belong on the chordate branch of the tree. symmetry The trunk represents the earliest animals and the branches represent radial symmetry the probable evolution of the major phyla of animals from a common bilateral symmetry ancestor, as shown in Figure 8. anterior Anatomical features in animals’ body plans mark the branching posterior points on the evolutionary tree. For example, animals without tissues cephalization are grouped separately from animals with tissues, and animals without dorsal segments are grouped separately from animals with segments. The rela- ventral tionships among animals on this tree are inferred by studying similari- coelom ties in embryological development and shared anatomical features. This pseudocoelom acoelomate traditional phylogeny, with animals classified into 35 phyla, is still used protostome by most taxonomists. However, molecular data suggest other relation- deuterostome ships among animals. Recent molecular findings, based on compari- sons of DNA, ribosomal RNA, and proteins, indicate that the g Multilingual eGlossary relationships between arthropods and nematodes and between flat- worms and rotifers might be closer than anatomical features suggest. Reading Check Summarize the structure of an evolutionary tree. Figure 7 Although these animals look very different from each other, they all have features that place them on the chordate branch of the evolutionary tree. Mouse Ferret Chimpanzee 698 Chapter 24 Introduction to Animals 698_704_C24_S2_894586.indd 698 5/18/10 4:51 PM  Development of Tissues As animals evolved from the first multicellular forms, the first anatom- ical feature to indicate a major change in body plan was the develop- ment of tissues. Therefore, tissues mark the first branching point on the evolutionary tree. Notice in Figure 8 that the only animals without tis- sues are sponges. These animals descended from a common ancestor that lacked tissues, and they are on the no-true-tissue branch of the evolutionary tree. Follow the tissue branch of the evolutionary tree, and you will see that all other phyla have tissues. sponges cnidarians flatworms roundworms rotifers mollusks annelids arthropods echinoderms chordates segmentation segmentation protostome development deuterostome development pseudocoelom coelom acoelomate body cavity radial symmetry bilateral symmetry no true tissues tissues Figure 8 Ancestral animals are found at the base of the trunk of the evolutionary tree multicellularity and at every node in the tree. The branches of the tree are distinguished by major develop- ments in the phylogeny of animals. Present-day animals appear at the top of the tree. Interpret which group of organisms is most closely related to phylum Ancestral protist Arthropoda. Section 2 Animal Body Plans 699 698_704_C24_S2_894586.indd 699 5/18/10 4:52 PM  Anterior Ventral Dorsal Posterior Hummingbird— Sponge—asymmetry Jellyfish—radial symmetry bilateral symmetry Figure 9 Animals have different arrangements of body structures. The sponge Symmetry has an irregular shape and is asymmetrical. The Move along the tissue branch on the evolutionary tree in Figure 8, jellyfish has radial symmetry, and the humming- and you will find the next branching point to be symmetry. bird has bilateral symmetry. List objects in the classroom that have Symmetry (SIH muh tree) describes the similarity or balance among bilateral symmetry. body structures of organisms. The type of symmetry an animal has enables it to move in certain ways. Review Personal Tutor Asymmetry The sponge in Figure 9 has no tissue and has asymme- try. It is irregular in shape and has no symmetry or balance in its body structures. In contrast, animals with tissues have either radial or bilat- eral symmetry. Radial symmetry An animal with radial (RAY dee uhl) symmetry can be divided along any plane, through a central axis, into roughly Video BrainPOP equal halves. The jellyfish in Figure 9 has radial symmetry. Its tentacles radiate from its mouth in all directions, a body plan adapted to detecting and capturing prey moving in from any direction. Jellyfishes and most other animals with radial symmetry develop from only two embryonic cell layers—the ectoderm and the endoderm. Bilateral symmetry The bird in Figure 9 has bilateral symmetry. VOCABULARY In contrast to radial symmetry, bilateral (bi LA tuh rul) symmetry SCIENCE USAGE V. COMMON USAGE means the animal can be divided into mirror image halves only along Plane one plane through the central axis. All animals with bilateral symmetry Science usage: an imaginary line that develop from three embryonic cell layers—the ectoderm, the endoderm, divides a body form into two parts and the mesoderm. The dog can be divided into its ventral and dorsal parts by a plane. Cephalization Animals with bilateral symmetry also have an anterior, or head end, and a posterior, or tail end. This body plan Common usage: an aircraft is called cephalization (sef uh luh ZA shun)—the tendency to con- The pilot flew the plane from Cleve- centrate nervous tissue and sensory organs at the anterior end of the land to Chicago. animal. Most animals with cephalization move through their envi- ronments with the anterior end first, encountering food and other stimuli. In addition to cephalization, animals with bilateral symme- try have a dorsal (DOR sul) surface, also called the backside, and a ventral (VEN trul) surface, also called the underside or belly. 700 Chapter 24 Introduction to Animals 698_704_C24_S2_894586.indd 700 5/18/10 4:52 PM  Body Cavities In order to understand the next branching point on Review Personal Tutor the evolutionary tree, it is important to know about certain features of animals with bilateral symmetry. Fluid-filled cavity Body plans of animals with bilateral symmetry include the gut, which is either a sac inside the body or a tube that runs through the body, where food is digested. A saclike gut has one opening, a mouth, for taking in food and disposing of wastes. A tubelike gut has an opening at both ends, mouth and an anus, and is a complete digestive system that digests, absorbs, and stores food, and disposes of waste. Coelomates Between the gut and the outside body wall of most animals with bilateral symmetry is a fluid-filled body cavity. One type of fluid-filled cavity, the coelom (SEE lum), shown in Figure 10, has tissue formed from mesoderm that lines and Coelomate body plan encloses the organs in the coelom. You have a coelom, as do insects, fishes, and many other animals. There- Fluid-filled cavity fore, you are a coelomate. The coelom was a key adap- tation in the evolution of larger and more specialized body structures. Specialized organs and body systems that formed from mesoderm developed in the coe- lom. As more efficient organ systems evolved, such as the circulatory system and muscular system, animals could increase in size and become more active. Pseudocoelomates Follow the body cavity branch on the evolutionary tree in Figure 8 until you come to the pseudocoelomates, which are animals with pseudocoeloms. A pseudocoelom (soo duh SEE lum) is a fluid-filled body cavity that develops Pseudocoelomate body plan between the mesoderm and the endoderm rather than developing entirely within the mesoderm as in coelomates. Therefore, the pseudocoelom, as shown in Figure 10, is lined only partially with mesoderm. The body cavity of pseudocoelomates separates mesoderm and endoderm, which limits tissue, organ, and system development. Acoelomates Before the body cavity branch on the evolutionary tree in Figure 8, notice that the branch to the left takes you to the acoelomate animals. Acoelomates (ay SEE lum ayts), such as the flat- worm in Figure 10, are animals that do not have a Acoelomate body plan coelom. The body plan of acoelomates is derived from ectoderm, endoderm, and mesoderm—the Key: Endoderm Ectoderm Mesoderm same as in coelomates and pseudocoelomates. How- ever, acoelomates have solid bodies without a fluid- Figure 10 An earthworm has a coelom, a fluid-filled body cavity filled body cavity between the gut and the body wall. surrounded completely by mesoderm. The pseudocoelom of a roundworm Nutrients and wastes diffuse from one cell to develops between the mesoderm and endoderm. A flatworm has a solid body without a fluid-filled cavity. another because there is no circulatory system. Section 2 Animal Body Plans 701 698_704_C24_S2_894586.indd 701 5/18/10 4:52 PM  Development in Coelomate Animals mollusks annelids arthropods echinoderms chordates The evolutionary tree in Figure 11 begins at the coelomate branch. Notice that two major lines of development have been identified in coelomate animals. One is protostome development, which occurs in animals such as snails, earthworms, and spiders. The other is deutero- stome development, which occurs in animals such as sea urchins, dogs, and birds. Biologists can tell if animals are closely related based on protostome deuterostome their patterns of embryonic development. Protostomes In organisms that are protostomes (PROH tuh stohms), coelom the mouth develops from the first opening in the gastrula. As proto- stomes develop, the final outcome for each cell in the embryo cannot be altered. If one cell of the embryo is removed, the embryo will not develop Figure 11 This part of the evolutionary tree into a normal larva, as shown in Figure 12. In addition, in the eight-cell shows that protostomes and deuterostomes are stage of embryonic development, the top four cells are offset from the branches of coelomate animals. bottom four cells, giving the embryo a spiral appearance. As the embryo continues to develop, the mesoderm splits down the middle. The cavity between the two pieces of mesoderm becomes the coelom. Deuterostomes In organisms that are deuterostomes (DEW tihr uh stohms), the anus develops from the first opening in the gastrula. The mouth develops later from another opening of the gastrula. During the development of deuterostomes, the final outcome for each cell in the embryo can be altered. In fact, each cell in the early embryo, if removed, can form a new embryo, as shown in Figure 12. In contrast to protostome development, in the eight-cell stage of embryonic deutero- stome development the top four cells are directly aligned on the bottom four cells. As the embryo develops, the coelom forms from two pouches of mesoderm. Reading Check Determine whether you are classified as a protosome or a deuterostome. Explain. 2 Examine Body Plans ? Inquiry MiniLab What is the importance of a body plan? One way to classify animals is by body plan. Looking at cross sections of different animals can help you distinguish between the different body plans. Procedure 1. Read and complete the lab safety form. 2. Obtain prepared slides of cross sections of an earthworm and a hydra. Using a microscope, observe each slide under low-power magnification. 3. Sketch each cross section. 4. Obtain labeled diagrams of cross sections of each animal from your teacher. Make a list of how your sketches are like the diagrams and another list of how they are different. Analysis 1. Compare and contrast each animal’s type of body cavity. Are they acoelomate or coelomate? What do your observations tell you about the phylogeny of these animals? 2. Infer how the body plan of each animal is related to how each of these animals obtains food. 702 Chapter 24 Introduction to Animals 698_704_C24_S2_894586.indd 702 5/18/10 4:52 PM  Visualizing Protostome and Deuterostome Development Figure 12 Developmental differences characterize protostome and deuterostome development. Protostome Development Deuterostome Development A If one cell is removed from a protostome at the four-cell stage, the development of the embryo is altered. If a cell is removed in a deuterostome at this stage, each cell or group of cells is not altered Normal larvae develop and will develop into a normal embryo. Development altered B Another difference is apparent at the eight-cell stage. In protostomes, the four cells are between the other four cells. In deuterostomes, the cells align. Cells not aligned Cells aligned C A blastula forms in both types of development. Endoderm Mesoderm Ectoderm D Note the location of mesoderm Ectoderm as the gastrula forms. Mesoderm Endoderm Gut Gut E As the embryo continues to Gut Gut develop, the mesoderm splits in protostomes to form the coelom. Mesoderm In deuterostomes, the coelom is pouches formed from pouches of form Split in mesoderm that separate from mesoderm the gut. Anus Mouth F The opening in the gastrula, Coelom called a blastopore, becomes the Coelom mouth in protostomes and the anus in deuterostomes. Blastopore (mouth) Blastopore (anus) Concepts in Motion Animation Section 2 Animal Body Plans 703 698_704_C24_S2_894586.indd 703 5/18/10 4:52 PM  mollusks annelids arthropods echinoderms chordates segmentation segmentation coelom Scorpion Figure 13 Segmentation enables a scorpion to move its stinger in different Segmentation directions to attack prey or for defense. Examine the next branching point on the evolutionary tree in Figure 13. Segmentation is an important feature in the evolution of coelomate animals. Just as a chain is constructed from a series of links, segmented animals can be “put together” from a succession of similar parts. The segmentation, such as that seen in scorpions, has two advan- tages. First, segmented animals can survive damage to one segment because other segments might be able to carry out the damaged sec- tion’s function. Second, movement is more effective because segments can move independently. Therefore, the scorpion in Figure 13 has more flexibility and can move in ways that are very complex. Segments allow the scorpion to arch its tail over its back to sting prey. Section 2 Assessment Section Summary Understand Main Ideas ◗ Animal phylogeny can be compared to a 1. MAIN Idea Explain how body symmetry is related to the phylogeny of tree with branches. animals. ◗ The branches of a phylogenetic evolutionary 2. Name the features marking the main branching points on the evolutionary tree show the relationships among animals. tree of animals. ◗ Animal phylogeny can be determined, in 3. Illustrate how body cavities distinguish branches of development of part, by the animal’s type of body cavity or animals with bilateral symmetry. lack of a body cavity. 4. Compare and contrast deuterostome and protostome development. ◗ After gastrulation, two types of development Think Critically can occur in coelomate animals. 5. Diagram animals not shown in Figure 9 that have radial and bilateral ◗ Segmentation is an important feature in symmetry. Indicate the type of symmetry by showing planes passing some coelomate animals. through the animals. Label each animal as having either radial or bilateral symmetry. Biology 6. Write a paragraph summarizing the differences among coelomates, pseudocoelomates, and acoelomates. 704 Chapter 24 Introduction to Animals Assessment Online Quiz 698_704_C24_S2_894586.indd 704 5/18/10 4:52 PM  Section 3 Reading Preview Essential Questions Sponges and Cnidarians ◗ What are the characteristics of sponges and cnidarians? MAIN Idea Sponges and cnidarians were the first animals to ◗ How are sponges and cnidarians evolve from a multicellular ancestor. alike and different? Real-World Reading Link Have you ever double-bagged your groceries? If so, ◗ What is the ecological importance of you have an idea of how a sponge is structured—a layer, or sac, of cells within sponges and cnidarians? another sac of cells. These sacs of cells are among the first animals to evolve Review Vocabulary from the common ancestor of all animals. diploid: cell with two of each kind of chromosome Sponges New Vocabulary If you examine a living sponge, you might wonder how these animals filter feeder do so much with so little. They have no tissues, no organs, and most sessile have no symmetry. You can break apart a sponge into its individual cnidocytes cells and those cells will come together again to form a sponge. Other nematocyst animals cannot do this. gastrovascular cavity Locate sponges on the evolutionary tree in Figure 14. They are in nerve net polyp the phylum Porifera (po RIF uh ruh), which contains between 5000 and medusa 10,000 members. Most live in marine environments. Biologists hypoth- esize that sponges evolved from the colonial choanoflagellates because g Multilingual eGlossary sponges have cells that look similar to these protist cells. Body structure Notice the asymmetrical appearance and bright colors of the sponge in Figure 14. It is difficult to think that these are animals, especially if you see one washed up on a beach where it might appear as a black blob. Recall that tissues form from ectoderm, endo- Figure 14 The sponges in the photograph derm, and mesoderm in a developing embryo. Sponge embryos do not are animals that take in and digest food, grow, develop endoderm or mesoderm, and, therefore, sponges do not develop and reproduce, even though they lack true tissues. How does a sponge’s body function without tissues? tissues. sponges cnidarians flatworms roundworms rotifers mollusks annelids arthropods echinoderms chordates no true tissues tissues multicellularity Ancestral protist Section 3 Sponges and Cnidarians 705 705_715_C24_S3_894586.indd 705 5/18/10 4:54 PM  Pore Collar cell Epithelial- like cell Direction of water flow through pores Osculum Spicule Archaeocyte Figure 15 Sponges have no tissues or organs and have a body made of two layers of cells. Two layers of independent cells with a jellylike substance between the layers accomplish all of the life functions of sponges. As illustrated Concepts in Motion in Figure 15, epithelial-like cells cover the sponge and protect it. Collar Animation cells with flagella line the inside of the sponge. As collar-cell flagella whip back and forth, water is drawn into the body of the sponge through pores. These pores give sponges their phylum name Porifera, which means “pore-bearer.” Water and waste materials are expelled from the sponge through the osculum (AHS kyuh lum), which is the mouthlike opening at the top of the sponge. Study Tip Feeding and digestion When an organism, such as a sponge, gets its food by filtering small particles from water, it is called a Think Aloud Read the text and filter feeder. Even though this might sound like a process that is not captions aloud. As you read, say aloud your questions and comments. For very active, consider that a sponge only 10 cm tall can filter as much instance, when you come to the as 100 L of water each day. Although sponges have free-swimming mention of Figure 15, look at the larvae, the adults move very little. Adaptations for filter-feeding are figure and say how it relates to the common in animals that are sessile (SES sul), meaning they are text. attached to and stay in one place. As nutrients and oxygen dissolved in water enter through the pores in a sponge’s body, food particles cling to the cells. Digestion of nutrients takes place within each cell. Reading Check Infer why filter feeding is an adaptive advantage for sponges. 706 Chapter 24 Introduction to Animals 705_715_C24_S3_894586.indd 706 5/18/10 4:54 PM  Demosponge Figure 16 Bath sponges are harvested from the sea and processed for human use. Support Within the jellylike material that lies between the two cell lay- ers of a sponge are amoeba-like cells—cells that can move and change shape. These amoeba-like cells are called archaeocytes (ar kee OH sites) and are illustrated in Figure 15. These cells are involved in digestion, production of eggs and sperm, and excretion. Archaeocytes also can become specialized cells that secrete spicules (SPIH kyuhls), the support structures of sponges. Spicules are small, needlelike structures made of calcium carbonate, silica, or a tough fibrous protein called spongin. Sponge diversity Biologists place sponges into three classes based Video BrainPOP on the type of support system each has. Most sponges belong to class Demospongiae (deh muh SPUN jee uh), the demosponges, and have spicules composed of spongin fibers, silica, or both. Natural bath sponges, like the ones in Figure 16, have spongin support. Class Cal- carea (kal KER ee uh) consists of sponges with spicules composed of cal- cium carbonate. Calcareous sponges, like the one in Figure 17, often have a rough texture because the calcium carbonate spicules can extend through the outer covering of the sponge. The sponges in class Hexactinellida (heks AK tuh nuh LEE duh) are called glass sponges and have spicules composed of silica. These spicules join together to form a netlike skeleton that often Figure 17 Calcareous sponges are small looks like spun glass, as illustrated in Figure 17. and have a rough texture. The skeletons of glass sponges look like brittle spun glass. Calcareous sponge Glass sponge skeleton Section 3 Sponges and Cnidarians 707 705_715_C24_S3_894586.indd 707 5/18/10 4:54 PM  B Sperm are caught by the collar A Sperm are released into the water cells of another sponge, and and float on water currents to eggs are fertilized internally. other sponges. Free-swimming larvae are released. C The larvae swim using tiny cilia. E A sessile larva develops into an adult that can reproduce. D A larva eventually settles on a surface. Figure 18 Sexual reproduction in sponges requires water currents to carry sperm Response to stimuli Sponges do not have nervous systems. They do from one sponge to another. have epithelial-like cells that detect external stimuli, such as touch or Evaluate whether fertilization is chemical signals, and respond by closing their pores to stop water flow. internal or external in sponge sexual reproduction. Reproduction Sponges can reproduce asexually by fragmentation, through budding, or by producing gemmules (JEM yewlz). In fragmen- tation, a piece of sponge that is broken off due to a storm or other event develops into a new adult sponge. In budding, a small growth, called a bud, forms on a sponge, drops off, and settles in a spot where it grows into a new sponge. Some freshwater sponges form seedlike particles called gemmules during adverse conditions like droughts or freezing temperatures. Gemmules contain sponge cells protected by spicules that will survive and grow again when favorable conditions occur. VOCABULARY Most sponges reproduce sexually, as illustrated in Figure 18. Some ACADEMIC VOCABULARY sponges have separate sexes, but most sponges are hermaphrodites. Survive Recall that a hermaphrodite is an animal that can produce both eggs to remain alive and sperm. During reproduction, eggs remain within a sponge, while Sponge gemmules survive despite sperm are released into the water. Sperm released from one sponge can adverse conditions. be carried by water currents to the collar cells of another sponge. The collar cells then change into specialized cells that carry the sperm to an egg within the sponge body. After fertilization occurs, the zygote devel- ops into a larva that is free-swimming and has flagella. The larva eventu- ally attaches to a surface, then develops into an adult. Reading Check Describe the methods by which sponges reproduce. 708 Chapter 24 Introduction to Animals 705_715_C24_S3_894586.indd 708 5/18/10 4:54 PM  Sponge ecology Although spicules and toxic or dis- tasteful compounds in sponges discourage most potential predators, sponges are food for some tropical fishes and turtles. Sponges also are common habitats for a variety of worms, fishes, shrimp, and colonies of symbiotic green algae. Some sponges even live on and provide camouflage for mollusks, as shown in Figure 19. Sponges also are beneficial to humans. Sponges with spicules made of spongin fibers often are used for household scrubbing purposes. Medical research is focus- ing on sponge chemicals that appear to discourage prey and prevent infection. Ongoing studies of these sponge chemicals as possible pharmaceutical agents have shown that they might have antibiotic, anti-inflamatory, or antitumor possibilities. They also might have potential importance as respiratory, cardiovascular, and gastroin- testinal medicines. Connection to Health For example, researchers dis- covered a powerful antitumor substance in the deep Figure 19 This crab hides from predators by carrying a living sponge on its back. The crab uses two pairs of legs to hold the water sponge shown in Figure 20. This substance, dis- sponge in place. codermolide (disk uh DER muh lide), stops cancer cells from dividing by breaking down the nucleus and rear- ranging the microtubule network. Recall that micro- tubules are part of a cell’s skeleton and help the cell maintain its shape. Note the differences in the nuclei and microtubules between the untreated and treated cancer cells in Figure 20. Figure 20 Discodermolide, a substance taken from the sponge Discodermia dissoluta, breaks down the nucleus in a cancer cell and rearranges its microtubules. LM Magnification: unavailable Nucleus Microtubules LM Magnification: unavailable Untreated cancer cells Microtubules Nucleus Discodermia dissoluta Treated cancer cells Section 3 Sponges and Cnidarians 709 705_715_C24_S3_894586.indd 709 5/18/10 4:54 PM  sponges cnidarians flatworms roundworms rotifers mollusks annelids arthropods echinoderms chordates radial symmetry Jellyfish—free floating Sea anemone—sessile Ancestral protist Figure 21 Cnidarians have radial symmetry and can be free floating or sessile. Cnidarians Explain how radial symmetry helps a cnidarian Imagine that you go snorkeling around a coral reef, and you obtain food. wear a bodysuit to protect yourself from the stings of jelly- fishes that float on the water. Later, when you go ashore to visit a tidepool, you might see colorful sea anemones that look somewhat like flowers. The jellyfish and sea anemone in Figure 21 belong to phylum Cnidaria (ni DARE ee uh). This phylum consists of about 10,000 species, most of which are marine. Body structure Like sponges, cnidarians (ni DARE ee uns) have one body opening and most have two layers of cells. However, in cnidarians, the two cell layers are organized into tissues with specific functions. The outer layer functions in protecting the internal body, while the inner layer functions mainly in digestion. Because cnidarians have tissues, they also have symmetry. As shown in Figure 21, cnidarian bod- ies have radial symmetry. Recall that radial symmetry enables slow moving or sessile animals to detect and capture prey from any direction. Cnidarians are adapted to aquatic Figure 22 Stinging cells that contain nematocysts are floating or sessile attachment to surfaces under the water. discharged from the tentacles of cnidarians when prey touches them. Feeding and digestion Cnidarian tentacles are armed with stinging cells called cnidocytes (NI duh sites). Cni- Cnidocyte darians get their name from these stinging cells. Cnido- Threadlike tube cytes contain nematocysts, as shown in Figure 22. A Trigger nematocyst (nih MA tuh sihst) is a capsule that holds a coiled, threadlike tube containing poison and barbs. Nematocyst Connection to PhysicsA nematocyst works like a tiny but very powerful harpoon. Remember that osmosis is the diffu- sion of water through a selectively permeable membrane. The pressure provided by this flow of water is called osmotic pressure. The water inside an undischarged nematocyst is under an osmotic pressure of more than 150 atmospheres. This pressure is about 20 times the pressure in an inflated bicycle tire. 710 Chapter 24 Introduction to Animals 705_715_C24_S3_894586.indd 710 1/11/12 9:57 AM  In response to being touched or to a chemical stimulus, the permeability of the nematocyst membrane increases, allowing more water to rush in. As the osmotic pressure increases, the nematocyst discharges forcefully. A barb is capable of penetrating a crab shell. Nematocyst discharge is one of the fastest cellular proc- Tentacles esses in nature. It happens so quickly—in just 3/1000ths of a second—that it is impossible to escape after touching these Mouth/anus cells. After capture by nematocysts and tentacles, the prey is brought to the mouth of the cnidarian. Two layers The inner cell layer of cnidarians surrounds a space called the gastrovascular (gas troh VAS kyuh lur) cavity, Gastrovascular illustrated in Figure 23. Cells lining the gastrovascular cavity cavity release digestive enzymes over captured prey. Undi- gested materials are ejected through the mouth. Recall that digestion occurs within each cell of a sponge. However, in cnidarians, digestion takes place in the gut cavity, a major evolutionary adaptation. Figure 23 A cnidarian’s mouth leads directly into its Response to stimuli In addition to cells adapted for gastrovascular cavity. Because the digestive tract has only one opening, wastes are expelled through the mouth. digestion, cnidarians have a nervous system consisting of a nerve net that conducts impulses to and from all parts of the body. The impulses from the nerve net cause contractions of musclelike cells in the two cell layers. The movement of tenta- cles during prey capture is the result of contractions of these musclelike cells. Cnidarians have no blood vessels, respiratory systems, or excretory organs. Look at Table 1 to compare the structures and functions of sponges and cnidarians. Reading Check Contrast a cnidarian’s response to stimuli from a sponge’s response. Comparison of Sponges Table 1