|
|
26-1 Introduction to the Animal Kingdom
Table of Animal Phyla The Animal World At A Glance
Of all the kingdoms of organisms, the animal kingdom is the most diverse in form. Some animals have forms that are comfortingly familiar. Others resemble creatures from a nightmare or a horror movie. Some animals are so small that they can live inside our bodies. Others are many meters long and live in the depths of the sea. Animals can be black, white, beautifully colored, or nearly transparent. Animals walk, swim, crawl, burrow, and fly all around us. In every case, each animal performs the essential functions of life in its own special way.
You will soon become acquainted with several major divisions in the animal kingdom. One division that we refer to often is that between vertebrates and invertebrates. Vertebrates, such as humans, have a backbone, or vertebral column. Invertebrates, are the subjects of this unit, have no backbone.
As different as they are, all animals share certain basic characteristics. Animals are heterotrophs, which means that they do not make their own food. Instead, they obtain the nutrients and energy they need by feeding on organic compounds that have been made by other organisms. Animals are multicellular, which means that their bodies are composed of more than one cell. And animal cells are eukaryotic-they contain a nucleus and membrane-enclosed organelles. Unlike plant cells or fungus cells, animal cells do not have cell walls. We can thus define an animal as a multicellular eukaryotic heterotroph whose cells lack cell walls.
Cell Specialization and Division of Labor
The bodies of animals contain many types of specialized squirrel is munching on a hazelnut, cells. Each specialized cell has a shape, physical structure, and the crayfish is nibbling on a chemical composition that make it uniquely suited to perform a particular function within a multicellular organism. For this reason, groups of specialized cells carry out different tasks for the organisms phenomenon known as division of labor.
You may wonder what advantage there is in dividing up different tasks among specialized cells. After all, monerans and protists do just fine as single cells! But large numbers of cells growing together simply cannot function the way single cells do. Recall from Chapter 8 that cells require a certain amount of surface area to take in food and oxygen and remove wastes. Cells that grow together have little, if any, of their surface exposed to the environment. They would soon be starved for I food and oxygen and smothered in carbon dioxide and other wastes if there were no efficient systems to carry out essential functions such as feeding, respiration, and elimination of wastes. In multicellular organisms, efficient systems require specialization. Specialized cells can perform their tasks more efficiently than unspecialized cells.
What Animals Must Do to Survive
In order to survive, animals must be able to perform a number of essential functions. For each animal group we study in the next several chapters, we shall examine these functions and describe the cells, tissues, organs, and organ systems that perform them. To help you make a checklist of those functions, we shall briefly describe them here.
FEEDING
Animals have evolved a variety of ways to feed. Herbivores, or animals that eat plants, may feed on roots, stems, leaves, flowers, or fruits. Some herbivores even feed on the nutrient-rich fluids in plant vascular tissues. Carnivores, or organisms that eat animals, may also feed on any part of their prey-fat, muscle, bone marrow, or even blood. Parasites live and feed either inside or attached to outer surfaces of other organisms, thereby doing harm to their hosts. Many aquatic animals, called filter feeders, strain tiny floating plants and animals from the water around them. And many animals feed not on living organisms but on tiny bits of decaying plants and animals called detritus (dee-TRIGHT-uhs). Detritus feeders are easy to overlook, but they are vitally important members of the living world.RESPIRATION
As you learned in Chapter 6, living cells consume oxygen and give off carbon dioxide in the process of cellular respiration. Thus entire animals must respire, or breathe, in order to take in and give off these gases. Small animals that live in water or in moist soil may respire through their skin. For large active animals, however, respiration through the skin is not efficient enough. The respiratory systems these animals have evolved take many different forms in adaptations suited to different habitats.INTERNAL TRANSPORT
Some aquatic animals, such as small worms, can function without an internal transport system. But once an animal reaches a certain size, it must somehow carry oxygen, nutrients, and waste products to and from cells deep within its body. Thus many multicellular animals have evolved a circulatory system in which a pumping organ called a heart forces a fluid called blood through a series of blood vessels. You will see in the next several chapters that circulatory systems can be simple or quite complex.EXCRETION
Cellular metabolism produces chemical wastes such as ammonia that are harmful and must be eliminated. Small aquatic animals depend on diffusion to carry wastes from their tissues into the surrounding water, which then carries the wastes away. But larger animals, both in water and on land, must work to remove poisonous metabolic wastes. As we study animals from worms to mammals, we shall follow the development of the excretory systems that store and dispose of these wastes.RESPONSE
Animals must keep watch on their surroundings to find food, spot predators, and identify others of their own kind. To do this, animals use specialized cells called nerve cells, which hook up together to form a nervous system. Sense organs, such as eyes and ears, gather information from the environment by responding to light, sound, temperature, and other stimuli. The brain, which is the nervous system's control center, processes the information and regulates how the animal responds. The complexity of the nervous system varies greatly in animals.MOVEMENT
Some animals are sessile, which means that they live their entire adult lives attached to one spot. But many animals are motile, which means that they move around. To move, most animals use tissues called muscles that generate force by contracting. In the most successful groups of animals, muscles work with a skeleton, or the system of solid support in the body. Insect and their relatives wear their skeleton on the outside of their body. These are called exoskeletons. Reptiles, birds and mammals, have their skeletons on the inside of their body. These are called endoskeleton. We call the combination of an animal's muscles and skeleton its musculo-skeletal system.REPRODUCTION
Animals must reproduce or their species will not survive. Because reproduction is so important, and because animals use many different methods to reproduce, we shall spend a lot of time studying reproduction. Some animals, such as jellyfish, switch back and forth between sexual and asexual reproduction. (Note that this is not the same as alternation of generations in plants, during which diploid (2N) and haploid (N) generations alternate. The sexual and asexual generations in animals are both diploid.) Many animals that reproduce sexually bear their young alive. Others lay eggs. The eggs of some species hatch into baby animals that look just like miniature adults. As they grow, these baby animals increase in size but do not change in overall form. This type of development is called direct development. In other species, eggs hatch into larvae (singular: larva), which are immature stages that look and act nothing like the adults. As larvae grow, they undergo a process called metamorphosis in which they change shape dramatically. This type of development is called indirect development.As we explore the invertebrate phyla, keep in mind that these phyla share an evolutionary heritage. In Chapter 30, the relationships between the different phyla of invertebrates will be represented in an evolutionary tree of the animal kingdom. This evolutionary tree will show our best understanding of the way in which animal phyla are related to one another. For now, focus on tracing a few important evolutionary trends and patterns as you move from one animal phylum to the next.
The levels of organization become higher as animals become more complex in form. The essential functions of less complex animals are carried out on the cell or tissue level of organization. As you move on to more complex animals, you will observe a steady increase in the number of specialized tissues. You will also see those tissues joining together to form more and more specialized organs and organ systems.
Some of the simplest animals have radial symmetry; most complex animals have bilateral symmetry. Some of the simplest animals, such as sea anemones, have body parts that repeat around an imaginary line drawn through the center of their body. These animals exhibit radial symmetry. See Figure 26-7. Animals with radial symmetry never have any kind of real "head." Many of them are sessile, although some drift or move about in a more or less random pattern. Complex invertebrates and all vertebrates have body parts (at least outside body parts such as arms and legs) that repeat on either side of an imaginary line drawn down the middle of their body. One side of the body is the mirror image of the other. These animals are said to have bilateral symmetry. Animals with bilateral symmetry have specialized front and back ends as well as upper and lower sides. The anterior is the front end and the posterior is the back end. The dorsal is the upper side and the ventral is the lower side.
More complex animals tend to have a concentration of sense organs and nerve cells in their anterior (head) end. Because animals with bilateral symmetry usually move with their anterior end forward, this end encounters new parts of the environment first. As you might imagine, natural selection favors animals that can sense the nature of the environment into which they are moving before their entire body is exposed to the new environment. It is not wise to back up into a potentially dangerous situation! Thus sense organs tend to gather at the anterior end. As the sense organs collect up front, so do the nerve cells that process information and "decide" what the animal should do. Eventually, the anterior end is different enough from the posterior end that we call it a head. This gathering of sense organs and nerve cells into the head region is called cephalization (cephalo- means head).
Cephalization becomes more pronounced as animals become more complex. Nerve cells in the head gather into clusters that process the information gathered by the nervous system and control responses to stimuli. Small clusters of nerve cells are called ganglia (singular: ganglion). In the most complex animals, large numbers of nerve cells gather together to form larger structures called brains.
Section Objectives
• Describe the structure of a sponge.
• Discuss how sponges perform
Porifera Lesson Phylum Porifera DIAGRAM-Porifera-Sponge DIAGRAM-Scypha
Sponges are among the most ancient of all animals that are alive today. The first sponges date back to the beginning of the Cambrian Period (about 580 million years ago), when the first traces of multicellular animals appeared in the fossil record. Most sponges live in the sea, although a few live in freshwater lakes and streams. Sponges inhabit almost all areas of the sea from the polar regions to the tropics and from the low-tide line down into water several hundred meters deep. Sponges belong to the phylum Porifera (por-IHF-er-ah). This name, which literally means pore-bearers, is appropriate because sponges have tiny openings all over their body. Sponges were once thought to be plants, which is easy to understand in light of the fact that adult sponges are sessile and show little detectable movement. As far as modern biologists are concerned, sponges are clearly multicellular animals -sponges are heterotrophic, have no cell walls, and contain several specialized cell types that live together. But sponges are very different from all other animals. Sponges have nothing that even vaguely resembles a mouth or gut, and they have no specialized tissues or organ systems. For these reasons, most biologists believe that sponges evolved from single-celled ancestors separately from other multicellular animals. The evolutionary line that gave rise to sponges was a dead end that produced no other groups of animals.
The body plan of a typical sponge is simple. Refer to Figure 26-10 as you read about the structure of a sponge. The body of a sponge forms a wall around a central cavity. In this wall are thousands of openings, or pores. A steady current of water moves through these pores into the central cavity. This current is powered by the flagella of cells called collar cells. The water that gathers in the central cavity exits through a large hole called the osculum (AHS-kyoo-luhm). The current of water that flows through the body of a sponge delivers food and oxygen to the cells and carries away cellular waste products. The water also transports gametes or larvae out of the sponge's body.
Many sponges manufacture thin, spiny spicules that form the skeleton of the sponge. A special kind of cell called an amebocyte (ah-MEE-boh-sight) builds the spicules from either chalklike calcium carbonate (CaCO,) or grasslike silica (SiO,). These spicules interlock to form beautiful and delicate skeletons, such as the Venus' flower basket shown in Figure 26-11 on page 562. The softer but stronger sponge skeletons that we know as natural bath sponges consist of fibers of a protein called spongin. Some sponges have skeletons that are made up of both spongin and spicules.
Sponges are filter feeders that sift microscopic particles of food from the water that passes through them. As the water moves through the sponge, tiny food particles stick to the collar cells. The trapped particles are then engulfed by the collar cells (endocytosis), where they may be digested. If the collar cells do not digest the food, they pass it on to the amebocytes. When the amebocytes are finished digesting the food particles, they wander around, delivering digested food to other parts of the sponge. Note that all digestion in sponges is intracellular; as its respiratory, excretory, and internal transport system. As water passes through the body wall, sponge cells remove oxygen from it and give off carbon dioxide into it. Metabolic wastes produced by cellular respiration (such as ammonia) are also released into the water, which carries them away. The amount of water that is pumped through a sponge is amazing. A sponge 10 centimeters in height and I centimeter in diameter was found to pump 22.5 liters of water per day through its body.
The water that flows through the body of the sponge also plays a role in sexual reproduction. Although eggs are kept inside the body wall of the sponge, sperm are released into the water flowing through the sponge and are thus carried out into the open water. If those sperm are taken in by another sponge, they are picked up by amebocytes and carried to that sponge's eggs, where fertilization occurs. The zygote (fertilized egg) that results develops into a larva that swims and can be carried by currents for a long distance before it settles down and grows into a new sponge.
Sponges reproduce asexually as well as sexually. Faced with cold winters, some freshwater sponges produce structures called gemmuls (JEHM-yooIZ). Gemmules are sphere-shaped collections of amebocytes surrounded by a tough layer of spicules. Gemmules can survive long periods of freezing temperatures and drought, which would kill adult sponges. When conditions again become favorable, gemmules grow into new sponges. Sponges can also reproduce asexually by budding. In this process, part of a sponge simply falls off the parent and larva grows into a new sponge.
Budding is one indication of the sponges' remarkable powers of regeneration (the ability to regrow a lost or damaged part). In fact, if you were to grind up a sponge, separate its cells by passing them through a filter, and place the cells in a container of water, the cells would clump together and grow into several new sponges! It is not surprising, therefore, that sponges can easily repair torn body parts.
Why do sponges have few predators?The living material of a sponge is thinly distributed through a scaffold of indigestible skeletal material. Eating a sponge would be a bit like eating a mouthful of glass toothpicks, or a bundle of fishing line, dipped in a thin soup
What are the advantages and disadvantages of separate sexes?The advantages of separate sexes are mainly tied to the maintenance of a high degree of adaptability, evolutionary flexibility and genetic diversity. The offspring of two different individuals are likely to be more genetically diverse and carry a lower proportion of accumulated genetic damage than the offspring of a single monoecious organism. In addition, dioecious reproduction often involves mate selection and competition, which allows only the healthiest, most evolutionarily fit individuals to reproduce. Within-species competition for access to mates only occurs when the sexes are separate, and is an important evolutionary force.
The most important advantage of monoecious organisms is the ability to reproduce quickly, without having to endure the energetically wasteful and often self-endangering process of searching for and evaluating a mate. Even monoecious organisms that require two individuals for reproduction to occur (for example, earthworms) use less energy looking for a mate because they do not need to be of the opposite sex (any other individual will do). Also, since both organisms produce offspring, fecundity (total number of offspring produced) is high. The disadvantages faced by monoecious organisms include the reduced amount of genetic diversity generated in species with self-fertilization and the loss of any useful, evolution-generating within-species competition for mates of the opposite sex. Can you think of others?
What is the adaptive value of a ciliated, motile larva?Dispersal is an important characteristic of many species that helps them to invade new environments, to avoid overcrowding and excessive competition, and to promote genetic diversity by preventing inbreeding (reproduction between related individuals). Many organisms have a life cycle stage that is specially adapted for travel over long distances. In the Cnidaria, the planula larva is used for this purpose. Its small size and mobility allow it to be carried long distances by ocean currents, then settle and develop into an adult when it encounters a favorable habitat. What are the possible disadvantages of dispersal
How Sponges Fit into the WorldSponges are often the most common forms of life in dark places such as the walls of underwater caves and on dock pilings. Many other marine animals-certain kinds of worms, shrimp, snails, and starfish, for example-live on, in, and under sponges. Sponges are also involved in symbiotic relationships with organisms that are not animals. Certain sponges contain symbiotic bacteria, blue-green bacteria, or plantlike protists. The photosynthetic symbionts provide food and oxygen to the sponge and remove wastes. Although sponges produce spicules and protective chemicals that discourage most animals from feeding on them, sponges are important parts of the diets of certain snails, starfish, and fishes.
The family of sponges known as the boring sponges are particularly important in "cleaning up" the ocean floor. Special amebocytes in these sponges release chemicals that allow the sponges to bore, or drill, tunnels through old shells and pieces of coral. These tunnels weaken the shells and coral and thus help break them down.
Since the time of the Greeks and Romans, humans have used the dried and cleaned bodies of some sponges in bathing. Most sponges you see in supermarkets today are artificial, but natural bath sponges are still available. Recently, scientists have found uses for parts of the sponge other than its skeleton. In a series of exciting new developments, scientists are learning to use several chemicals manufactured by sponges.
Because sponges cannot move, they must protect themselves from their enemies in other ways. Bacteria, algal spores, and many tiny organisms are constantly looking for surfaces on which to settle. To protect themselves from being overgrown by these organisms, sponges manufacture numerous compounds that are toxic to such organisms. These chemicals also discourage many animals from chewing on sponges. Researchers have found that many of these chemicals are powerful antibiotics that can be used to fight bacteria and fungi that cause disease. Other sponge chemicals act against viruses almost as well as antibiotics fight bacteria. One compound taken from a Caribbean sponge may be useful against leukemia and herpes viruses. Another may help fight certain forms of arthritis. Still other sponge chemicals may be effective against the bacteria that cause strep throat and those that become resistant to penicillin. Although most of these drugs are still in the experimental stage, scientists hope that they will soon be ready for human use.
Section objectives
Cnidarian Systemics Cnidarian Morphology Cnidarian Characteristics Phylum Cnidaria Diagram-Cnidaria
The phylum Cnidaria (nigh-DAIR-ee-ah) includes many animals with brilliant colors and unusual shapes. Delicate jellyfish float in ocean currents. Brightly colored sea anemones cling to rocks, looking more like underwater flowers than animals. These beautiful and fascinating animals are found all over the world, but most species live only in the sea.
Cnidarians are soft-bodied animals with stinging tentacles arranged in circles around their mouth. Some cnidarians live as single individuals. Others live as groups of dozens or even thousands of individuals connected into a colony. All cnidarians exhibit radial symmetry and have specialized cells and tissues. Many cnidarians have life cycles that include two different-looking stages, the sessile flowerlike polyp .(PAH-lihp) and the motile bell-shaped medusa (meh-DOO-sah).
The body plans of a typical cnidarian polyp and a medusa are shown in Figure 26-15. As you can see, both polyps and medusae have a body wall that surrounds an internal space called the gastrovascular cavity. It is in the gastrovascular cavity that digestion takes place. The body wall consists of three layers: epidermis, mesoglea, and gastroderm. The epidermis is a layer of cells that covers the outer surface of the cnidarian's body. The gastroderm is a layer of cells that covers the inner surface, lining the gastrovascular cavity. Between these two cell layers is the mesoglea (mehz-oh-GLEE-ah). The mesoglea ranges from a thin noncellular membrane to a thick jellylike material that may contain wandering amebocytes. In general, the mesoglea is a thin layer in polyps and a thick layer in medusae.
Form and Function in Cnidarians
Almost all cnidarians capture and eat small animals by using stinging structures called nematocysts (neh-MAT-ohsihsts), which are located on their tentacles. Each nematocyst is a poison-filled sac containing a tightly coiled "springloaded" dart. When another animal touches a nematocyst, the dart uncoils as if it had exploded and buries itself in the skin of the animal. The dart carries with it enough poison to paralyze or kill the prey. Once the prey is rendered helpless, the cnidarian's tentacles push the food through the mouth and into the gastrovascular cavity. There the food is gradually broken up into tiny pieces. These food fragments are taken up by special cells in the gastroderm that digest them further. The nutrients are then transported throughout the body by diffusion. Any materials that cannot be digested are passed back out through the mouth, which is the only opening in the gastrovascular cavity, several hours later.
Although most cnidarians are considered carnivores, many do not actually "eat" much, thanks to an extraordinary symbiosis, which we talked about in Chapter 18. In many cnidarians, tiny photosynthetic protists grow right inside the living cells of the gastroderm. This relationship between autotrophic protist and heterotrophic animal works very efficiently. The photosynthetic protists use the carbon dioxide and other metabolic wastes produced by the cnidarian's cells to manufacture oxygen and organic compounds such as carbohydrates and proteins. The protists use some of the oxygen and organic compounds themselves and release the rest into the tissues of their cnidarian hosts. Many cnidarians depend on this symbiosis to such an extent that they can live only in bright sunlight! These cnidarians will slowly starve if kept in a darkened laboratory tank, even if they are fed pieces of shrimp and fish. - Because most cnidarians are only a few cell layers thick, they have not had to evolve many complicated body systems in order to survive. Some colonial cnidarians and some jellyfish have long, tube-shaped, branching gastrovascular cavities that help carry partially digested food through their bodies. Because these animals live in clean constantly flowing water, they can respire and eliminate waste products by diffusion directly through their body walls. There is no organized internal transport network or excretory system in cnidarians.
Cnidarians also lack a centralized nervous system and anything that could be called a brain. They have simple nervous systems called nerve nets. The nerve net is concentrated around the mouth, but it does spread throughout the body.
Information about the environment is transmitted to the rest of a cnidarian's nervous system by specialized sensory cells. Both polyps and medusae have sensory cells in the epidermis that detect chemicals from food and the touch of foreign objects. In medusae, some groups of sensory cells are organized into simple organs. These organs, which are called statocysts and ocelli, are arranged around the rim of a medusa's bell. Statocysts are involved with balance-they help an organism determine which way is up. Ocelli (oh-SEHL-igh; singular: ocellus), or eyespots, detect the presence of light.
Cnidarians lack the muscle cells that most other animals use to move about. But many of the epidermal cells in cnidarians can change shape when stimulated by the nervous system. Thus these cells serve the same function as muscles. Cnidarian polyps can expand, shrink, and move their tentacles by relaxing or contracting these epidermal cells. In medusae, contractions of the special epidermal cells change the bell-shaped body, causing it to "close" like a folding umbrella. The "closing" of the body pushes water out of the bell. This moves a medusa forward by jet propulsion.
Most cnidarians can reproduce both sexually and asexually. As you can see in Figure 26-17, polyps can produce new polyps asexually by budding. Budding begins with a swelling on the side of an existing individual. This swelling eventually grows into a complete polyp. Many polyps also reproduce asexually by budding off tiny medusae. When the medusae mature, they reproduce sexually by releasing gametes into the water. Depending on the species, fertilization occurs either in open water or inside an egg-carrying medusa. The zygote (fertilized egg) grows into a ciliated larva that swims around for some time. Later, the larva settles down, attaches to a hard surface, and changes into a polyp that begins the cycle again.
DIAGRAM-Hydra
DIAGRAM-ObeliaThe class Hydrozoa (high-droh-ZOH-ah) is made up of cnidarians that spend most of their lives as polyps, although they reproductive polyps will eventually usually have a short medusa stage. As you can see in Figure 26-18, most hydrozoan polyps grow in branching sessile colonies. Hydrozoan colonies range in length from a few centimeters to more than a meter. In each of these colonies, specialized polyps perform particular functions, such as feeding, reproduction, or defense. Reproductive polyps produce free-swimming medusae by budding. These medusae are usually less than 2 centimeters in diameter. Soon after they form, the medusae produce both eggs and sperm and then die.
The most common freshwater hydrozoans are the hydras. Hydras are not typical hydrozoans because they live as solitary polyps and lack the medusa stage in their life cycle. Unlike most other polyps, hydras can move around with a curious somersaulting movement. Hydras can reproduce either asexually by budding or sexually by producing eggs and sperm in their body walls. In most species of hydras, the sexes are separate. In other words, individuals are either male or female. However, a few species are hermaphroditic. A hermaphrodite is an individual that has both male and female reproductive organs and thus produces both sperm and eggs
One unusual hydrozoan is the Portuguese man-of-war. These animals form floating colonies that contain several specialized kinds of polyps. In each Portuguese man-of-war, one polyp forms a balloonlike float that keeps the colony on the surface. This float may be up to 30 centimeters long. Some of the polyps in the colony produce long stinging tentacles that hang several meters below the float and paralyze and capture prey. Some polyps digest the food held by the tentacles, and still others do nothing but make eggs and sperm. Portuguese man-of-war nematocysts are strong enough to sting humans very badly, so swimmers and beach-goers must take care when these animals are spotted near shore.
Why might the lack of a medusa stage be adaptive to Hydra?The absence of a medusa stage is atypical of hydrozoans in general, but has probably evolved in Hydra because it would be a waste of valuable calories to pursue a polymorphic life cycle when a) Hydra typically lives in small bodies of water that it can colonize quickly without the use of a motile stage. b) It often lives in pools that are only temporary or will be frozen in the winter. Hydra therefore needs to complete its reproductive cycle quickly and this may not be possible if several different stages are involved.
Hydra Investigation Introduction to Hydra The Double Life of Obelia
DIAGRAM-Life Cycle of Jellyfish
The class Scyphozoa (sigh-foh-ZOH-ah) contains the true jellyfish. Jellyfish go through the same life-cycle stages as hydrozoans. However, in scyphozoans the medusa is large and long-lived, and the polyp is restricted to a tiny larval stage. Some jellyfish, such as the lion's mane, which is found in the north Atlantic, often grow up to 2 meters in diameter. The largest jellyfish ever found was more than 3.6 meters in diameter and had tentacles more than 30 meters long. The nematocysts of most jellyfish are harmless to humans, but a few can cause painful stings. One tiny Australian jellyfish has a toxin powerful enough to cause death in 3 to 20 minutes!
What is the composition of the "jelly" in a "jellyfish"?
Cnidarians have two true tissue layers that are separated by various amounts of a non-cellular gelatinous material called mesoglea. In the scyphozoan jellyfish, this layer is especially extensive and helps maintain the shape and elasticity of the organism. It consists mainly of water held as a gel in a matrix of fibrous collagen-like material. Some amoeboid and other cells may be present, which is why this layer is sometimes called "mesenchyme" (the "chyme" indicates the presence of living cellular material).
DIAGRAM-Coral
The class Anthozoa (an-thoh-ZOH-ah) contains sea anemones and corals, which are among the most beautiful and ecologically important invertebrates. Anthozoans have only the polyp stage in their life cycles. Adult polyps reproduce sexually by producing eggs and sperm that are released into the water. The zygote grows into a ciliated larva that settles to the ocean bottom and becomes a new polyp. Many anthozoans also reproduce asexually by budding.
Sea anemones are solitary polyps that live in the sea from the low-tide line to great depths. Although sea anemones can catch food with the nematocysts on their tentacles, many shallow-water species depend heavily on their photosynthetic symbionts. Some sea anemones can grow up to a meter in diameter.
Corals grow in shallow tropical waters around the world. Coral polyps are very similar in form to sea anemones. However, corals produce skeletons of calcium carbonate (CaCo3), or limestone. Although a few corals are solitary, most are colonial. As a coral colony grows, new polyps are produced by budding, and more and more limestone is laid down. Coral hard skeletons of calcium carbonate. colonies grow very slowly, but they may live for hundreds, or Most stony corals are colonial. even thousands, of years. Together, countless coral colonies produce huge structures called coral reefs. Some of these reefs are enormous and contain more rock and living tissue than even the largest human cities. The Great Barrier Reef off the coast of Australia is more than 2000 kilometers long and some 80 kilometers wide.
How Cnidarians Fit into the World
Cnidarians form a number of interesting symbiotic relationships with other animals. Certain fish, shrimp, and other small animals live among the tentacles of large sea anemones. The sea anemone protects and provides scraps of food for these symbionts, which are unaffected by the sea anemone's nematocysts. In turn, the symbionts are thought to help clean the sea anemone and protect it from certain predators.
Corals and the reefs they form are extremely important in the ecology of tropical oceans. Because coral reefs are built from many separate coral colonies attached together, they contain tunnels, caves, and deep channels. In these recesses live some of the most beautiful and fascinating animals in the world.
Corals are important to humans in many ways. Coral reefs provide a home for food fishes and other edible animals, as well as for organisms that produce valuable shells, pearls, and other products. Reefs also protect the land from much of the action of waves. When coral reefs are destroyed or severely damaged, large amounts of shoreline may be washed away. Fossil reefs offer important clues to geologists about the locations of oil deposits. Large blocks of coral have been used to build houses and to filter drinking water. Humans have long used certain corals to make jewelry and decorations.
Some cnidarians are used in medical research. Corals, like sponges, produce chemicals to protect themselves from being infected, overgrown, or settled upon by other organisms. Some of these chemicals may provide us with anti-cancer drugs, and others may help us learn more about cancer itself. The nerve toxins produced in cnidarian nematocysts are another powerful research tool. Whenever a compound poisons a biological system, studies of how the poison operates reveal a lot about how the system works. Cnidarians such as the sea wasp jellyfish produce several strong nerve poisons that have already helped scientists better understand nerve-cell function.
26-3 SECTIOIN REVIEW
1. What is a cnidarian? What kind of symmetry do cnidarians have?
2. List the three classes of cnidarians. Give examples of each class.
3. Describe the life cycle of a typical cnidarian.
4. Discuss symbiotic between and other interactions between cnidarians and other living things.
5. A medusa usually has specialized sense organs. It may also have nerves encircle its body and structures that control body contractions. Explain why a medusa needs a more complex nervous system than a polyp? (Hint: How does the lifestyle of a medusa differ from that of a polyp?)
Section Objectives
Platyhelminthes Lesson Phylum Platyhelminthes DIAGRAM-Flatworms DIAGRAM-Tapeworm
When most people think of worms, they think of earthworms-long, squiggly creatures that spend their time making tunnels in the ground. But there are many animals called worms that look nothing like earthworms. Many live in fresh water, a large number live in the ocean, and lots of them are important to humans. The two phyla of wormlike animals that we shall examine in this section are much simpler in structure than earthworms. They are known as unsegmented worms because their bodies are not divided into special segments. The phylum Platyhelminthes (pla-tee-hehl-MIHN-theez) consists of simple animals called flatworms. The phylum Nematoda (nee-mah-TOHD-ah) consists of long, thin worms called roundworms.
The members of the phylum Platyhelminthes are the simplest animals with bilateral symmetry. Most members of this phylum exhibit enough cephalization, or development of the anterior end, to have what we call a head. Because flatworms really are flat, the name of the phylum is quite appropriate (platy- means flat and helminth means worm). Many flatworms are no more than a few millimeters thick, although they may be up to 20 meters long. Flatworms have more developed organ systems than either sponges or cnidarians.
Form and Function in Flatworms
Flatworms feed in either of two very different ways. Some are aquatic and free-living, which means that they wander around in streams, lakes, and oceans. These worms may be carnivores that feed on tiny aquatic animals, or they may be scavengers that feed on recently dead animals. (You can probably catch flatworms in a local stream by leaving a piece of liver in the water overnight.) Free-living flatworms have a gastrovascular cavity with one opening at the end of a muscular tube called a pharynx (FAR-ihnks). See Figure 26-24. They use the pharynx to suck food into the gastrovascular cavity. The gastrovascular cavity forms an intestine with many branches along the entire length of the worm. In the intestines, enzymes help break down the food into small particles. These particles are taken inside the cells of the intestinal wall, where digestion is completed. Because the intestine branches into nearly all parts of the body, completely digested food can diffuse to other body tissues. Like cnidarians, flatworms expel undigested materials through the mouth.
Many other flatworms are parasites that feed on blood, tissue fluids, or pieces of cells inside the body of their host. Some of these animals have a pharynx that pumps food into a pair of dead-end intestinal sacs where the food is digested. But in many parasitic flatworms, the digestive tract is simpler than in free-living forms. Tapeworms, which live within the intestines of their host, do not have any digestive tract at all. They have hooks and/or suckers with which they latch onto the intestinal wall of the host. From this position, they can simply absorb the food that passes by-food that has already been broken down by the host's digestive enzymes.
Flatworms lack any kind of specialized circulatory or respiratory system. Because they are so flat, they can depend on diffusion to transport oxygen and nutrients to their tissues. And they can get rid of carbon dioxide and most other metabolic wastes by allowing them to diffuse out through their body walls. Freshwater flatworms such as planarians have structures called flame cells that help them get rid of extra water. Many flame cells join together to form a network that empties through tiny pores in the animal's skin.
Free-living flatworms have nervous systems that are much more developed than those of cnidarians and sponges. They have a definite head in which a simple brain is located. This brain is the control center of a simple nervous system that stretches throughout the body. One or more long nerve cords run from the brain down the length of the body on either side. Shorter nerve cords run across the body. Many flatkworms have one or more pairs of light-sensitive organs called ocelli, or eyespots. These eyespots do not see objects as our eyes do; they simply detect whether the animal is in light or in darkness. Most flatworms have cells that are sensitive to chemicals found in food, and other cells that tell the worm which way the water around them is flowing. These cells are usually scattered all over the body. The nervous system of free-living flatworms allows them to gather information from their environment-information that they use to locate food and to find dark hiding places beneath stones and logs during the day.
Parasitic flatworms often do not have much of a nervous system. As you can imagine, there is not much need for a nervous system in an organism that, mainly hangs onto an intestinal wall and absorbs food! In fact, in tapeworms the nervous system has completely disappeared as the worms have adapted to their parasitic lifestyle.
Free-living flatworms usually use two means of locomotion at once. Cilia on their epidermal cells help them glide through the water and over the bottom. Muscle cells controlled by the nervous system allow them to twist and turn so that they are able to react to environmental conditions rapidly.
Reproduction in free-living flatworms can be either sexual or asexual. Most free-living flatworms are hermaphrodites, which means that they have both male and female organs. During sexual reproduction, the worms join in pairs. One worm delivers sperm to the other worm while receiving sperm from its partner at the same time. The eggs, which are laid in small clusters, hatch within a few weeks. Asexual reproduction by fission is also common among free-living flatworms. Most of these worms have incredible abilities of regeneration. In one form of asexual reproduction, a worm will simply "fall to pieces" and each piece will grow into a new worm! Parasitic flatworms do not reproduce asexually. They often have complicated life cycles, as you will see shortly.
PLANARIANS
The free-living flatworms belong to the class Turbellaria. The most familiar members of this class are planarians, the "cross-eyed" freshwater worms. Turbellarians vary greatly in color, form, and size. See Figure 26-27. Although most turbellarians are less than I centimeter in length, some giant land planarians, which are found in moist tropical areas, can attain lengths of more than 60 centimeters!FLUKES
DIAGRAM-Cow Liver Fluke DIAGRAM-Human Liver FlukeThe members of the class Trematoda are parasitic flatworms known as flukes. Some flukes are external parasites that live on the skin, mouth, gills, or other outside parts of a host. Most flukes, including the ones that affect humans, are internal parasites that infect the blood and organs. These flukes have complicated life cycles that involve at least two different host animals. Although flukes are less than a centimeter long, the damage they cause to their host during their life cycle sounds like the script for a horror movie! Refer to Figure 26-28 as you read about the life cycle of a blood fluke. Keep in mind that the pattern of multiple hosts is typical of most parasitic flukes and, indeed, of many parasites in general.
Blood flukes are found primarily in Southeast Asia, North Africa, and other tropical areas. As you might expect, blood flukes live in the blood-specifically, the blood within the tiny blood vessels of the intestines. Humans are the primary hosts of blood flukes that belong to the genus Schistosoma. (The primary host of a parasite is the host organism in which adult parasites are found and in which sexual reproduction of the parasite occurs.)
Most flukes are hermaphrodites and undergo sexual reproduction in a manner similar to that of free-living flatworms. (However, the sexes are separate in Schistosoma.) Flukes produce many more eggs than free-living flatworms-about 10,000 to 100,000 times as many! Blood flukes lay so many eggs that the tiny blood vessels of the host's intestine break open. The broken blood vessels leak both blood and eggs into the intestine. The eggs are not digested by the host and thus become part of the feces. In developed countries, where there are toilets and proper sewage systems, these eggs are usually destroyed in the sewage treatment process. But in many undeveloped parts of the world, human wastes are simply tossed into streams or even used as fertilizer.
Once the fluke eggs get into the water, they hatch into swimming larvae. When these larvae find a snail of the correct species, they burrow inside it and digest its tissues. The snail is an intermediate host for the fluke. Although sexual reproduction does not occur in an intermediate host, this host is still an essential part of the parasite's life cycle. In the intermediate host (in this case, a snail), the flukes reproduce asexually. The resulting new worms break out of the snail and swim around in the water. If they find a human, the worms bore through the skin and eat their way to the blood vessels. In the blood, they get carried around through the heart and lungs to the intestine, where they live as adults.
People infected with blood flukes get terribly sick. They become weak and often die-either as a direct result of the fluke infection or because they cannot recover from other diseases in their weakened condition. Blood flukes cause some of the most serious health problems in the world today. But because the species dangerous to humans live only in the tropics, most people in the United States know nothing about them-even though hundreds of millions of people suffer from blood flukes.
There are only one or two kinds of blood flukes in lakes and streams of the United States. These flukes normally have fishes or water birds as their primary hosts. If these worms find human swimmers, they try to burrow through the skin. This causes what is known as "swimmers itch." But because they are not adapted as human parasites, the worms cannot live in human bodies. The itch goes away after a time and the body repairs the damage.
TAPEWORMS
Members of the class Cestoda are long, flat parasitic worms that live a very simple life. They have a head called a scolex (SKOH-leks) on which there are several suckers and a ring of hooks. These structures attach to the intestinal walls of humans and other animals. Inside the intestine, these worms are surrounded with food that their primary host has already digested for them. The worms absorb this food through their body walls. Adult human tapeworms can be up to 18 meters long! Tapeworms almost never kill their hosts, but they do use up a lot of food. For this reason, hosts may lose weight and become weak. Behind the scolex of the tapeworm is a narrow neck region that is constantly dividing to form the many proglottids (proh-GLAH-tihds), or sections, that make up most of the body of the tapeworm. As you can see in Figure 26-30, the youngest and smallest proglottids are at the anterior (head) end of the tapeworm, and the largest and most mature proglottids are at the posterior (tail) end. Proglottids contain little more than male and female reproductive organs. Sperm produced by the testes, or male reproductive organs, can fertilize eggs in the proglottids of other tapeworms or of the same individual. Fertilized tapeworm eggs are released when mature proglottids break off the posterior end of the tapeworm and burst open. A mature proglottid may rupture either in the host's intestine or after it has been passed out of the host's body with the feces. A single proglottid may contain over 100,000 eggs, and a single worm can produce more than half a billion eggs each year!
If food or water contaminated with tapeworm eggs is consumed by cows, pigs, fishes, or other intermediate hosts, the eggs enter the intermediate host and there hatch into larvae. These larvae grow for a time and then burrow into the muscle tissue of the intermediate host and form a dormant protective stage called a cyst. If a human eats raw or incompletely cooked meat containing these cysts, the larvae become active within the human host. Once inside the intestine of the new host, they latch onto the intestinal wall and grow into adult worms.
Nematoda Lesson Phylum Nematoda Phylum Nematoda DIAGRAM-Nematatode DIAGRAM-Ascaris
Members of the phylum Nematoda, which are known as roundworms, are among the simplest animals to have a digestive system with two openings-a mouth and an anus. Food enters through the mouth, and undigested food leaves through the anus. Roundworms, which range in size from microscopic to a meter in length, may be the most numerous of all multicellular animals. It is difficult to imagine just how many roundworms there are around us all the time. A single rotting apple can contain as many as 90,000 roundworms! And a small bucketful of garden soil or pond water may house more than a million roundworms.
Form and Function in Roundworms
Most roundworms are free-living. Free-living roundworms are found in virtually all parts of the Earth-in soil, salt flats, and aquatic sediments; in polar regions and in the tropics; in fresh water, oceans, and hot springs. There are, however, many species of parasitic roundworms. Parasitic roundworms affect almost every kind of plant and animal.
All roundworms have a long tube-shaped digestive tract with openings at both ends. This system is very efficient because food can enter through the mouth and continue straight through the digestive tract. Any material in the food that cannot be digested leaves through an opening called the anus.
Free-living roundworms are often carnivores that catch and eat other small animals. Some soil-dwelling and aquatic forms eat small algae, fungi, or pieces of decaying organic matter. Some actually live on the organic matter itself. Others digest the bacteria and fungi that break down dead animals and plants. Many roundworms that live in the soil attach to the root hairs of green plants and suck out the plant juices. These parasitic worms cause tremendous damage to many crops all over the world. Roundworms are particularly fond of tomato plants. For this reason, many tomato plants have been specially bred to be resistant to roundworms. Other roundworms live inside plant tissues, where they cause considerable damage.
Like flatworms, roundworms breathe and excrete their metabolic wastes through their body walls. They have no internal transport system and thus depend on diffusion to carry nutrients and wastes through their body.
Roundworms have simple nervous systems. They have several ganglia, or groups of nerve cells, in the head region, but they lack anything that can really be called a brain. Although roundworms have several types of sense organs, these are simple structures that detect chemicals given off by prey or hosts. Several nerves extend from the ganglia in the head and run the length of the body. These nerves transmit sensory information and control movement. The muscles of roundworms run in strips down the length of their body walls. Aquatic roundworms contract these muscles to move like snakes through the water. Soil-dwelling roundworms simply push their way through the soil by thrashing around.
Roundworms reproduce sexually. Most species of roundworms have separate males and females, but a few species are hermaphroditic. Fertilization takes place inside the body of the female. Roundworms that are parasites on animals often have complex life cycles. Two or three hosts may be involved in the life cycle of some roundworms. In other roundworms, such as Ascaris, the stages of the life cycle take place in different organs of one host.
Ascaris is a parasitic roundworm that lives in humans. Species that are closely related to Ascaris affect horses, cattle, pigs, chickens, dogs, cats, and many other animals. Ascaris and its relatives, which are collectively known as ascarids, have life cycles that are similar to one another. One of the reasons puppies are wormed while they are young is to rid them of the ascarid that affects dogs.
Adult ascarid worms live in the intestines, where they produce many eggs that leave the host's body in the feces. If food or water contaminated with these feces is eaten by another host, the eggs hatch in the small intestine of the new host. The young worms burrow into the walls of the intestines and enter surrounding blood vessels. Carried around in the blood, the tiny worms end up in the lungs. Here they break out into the air passages and climb up into the throat, where they are swallowed. Carried back into the intestines, they mature and the cycle repeats itself.
How Unsegmented Worms Fit into the World
Unsegmented worms do not exert much positive influence on the daily lives of humans, and thus they are easy to ignore. Most unsegmented worms lead inoffensive lives. They eat small organisms and are eaten by larger organisms; some help aerate the soil with their burrows. However, unsegmented worms are generally known by the parasitic rather than the free-living members of their phylum. We have already talked about parasitic flatworms. In this section we shall focus our attention on parasitic roundworms, which are responsible for some of the most painful and horrific diseases known. Parasitic roundworms include hookworms, trichinosis-causing worms, filarial worms, eye worms, and a host of others too numerous to be mentioned here.
Hookworms are serious human intestinal parasites that are often found in the southern United States and are common in tropical countries. As many as one fourth of the people in the world today are infected with hookworm! Hookworm eggs hatch outside the body of the host and develop in the soil. If they find an unprotected foot, they use sharp teeth and hooks to burrow into the skin and enter the bloodstream. Like Ascaris, these worms travel through the blood to the lungs and then down the throat to the intestines. There, the adult worms dig into the intestinal wall and suck the blood of the host. These worms can devour enough blood to cause weakness and poor growth.
Trichinosis (trihk-ih-NOH-sihs) is a terrible disease caused by the roundworm Trichinella.. Adult worms, which are hard to see without a microscope, live and mate in the intestines of the host. Females carrying fertilized eggs burrow into the intestinal wall, where each releases up to 1500 larvae. These larvae travel through the bloodstream, from which they eventually exit through small blood vessels, and then burrow into organs and tissues. This causes terrible pain for the host. The larvae then form cysts in the host's muscle tissue and become inactive.
The only way these encysted worms can complete their life cycle is if infected muscle tissue is eaten. This means that hosts for Trichinella must be carnivorous-animals that do not eat infected meat do not get trichinosis. Two very common hosts for Trichinella are rats and pigs. (Rats eat any meat they can find, and may even eat each other. Pigs regularly catch and eat rats and other small animals.) Humans get trichinosis almost exclusively by eating raw or incompletely cooked pork. Filarial worms, which are found primarily in tropical regions of Asia, are threadlike worms that live in the blood and lymph vessels of birds and mammals such as humans. They are transmitted from one primary host to another through biting insects, especially mosquitoes. In severe infections, large numbers of filarial worms may block the passage of fluids within the lymph vessels. This causes elephantiasis, a condition in which an affected part of the body swells enormously. Fortunately, extreme cases of elephantiasis are now rare.
Eye worms are closely related to the filarial worms that cause elephantiasis. They are
found in Africa and affect both humans and baboons. Eye worms live in and burrow through
the tissues just below the skin of their host. In their travels, the worms occasionally
move across the surface of the eye-hence the name eye worm.
SUMMARIZING THE CONCEPTS
The key concepts in each section of this chapter are listed below to help you review the chapter content. Make sure you understand each concept and its relationship to other concepts and to the theme of this chapter.
26-1 Introduction to the Animal Kingdom
26-2 Sponges
26-3 Cnidarians
26-4 Unsegmented Worms
REVIEWING KEY TERMS
Vocabulary terms are important to your understanding of biology. The key terms listed below are those you should be especially familiar with. Review these terms and their meanings. Then use each term in a complete sentence. If you are not sure of a term's meaning, return to the appropriate section and review its definition.
26-1 Introduction to the bilateral spicule hermaphrodite
Animal Kingdom symmetry amebocyte
vertebrate anterior spongin 26-4 Unsegmented Worms
invertebrate posterior gemmule unsegmented
division of labor dorsal budding worm
herbivore ventral Platyheiminthes
carnivore cephalization 26-3 Cnidarians flatworm
parasite ganglion Cnidaria Nematoda
filter feeder polyp roundworm
detritus feeder 26-2 Sponges medusa pharynx
larva Porifera gastrovascular
metamorphosis collar cell cavity
radial symmetry osculum nematocyst
CONTENT REVIEW
Multiple Choice Choose the letter of the answer that best completes each statement.
1. All animals are a. unicellular. b. sessile. c. radially symmetric. d. heterotrophic.
2. A hydra is best described as a a. herbivore. b. carnivore. c. parasite. d. filter feeder.
3. In which animal would you expect to observe cephalization? a. jellyfish b. sponge c. roundworm d. sea anemone
4. Which animal is free-living? a. Hydra b. Trichinella c. Schistosoma d. Ascaris
5. Animals in the phylum Cnidaria include a. flukes. b. roundworms c. medusae. d. sponges.
6. Which animal is most likely to possess ocelli, statocysts, and a nerve net? a. sponge b jellyfish c. coral d. flatworm
7. Which animal lacks a digestive system and digestive organs? a. jellyfish b. hookworm c. planarian d. tapeworm
8. An immature animal that looks and acts nothing like the adult of that species is called a a. gemmule. b. larva c. bud. d. proglottid.
True or False
Determine whether each statement is true or false. T it is true. If it is false,
change the underlined word or words to make the
Word Relationships
In each of the following sets of term, three of the terms are related. One term does not belong. Determine the
Characteristic common to three of the terms, then identify the term that does not belong.
CONCEPT MASTERY
Use your understanding of the concepts developed in the chapter to answer each of the following in a brief paragraph.
1. Draw a human, a sea anemone, and, a dog. Label each drawing using as many of the following terms as are appropriate: radial symmetry, bilateral symmetry, anterior, posterior, dorsal, ventral, sessile, motile.
2. Suppose you placed a harmless purple colored mixture of red dye and blue dye in the water beside a vase-shaped sponge. After a while, you noticed blue dye coming out of the top of the sponge. Describe how the blue dye got from the outside environment into the sponge. Propose an explanation for what happened to the red dye. How might you determine if your explanation about the red dye is correct?
3. Explain how flukes and tapeworms display the following parasitic adaptations: (a) organs for attachment to the host, (b) reduced sense organs, (c) modifications in food-getting, (d) increased reproductive capabilities and well-developed reproductive organs, (e) larvae that allow the transfer from one host to another.
4. At one time, diet pills containing tapeworm eggs were sold. Why would such pills work? Why are such pills dangerous.
