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CyberEd® Student Study Guide Cnidarians CyberEd® Student Study Guide: Cnidarians The following National Science Education Standards relate to this study guide: BIOLOGICAL EVOLUTION o Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification. o The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors. THE INTERDEPENDENCE OF ORGANISMS o Energy flows through ecosystems in one direction, from photosynthetic organisms to herbivores to carnivores and decomposers. o Organisms both cooperate and compete in ecosystems. The interrelationships and interdependencies of these organisms may generate ecosystems that are stable for hundreds or thousands of years. Page 1 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Table of Contents Introduction ..................................................................................................................... 3 Cnidarian Evolution ......................................................................................................... 5 Common Cnidarian Features .......................................................................................... 6 Cnidarian Senses, Movement and Digestion ................................................................ 11 Class Anthozoa: Corals ................................................................................................. 13 Class Anthozoa: Sea Anemones ................................................................................... 18 Class Hydrozoa: Hydroids ............................................................................................. 22 Class Hydrozoa: Hydras and Portuguese Man-of-Wars ................................................ 25 The Jellyfish .................................................................................................................. 29 Review .......................................................................................................................... 34 Resources Referenced in this Study Guide Software Programs CyberEd Biology Course Title: Cnidarians Quizzes & Tests All quizzes and tests referenced in this Study Guide can be located in the Teaching Resources section of the CyberEd Oasis website. Lab Activities All lab activities referenced in this Study Guide can be located in the Teaching Resources section of the CyberEd Oasis website. Web Links All web links referenced in this Study Guide can be located in the Teaching Resources section of the CyberEd Oasis website. Page 2 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Introduction Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 1 - 4 and then answer the following questions. Learn where cnidarians fit in the classification scheme of life. Multimedia Presentation Scenes 1 – 2 1. A Diverse Phylum: These pictures show various cnidarians. Discuss this diversity in the table below. Some animals included in phylum Cnidaria What this program examines Number of Cnidarian species, so far Habitat jellyfish, sea anemones, corals, and hydras, among others cnidarian physical similarities and differences, habitats and behaviors, and relationships with the organisms around them Scientists have identified over 9000 living species in the Cnidaria phylum, to date. All cnidarians are totally aquatic. Most species live in the salty oceans. Some species, such as the hydras, are found in fresh water streams and lakes. Although a few species float along the surface and others are found in deep, cold waters, the majority of cnidarians live in the shallow depths of warm, tropical oceans. Page 3 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 3 – 4 2. Animal Essentials: These pictures show where cnidarians fit in the animal kingdom. In the table below, explain the defining characteristics of animals and the distinction between vertebrates and invertebrates. Three defining characteristics Vertebrates Invertebrates Relative abundance of the two Evolutionary relationship of the two 1. All animals are made of eukaryotic cells, which are cells that possess a nuclear envelope. 2. Animal cells DO NOT have a cell wall, which is a rigid structure found encompassing only the cells of plants and bacteria. 3. All animals are heterotrophs, meaning they must obtain nutrient energy by consuming other organisms. Vertebrates are animals which possess a vertebral column, or spine, such as humans and sharks. Invertebrates such as cnidarians lack a vertebral column. Scientific estimates suggest that 95 to 98 percent of all animal species are invertebrates. Fossils provide evidence that the vertebrates most likely evolved from invertebrate ancestors. Page 4 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Cnidarian Evolution Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 5 - 9 and then answer the following questions. Learn how cnidarians fit into the time scheme of life. Multimedia Presentation Scene 5 1. Time Frame: This underwater view portrays early animal life in the oceans. Discuss cnidarians in the context of evolutionary time. Fossils indicate the earliest animals were invertebrates that lived in the oceans about 600 million years ago, a relatively recent era considering the Earth’s 4.5 billion year history. Some of the early animals such as Hallucigenia or Wiwaxia appear strange and exotic when compared to modern species. Others resemble the modern day cnidarians. The cnidarians of today exhibit many of the same structural and functional traits possessed by their ancestors millions of years ago. Scene 6 2. In the table below, discuss the biological meaning of the terms, primitive and advanced. What these terms mean What these terms do not mean These terms are relative comparisons. Primitive characteristics are thought to have originated further back in time. Advanced characteristics evolved later in the species’ history. Members of the phylum Cnidaria resemble the earliest animals known to exist; therefore, their characteristics are said to be relatively primitive. Animals with primitive characteristics should not be considered disadvantaged or less successful than animals with more advanced characteristics. The cnidarians’ primitive characteristics have proven successful for hundreds of millions of years. Page 5 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Common Cnidarian Features Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 7 - 18 and then answer the following questions. Multimedia Presentation Learn about radial symmetry, cnidarian tissues, sac-like bodies, tentacles and cnidocyte cells. Scenes 7 – 9 1. Anemone Pie: This picture shows an anemone sliced along planes of symmetry. In the table below, explain cnidarian body symmetry and level of organization, comparing it to human symmetry and level of organization. What body symmetry means Human body symmetry Body symmetry refers to the spatial arrangement of body parts. If an organism’s body can be divided at least once into equal but opposite parts, then it possesses symmetry determined by the arrangement of planes of symmetry. Humans have only one plane of symmetry and are bilaterally symmetrical with equal right and left halves on each side of the plane. Cnidarian symmetry Cnidarians can be sliced in half by any plane that passes through the center axis of the organism. This type of symmetry is called radial symmetry. Human body level of organization In humans, cells are organized into tissues, tissues are organized into organs, and organs are organized into organ systems. Cnidarian body level of organization Most cnidarians function at the tissue level of organization. A tissue is a group of similar cells working together to perform a specific function. A few cnidarian species possess primitive organs. Page 6 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 10 – 11 2. It’s in the Bag: This illustration shows cut away views of cnidarian bodies. Explain cnidarian tissues and structures in the table below. Types and functions of tissue Cnidarians possess only two tissue types. One is the outermost epidermal layer, or epidermis, which protects the animal from the outside environment. The other tissue is the innermost gastrodermal layer, or gastrodermis, which absorbs food nutrients. Mesoglea The mesoglea is a gelatinous matrix sandwiched between the epidermal and gastrodermal tissue layers. It is made mostly of water and may contain some unorganized cells and fibers. It is not considered a true tissue layer, but rather, a packing material that allows many chemicals to diffuse through it. Cnidarian bodies have only one opening, called the mouth. The mouth functions as an entrance for food as well as an exit for waste. The mouth leads to internal pocket called a gastrovascular cavity. Mouth Body shape The cnidarian body resembles a sac or bag. Biologists call this single opening type of body a sac-like body. Page 7 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 12 3. Inverted Forms: These pictures show a jellyfish and a sea anemone. Explain how they demonstrate the two basic types of cnidarian body form. The sac-like bodies of the cnidarians are found in one of two forms. The jellyfish shows the umbrella-like medusa form, the sea anemone exhibits the column-like polyp form. The medusa form is an upside down, compressed version of the polyp form. Cnidarians in the medusa form generally move about their environment by swimming in a pulsating manner. The polyp form cnidarians have various methods of mobility, or are sessile, remaining in one place. Most cnidarians show one form or the other; some cnidarian species can exist in each form during their life cycle. Scenes 13 – 14 4. Tentacle Touch: These pictures feature cnidarian tentacles. Explain the variety, structure and function of cnidarian tentacles. All cnidarians possess tentacles around the mouth. Tentacles may be short and stubby like those of the corals and sea anemones, or long and slender like those of most jellyfish. Some jellyfish species have tentacles that reach 50 meters or more. Tentacles are extensions of the cnidarian’s body, usually containing epidermal tissue, gastrodermal tissue, and mesoglea. Although often dangling in the water currents, these tentacles also have the capacity to extend and contract, allowing for hunting, movement and self defense. Page 8 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 15 – 17 5. Pop Goes the Sting: This illustration shows the mechanism and distribution of cnidocyte cells. In the table below, explain how they work. Where found Cnidocyte structure Cnidocyte effect Trigger and lid action Stinging, sticking and tangling devices Fate of fired cell Types of stimuli embedded within the epidermis of all cnidarians and usually very concentrated along the tentacles Cnidocyte cells contain a capsule-like organelle called a nematocyst, which houses a hollow filament. Depending on their function, filaments may be covered in a sticky adhesive or they may be equipped with barbs capable of injecting poisons. When triggered the tiny filaments spring outward explosively, either sticking to a surface or delivering a fatal sting. If human skin accidentally comes in contact with certain jellyfish, the cnidocyte cells deliver painful and sometimes fatal stings. The trigger is a modified flagellum called a cnidocil. The lid is called the operculum. When the cnidocil is triggered by the proper stimulus, the capsule membrane instantly allows water to flood into the nematocyst’s inner chamber. The operculum then flips open and the filament is rapidly ejected outward. If the nematocyst organelle is designed to sting prey, tiny barbs will extend outward during discharge. These spikes can pierce and inject poison into prey. Other types of nematocysts may secrete a sticky substance which allows the tentacles to attach to certain surfaces. A third type of nematocyst may recoil like a spring, entangling the prey so that the tentacles may draw the food into the mouth. Once a nematocyst has been discharged, the cnidocyte is destroyed by the cnidarian’s body. specific tactile and chemical stimuli, usually in a specific order that distinguishes the prey so the nematocytes only fire when appropriate Page 9 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 18 6. Borrowed Defense: This nudibranch is making a meal of a cnidarian polyp. Explain how animals such as certain mollusks and flatworms can have cnidocytes. The cnidarians are the only animals that develop cnidocytes. The word Cnidaria was derived from the word cnidocyte. Some animals, such as the nudibranch shown in the picture, can eat cnidarians without triggering the nematocysts to fire. The nudibranch then incorporates the cnidarian’s cnidocytes into its own body. Thus, cnidocytes can be found in other types of animals although only cnidarians can develop these discharging cells. Page 10 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Cnidarian Senses, Movement and Digestion Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 19 - 23 and then answer the following questions. Multimedia Presentation Learn how cnidarian sensory cells stimulate movements, including digestive reactions. Scene 19 1. Web of Nerves: This illustration portrays the cnidarian nerve net. Describe how it works. The nerve net coordinates the movement of the tentacles and the body. This intricate structure is composed of primitive nerve cells called protoneurons distributed in a web-like network underlying both the epidermal and gastrodermal tissues. Usually, when a part of the net is excited the impulse spreads in all directions rather than along a specific path. Even if only a single protoneuron is excited, the entire net quickly becomes stimulated. Although some cnidarians show more organization of nerve cells than others, none have a region centralized enough to be called a brain. The cnidarians do not even have a distinct body region that can be considered a head. Scene 20 2. Hair Trigger: This graphic shows a crab leg and its aura of chemicals touching a cnidarian sensory cell. Explain this process. Sensory cells embedded in the epidermis and mostly concentrated near the mouth and tentacle regions receive external messages. These sensors usually contain a small flagellum capable of receiving tactile and chemical stimuli, much like the cnidocil of the cnidocyte cells. The proper stimuli trigger corresponding nerve impulses across the nerve net throughout the body. Page 11 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 21 3. Contraction Action: This graphic depicts the contracting parts of cnidarians. Explain their structure and function. Although cnidarians do not have any true muscle tissue, tentacle movement and body contraction are both produced by two types of cells with contracting parts: the epitheliomuscular cells of the epidermis and the nutritive-muscular cells of the gastrodermis. Most of the cells in the epidermis are epitheliomuscular cells; an elongated, contracting portion of each epitheliomuscular cell is situated in the mesoglea. Similarly, while most of the nutritive-muscular cell is found in the gastrodermal layer, the contracting portion is found in or adjacent to the mesoglea. Protoneurons of the nerve net are connected to these muscle-like cells, stimulating muscle-like contractions. Scene 22 4. Digestive Reach: This graphic shows digestion taking place in the gastrodermis. Explain the processes of extracellular and intracellular digestion in cnidarians. When prey is pushed into the mouth area, gland cells embedded in the gastrodermis secrete digestive enzymes that break down the prey into smaller food particles. This is called extracellular digestion because it occurs outside of the cells. Once food is broken into smaller bits the nutritivemuscular cells reach out with pseudopodia and engulf the food particles for intracellular digestion. Nutrients diffuse throughout the mesoglea for use by the other cells of the cnidarian. Page 12 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lessons # 1 & 2 A review of cnidarian characteristics. Interactive Lesson Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lesson # 5 A review of hydrozoan digestion. Interactive Lesson Quiz: Introduction to Cnidarians Please take the quiz provided by your teacher. Quiz Quiz: Characteristics of Cnidarians Please take the quiz provided by your teacher. Quiz Page 13 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Class Anthozoa: Corals Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 23 - 32 and then answer the following questions. Learn about coral animals and coral colonies. Multimedia Presentation Scene 23 1. In the table below list the similar physical characteristics and the four major classes of cnidarians. Similar physical characteristics The four major classes sac-like body, radial symmetry, epidermal and gastrodermal tissue, mesoglea, tentacles, and cnidocytes Anthozoa, Hydrozoa, Scyphozoa, and Cubazoa Scene 24 2. Flower Animals: These pictures show two members of the class Anthozoa (one with a close-up). Identify them and discuss the general characteristics of colonies. Corals and sea anemones, among others, are “flower animals” in the class Anthozoa. While the corals are attached to their homes for life, the sea anemones can tumble, twist, or roll about their environment. The anthozoans can be found from tropical seas to polar oceans, and they may exist individually or as part of a colony. A colony is a group of similar organisms living in such close contact that they are interdependent, sharing responsibilities for survival. Some coral colonies form islands and reefs. Page 14 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 25 3. Coral Belt: These maps show the distribution of coral reefs and islands, and the location of the famous Great Barrier Reef. Discuss the reef ecosystem. Coral reefs and islands exist in tropical oceans. The Great Barrier Reef, a region of coral reef and coral island colonies, extends over one thousand miles and is home to thousands of animal and plant species. Coral reefs and islands are ecosystems, defined as communities of organisms interacting with each other and their physical environment. Coral reefs are surpassed only by tropical rain forest ecosystems in terms of species diversity, and they construct their own physical foundation. Scenes 26 – 28 4. Cup by Cup: This illustration shows how corals build reefs and islands. In the table below, explain the process. Foundation material Hermatypic corals and their cups Generation after generation Calcium carbonate, consisting of calcium, carbon, and oxygen, the same hard substance found in blackboard chalk, stalagmites, and mollusk shells. Calcium carbonate does not readily dissolve in sea water. As they grow, hermatypic corals secrete calcium carbonate to the outside of their lower epidermal tissue, forming a hard cup called the coral exoskeleton, within which the sessile coral polyp spends its entire life. The cup provides a structural skeleton and protection. If the polyp senses a threat it can pull itself into the cup. After coral animals die other polyps build new exoskeleton cups upon the old ones. Over thousands of years, huge deposits of calcium carbonate accumulate and eventually become intricate underwater reefs or islands harboring a plethora of sea life beside the corals. Page 15 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 29 5. Mutual Gains: This graphic depicts the mutualism between reef coral animals and the algae living inside them. Describe this relationship and how it influences the growth of coral reefs. Hermatypic corals produce large amounts of calcium carbonate with the help of singlecelled algae that live inside their bodies, a mutualistic relationship that benefits both organisms. The algae are provided a protected place to live, while the corals that house the algae are able to produce calcium carbonate many times faster than corals which lack algae. As the algae rely on photosynthesis for their survival, the mutualistic corals must reside close to the surface so that the algae they contain will receive sufficient sunlight. This is the reason why many of the world’s largest reefs are found in relatively shallow water. Scene 30 6. Coral Gallery: These pictures show a variety of coral colonies that are different from the hermatypic corals. Briefly describe them. Some corals, unlike the hermatypic corals, do not secrete skeleton cups. Thorny corals exhibit exotic exoskeletons that form tree-like structures festooned with spikes and thorns. The soft corals, sea fans, and sea pens form skeletons embedded within the mesoglea, an endoskeleton formed inside the animal. At a distance each of these colonies may look like one animal. Up close, it is apparent that these forms are elaborate networks of individual polyp corals. Page 16 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 31 – 32 7. Cutaway Coral: This graphic shows the inner workings of hermatypic corals. In the table below, discuss the components of the gastrovascular cavity. Pharynx The pharynx is a narrow passageway occupying the area between the mouth and the main gastrovascular area. Septa Septa are partitions within the pharynx and gastrovascular cavity. They increase the surface area of the gastrodermis, providing more nutritive-muscular cells to absorb the nutritional elements of food brought into the gastrovascular cavity. Within the septa, the contracting portions of the nutritive-muscular cells run longitudinally, allowing the coral to pull its body inside its exoskeleton cup. Some septa do not extend completely to the pharynx. The edges of these incomplete septa contain gland cells, which secrete digestive enzymes. In some species, the incomplete septa may even wield stinging cnidocyte cells. The lower end of the incomplete septa is often modified into thread-like structures called acontia, which contain gland cells and cnidocyte cells. When necessary, acontia can exit the mouth area and act as extra tentacles. This might happen if captured prey is too large to fit into the animal’s mouth. Gland cells located on the acontia threads can secrete their digestive enzymes to partially digest the prey outside of the polyp. When large food is sufficiently broken up, it is pulled into the gastrovascular cavity. Coral contraction Incomplete septa Acontia Page 17 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Class Anthozoa: Sea Anemones Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 33 - 40 and then answer the following questions. Learn about sea anemones. Multimedia Presentation Scene 33 – 34 1. Slow but Sure: These pictures show various methods of sea anemone mobility. In the table below discuss how sea anemones differ from corals, and how they move about their habitat. Comparison with corals animals and coral colonies Mobility via the pedal disc Class Anthozoan includes sea anemones, carnivorous animals with body plans similar to the coral polyps. They are larger than corals and do not secrete an exoskeleton cup. Unlike coral colonies, the majority of sea anemones live independent of other sea anemones, although they may be crowded close together. While corals are sessile, sea anemones are somewhat mobile. Sea anemones have a slightly enlarged lower part of the body called a pedal disc. They can slowly slide along a mucus carpet secreted by epidermal cells on the pedal disc. Mobility via swimming Some sea anemones swim by using spastic bending movements. Mobility via tumbling Some sea anemones can tumble with the currents along the ocean floor. Mobility via hitchhiking Some crabs can pry sea anemones from rocks and place them atop their backs. The sea anemone hitches a ride; the crab receives camouflage. Page 18 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 35 – 36 2. Squeezing the Incompressible: This graphic depicts the internal structure of a sea anemone, and what happens when it squeezes itself around its circumference. Explain both features. Sea anemones resemble corals in their internal structure, with a pharynx leading to the gastrovascular cavity, incomplete septa ending in acontia, and complete septa partitioning the animal. Additionally, sea anemones have well developed nutritive-muscular cells with contracting parts that run in a circular belt around the circumference of the animal. Circular contractions of these cells push against the noncompressible, water-filled hydrostatic skeleton, squeezing the body longer. Scene 37 3. Single Parent Family: This graphic shows two ways a sea anemone can produce identical genetic copies of itself. Discuss asexual reproduction in sea anemones and corals. In sea anemones, asexual reproduction frequently occurs by longitudinal fission, the animal dividing in half by creating a new body wall down the middle of itself. Asexual reproduction also occurs when a piece of the pedal disc is detached as the animal glides along. If the piece is left on a suitable habitat, it will grow into a new sea anemone. This is termed regeneration from fragmentation. Similarly, corals also have the ability to asexually reproduce if a piece of their body happens to break off and land in a hospitable spot. Page 19 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 38 – 39 4. Rendezvous for Two: This graphic illustrates contrasting modes of sexual reproduction in anthozoans. In the table below, explain sexual reproduction in general, and how it works with sea anemones and corals. Sexual reproduction Anthozoan sexes Internal fertilization External fertilization A new organism forms through the combination of male and female sex cells, or gametes. Male gametes are called sperm and female gametes are called eggs. When a sperm cell and an egg cell combine it is called fertilization. Some anthozoans can produce only sperm, or only eggs; other anthozoan species are hermaphrodites, producing sperm and egg alternately to avoid self fertilization. In species that utilize internal fertilization, the eggs reside on the septa inside the gastrovascular cavity, and are fertilized by sperm from another anthozoan. During external fertilization, the most common mode in anthozoans, sperm and egg from two animals are released to the open water where they combine. Some anthozoans exhibit mass fertilization, with thousands of individuals in the span of one night releasing their sperm and eggs simultaneously to the sea. Scene 40 5. Round and Round: This graphic depicts the life cycle of sea anemones. In the table below, explain the steps in that cycle. Fertilization Zygote Larva Adult body form Sex cells from two adults combine to produce a zygote. The zygote grows into a ciliated larva. Upon settling, the larva grows directly into a new adult. The only type of body plan exhibited by the adult sea anemone is the polyp type. There are no known anthozoans with the medusa body plan. Page 20 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Quiz: Anthozoans Please take the quiz provided by your teacher. Quiz Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lesson # 6 A review of anthozoan structures. Interactive Lesson Page 21 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Class Hydrozoa: Hydroids Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 41 - 45 and then answer the following questions. Learn about hydrozoan varieties, and hydroids in particular. Multimedia Presentation Scene 41 1. This picture shows three species of hydrozoans. In the table below, discuss class Hydrozoa. Hydrozoan habitats Life cycle body plans Comparison with anthozoans Solitary and colonial lifestyles Three different hydrozoans mostly marine, some freshwater Some Hydrozoan species show only the polyp form, some exhibit both polyp and medusa forms in their life cycle. Hydrozoan polyps generally are much smaller than anthozoan polyps, and in some species are significantly modified. Some hydrozoans are solitary; most are colonial, permanently attached to the ocean floor or hard surface. Some colonies float on the surface. colonial hydroids; the solitary hydras; and colonial Portuguese man-of-wars Page 22 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 42 – 44 2. Many from One: This illustration shows the development of an entire hydroid colony from one larva. In the table below, describe the process and the result. From larva to colony A ciliated larva settles on a suitable surface and grows into a polyp. The polyp puts out root-like structures called hydrorhiza which serve as an anchor. The anchored polyp undergoes asexual reproduction through budding—the growth of a new organism from the surface of the parent. This asexual budding produces an entire hydroid colony. Zooids, the gastrovascular cavity, and the perisarc Each new bud grows into an individual animal called a zooid. Zooids are connected by a common gastrovascular cavity through the base and stalk, walled with inner gastrodermal tissue, the mesoglea, and outer epidermal tissue. Cells with whip-like flagella line the gastrodermis, circulating water and nutrients throughout the colony. The base and stalk are covered by a sheath of protective material called the perisarc. In some hydroid species the perisarc also covers the individual zooids. Individual zooids in a colony have different functions and appearances. Gastrozooids are the food suppliers of the colony, stinging their prey with nematocyst studded tentacles and pushing the food into their mouths for digestion. Zooids called gonangiums are reproductive structures, producing medusoid buds that eventually break away from the colony. Zooid specialization Page 23 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 45 3. Both Body Types: This graphic depicts the full life cycle of hydroids. Starting with the medusoids, describe the progression. The medusoid buds of hydroids have the medusa body plan. They swim using rhythmic undulation. Medusoids develop into adult organisms a few millimeters in diameter called the hydromedusae, which conduct sexual reproduction. The female hydromedusae shed eggs to the open water and the males release sperm. This external fertilization results in ciliated larvae, which settle down to renew the hydroid life cycle by becoming hydroid polyps. Thus, the hydroid life cycle includes stages of both polyp and medusa body types. Page 24 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Class Hydrozoa: Hydras and Portuguese Man-of-Wars Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 46 - 51 and then answer the following questions. Learn about hydras and Portuguese man-of-wars. Multimedia Presentation Scene 46 – 47 1. Freshwater Creatures: This illustration shows the structure of another hydrozoan, the hydra, and some of the ways it moves about its habitat. In the table below, describe hydra bodies, habitats, and mobility. Body type, lifestyle and size Habitat Cnidarian features Mobility modes Hydras exist only as independent polyps a few centimeters in length. Unlike other cnidarians, they inhabit freshwater lakes and streams throughout the world, attaching to aquatic plants. Hydras are often used in research because they are widespread. Hydras have the cnidarian epidermis, mesoglea, and gastrodermis. Their tentacles are studded with stinging cnidocyte cells. They can slowly glide along their basal disc, or pull themselves forward with their tentacles, or perform acrobatic flip-flops, or float by producing a small gas bubble on the basal disc. Page 25 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 48 2. Head Start: this graphic shows the hydra’s namesake, the mythological monster Hydra, which could grow back a decapitated head. Discuss the methods for hydra asexual reproduction shown in the illustration. Halves of a cut hydra will grow into two hydras. Severed tentacles will soon grow back. This type of asexual reproduction is termed regeneration, and mainly functions to replace lost tentacles. Rarely is a new animal reproduced this way. More commonly, asexual reproduction in hydra takes place through budding. The new hydra first appears as a small bud off the side of the parent, eventually growing almost as large as the parent before detaching to float away and live on its own. In addition to budding, hydras also have the ability to regenerate from fragments, like the corals and sea anemones. Scene 49 3. Winter Break: This graphic depicts the climate-sensitive life cycle of hydra. Starting with sperm and egg, describe the process. Sexual reproduction of the hydra occurs internally. An ovary containing eggs is produced as an outgrowth on the female’s body. Sperm released to the water from the male hydra swim into the female hydra’s mouth and fertilize the egg. The fertilized egg develops while attached to the female hydra; a protective covering forms around it. Eventually, the developing embryo, enclosed in the protective covering, detaches from the parent. This cyst allows the new hydra embryo to survive harsh winter conditions. When conditions are favorable, the embryo is released and continues developing directly into an adult hydra. The hydra life cycle does not include any medusa type body plan. Page 26 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 50 – 51 4. Floating Colony: These pictures show a Portuguese man-of-war, the full length of the tentacles are not shown. In the table below, describe its organization, life style, and specialized parts. . Overall organization The Portuguese man-of-war is a colony of polyps, not a single organism. Pneumatophore form and function One of the highly modified polyps is a float, called a pneumatophore. The pneumatophore contains air rich in carbon dioxide. Hunting strategy Portuguese man-of-war colonies drift along the ocean surface, using the polyps dangling in the water to harvest small fish. The hanging tentacles are individual polyps, or zooids. Each zooid has a single, long tentacle that extends 10 meters or more. These tentacles deliver fatal stings to small fish, and can inflict serious injury upon humans. Zooid specialization Some of the zooids are designed for feeding and have a single mouth. Others lack a mouth and specialize in entangling prey. Still, other zooids are specially designed for reproduction. The man-of-wars, and some other colonies like them, are Comparison with the most modified of the hydrozoan cnidarians other hydrozoans Hanging tentacles Page 27 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Quiz: Hydrozoans Please take the quiz provided by your teacher. Quiz Page 28 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians The Jellyfish Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scenes # 52 - 60 and then answer the following questions. Learn about scyphozoans and cubazoans. Multimedia Presentation Scenes 52 – 53 1. Medusa Shapes: These pictures show two types of jellyfish. Discuss them. Jellyfish are not really fish but they do have a large layer of jelly-like mesoglea. The scyphozoans have bell-shaped medusa bodies; the cubazoans have cube-shaped medusa bodies. Both scyphozoans and cubazoans can be found floating or swimming in most oceans at most depths. Scyphozoans and cubazoans contain many of the same structures and functions. Most jellyfish are between 5 and 50 centimeters in diameter, and one species measures six feet across with tentacles as long as a ten story tall building. Jellyfish may be colorless or colorful; most are semi-transparent. Scene 54 2. Delicate Anatomy: This graphic depicts the body parts of a jellyfish. Describe the jellyfish anatomy, referring to the labels in the picture. Most jellyfish are delicate, graceful, animals, best known for their beautiful medusa forms. The top part of the jellyfish is the exumbrella; the lower is the subumbrella. Hanging from the subumbrella of most scyphozoan jellyfish is a structure called the manubrium. The opening of the manubrium is the mouth of the animal. Oral arms with oral lobes extend from the manubrium, appearing like tentacles. However, a jellyfish’s actual tentacles are located along the margin that separates the exumbrella from the subumbrella. The number of tentacles varies among different jellyfish. Page 29 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 55 – 56 3. Sensor Device: This illustration shows a rhopalium and its two components, the ocellus and the statocyst. In the table below, explain these sense organs. Rhopalia (singular: rhopalium) Rhopalia exist at regular intervals along the margin between the exumbrella and the subumbrella. They are among the most primitive animal sense organs, consisting of two different structures. Statocysts Statocysts in the rhopalia give the animal its sense of balance, allowing it to discern up from down. Statocysts are small round chambers lined internally with sensory hairs, enclosing tiny, hard particles called statoliths. As the animal tilts, the statoliths roll by gravitational pull inside the chamber, touching hairs which, in turn, send signals to the animal’s nerve net. Thus, the jellyfish can right itself. Ocelli in the rhopalia sense photon energy, allowing the animal to detect brightness. They are not considered true eyes, although in some cubazoans, ocelli are so well developed that biologists consider them primitive eyes. In most jellyfish, the ocelli are primitive structures which measure the relative light intensity in the water, assisting the jellyfish in determining danger, prey, or perhaps shelter. Ocellus (plural: ocelli) Page 30 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scenes 57 – 58 4. Fish Snack: This graphic shows a jellyfish eating a true fish. Describe the entire process, referring to the numbered steps. Tentacles A small fish bushes the tentacles of the jellyfish and is paralyzed by the powerful stings of the cnidocyte cells. This is shown at step 1. Oral arms Using cilia located along the oral arms, the jellyfish slowly moves the fish toward its mouth. This is shown at step 2. Gastric pouches As the prey is forced into the mouth, it encounters gastric pouches that lie just internally to the manubrium. The cells lining these pouches secrete digestive enzymes into the pouch space for extracellular digestion. This is shown at step 3. Radial canals Once the food is extracellularly digested, the nutrients are passed to the radial canals by water currents. The radial canals are hollow, tube-like extensions from the gastric pouches, leading radially toward the edge of the invertebrate. As the nutrients travel through the canals, they are absorbed by gastrovascular cells. The radial canals ultimately terminate at the ring canal, encircling the animal along its outermost margin. The ring canal also contains gastrovascular cells for nutrient absorption. Ring canal Page 31 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Scene 59 – 60 5. This diagram portrays the life cycle of a scyphozoan. In the table below, explain the phases in this life cycle. The two body type stages, overall larva to scyphistoma The typical jellyfish life cycle includes a short polyp stage and a predominant medusa stage. Strobila and ephyra Medusoid-like buds are produced asexually in a process termed strobilation. Each bud is called an ephyra, and the overall structure is known as a strobila. The ephyrae of the jellyfish strobila eventually detach and develop into free-swimming medusa jellyfish adults. Ephyra to adult Sexual reproduction Fertilization to larva When a ciliated larva settles on the ocean floor, it grows into a tentacled polyp known as a scyphistoma. Male and female jellyfish produce sperm and eggs within structures called gonads located in the gastric pouches. Sperm are released into the water in order to fertilize eggs residing within the female’s gastric pouch or on the fold of an oral lobe. Successful fertilization produces a zygote that either develops within the fold of an oral lobe or is released to develop in the open water. Eventually, this zygote becomes a free-swimming ciliated larva and the life cycle continues when the larva settles to the ocean floor. Page 32 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lessons # 8 & 9. A review of the scyphozoan features and lifecycle. Interactive Lesson Quiz: Scyphozoans & Cubazoans Please take the quiz provided by your teacher. Quiz Page 33 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Review Cnidarians Now load the CyberEd Biology Course Title: Cnidarians View scene # 61 and then answer the following questions. Review what you have learned about cnidarians. Multimedia Presentation Scene 61 1. In the table below, review what you have learned. Common Cnidarian external characteristics radial symmetry, tentacles, stinging cnidocyte Range of body types Some species have polyp bodies, some have medusa bodies, and some might even have both at different stages of their life cycle. Range of mobility While some are sessile animals, such as the colonial corals and colonial hydroids, others possess the ability to move about their aquatic environments, such as the hydras, jellyfish, and most sea anemones. Range of reproductive strategies Reproduction methods may be sexual, asexual, or both, depending on the species. Primitive characteristics The headless, sac-like body, with only one opening, is considered a primitive body plan within the animal kingdom. Other primitive invertebrate characteristics include their nerve net, gastrovascular cavity, and true tissue level of organization. Jellyfish possess primitive sense organs, but this is not typical of most cnidarians. Sense organs Page 34 © 2003 CyberEd, Inc. CyberEd® Student Study Guide: Cnidarians Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lessons # 3 & 4 A review of cnidarians. Interactive Lesson Cnidarians Please load the CyberEd Biology Course Title: Cnidarians Complete Interactive Lesson # 7 A review of sexual and asexual reproduction. Interactive Lesson Comprehensive Exam: Cnidarians Please take the exam provided by your teacher. Test Page 35 © 2003 CyberEd, Inc.