Reading Essentials for Biology Glencoe (794133), страница 71
Текст из файла (страница 71)
Also covers SC.D.1.4.4, SC.F.1.4.2, SC.G.1.4.1◗ Before You ReadImagine that you are an aquatic animal. Now imagine that for most of your life you had to remainin exactly the same place. What adaptations would you need to survive? List your thoughts on thelines below.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnWhat is a cnidarian?MouthGastrovascularcavityPolyp312Chapter 26What is the body structure of cnidarians?All cnidarians have the same basic body structure. Their bodiesare radially symmetrical. This means that they can be dividedalong any plane, through a central axis, into roughly equal halves.Cnidarians have one body opening.
Their bodies are made up oftwo layers of cells. The outer layer is protective. The inner layerof cells helps with digestion.Because a cnidarian’s body is only two layers of cell, no cell isever far from water. Oxygen dissolved in water diffuses directlyinto body cells. Carbon dioxide and other wastes move out of acnidarian’s body directly into the surrounding water.Most cnidarians have two different bodyforms during their life. The two bodyforms are the polyp and the medusa.
Apolyp (PAH lup) has a tube-shaped bodywith a mouth surrounded by tentacles. Amedusa (mih DEW suh; plural, medusae)Mouthhas an umbrella-shaped body, called abell. The tentacles hang down. Its mouthMedusais on the underside of the bell.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.IdentifyDetails As you read this section, highlight facts aboutcnidarians in one color. Highlight facts that apply to polypsin a different color. Highlightfacts about medusae in a thirdcolor.Cnidarians (ni DARE ee uns) are a group of invertebrates madeup of more than 9000 species of jellyfish, corals, sea anemones,and hydras.
They can be found worldwide and almost all live inmarine biomes.NameDateClassSection26.2Cnidarians, continuedIn cnidarians, one body form is usually easier to observe thanthe other. For example, in jellyfishes, the medusa is the body formyou observe.
The jellyfish polyp is small and not easily seen. Inhydras, the polyp is the form that is easier to see. A hydra’smedusa form is small and delicate. Corals and sea anemones haveonly polyp forms.How do cnidarians reproduce?All cnidarians have the ability to reproduce sexually and asexually. Sexual reproduction usually occurs in the medusa stage. Ifthere is no medusa stage, then the polyp can reproduce sexually.The illustration of the jellyfish below shows how the sexualmedusa stage alternates with the asexual polyp stage. MaleFemaleMaleA In cnidarian sexualreproduction, a malemedusa releases spermand a female medusareleases eggs into thewater.
Externalfertilization occurs.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.EggsSexualReproductionFertilizationSpermD One by one, the medusaeB The zygote grows and develops intobreak away from theparent polyp. When theymature, thecycle beginsagain.a blastula. The blastula becomes afree-swimming larva that eventuallysettles on a surface.BlastulaC In the asexual phase,a sessile polyp growsand begins to formbuds that becometiny medusae.AsexualReproductionBudREADING ESSENTIALSLarvaPolypChapter 26313NameDateClassSection26.2Cnidarians, continuedmedusae release sperm, and female medusae release eggs into thewater.
External fertilization occurs. Fertilization results in zygotes.Zygotes develop into embryos, and then into larvae. The freeswimming larvae settle and grow into polyps that reproduceasexually to produce new medusae. This may sound similar toalternation of generations in plant life cycles. However, in plants,the generations alternate between haploid and diploid. In cnidarians, medusae and polyps are diploid animals.Asexual reproduction can occur in either the polyp or themedusa stage. Polyps reproduce asexually by budding. Cnidariansthat remain in the polyp stage, such as corals and sea anemones,can reproduce sexually.1. How do cnidarians capture prey?________________________________________________________________________Cnidarians are predators. They capture their prey using nematocysts.
A nematocyst (nih MA tuh sihst) is a capsule that contains a coiled, threadlike tube. The tube can be sticky or barbed.It also may contain toxic substances. Nematocysts are located incells on the tentacles. When touched, nematocysts are fired offlike toy popguns, but much faster. The barbed tube either sticksto the prey, keeping it from escaping, or poisons the prey. Preyorganisms are then pulled in for digestion. The tentacles bringthe prey to the mouth by contracting.The inner cell layer of cnidarians surrounds a space called agastrovascular (gas troh VAS kyuh lur) cavity. Cells adapted fordigestion line the gastrovascular cavity and release enzymes thatbreak down the captured prey into small particles.
Whatever is notdigested is ejected back out of the mouth. Cnidarians are classifiedpartly based on whether or not there are divisions in the gastrovascular cavity, and if there are, how many divisions are present.Do cnidarians have a nervous system?2. Describe the nervous system of a cnidarian.________________________________________________________________________A cnidarian has a simple nervous system. It does not have acontrol center or brain such as other animals. The nervous systemconsists of a nerve net that conducts impulses to and from allparts of the body. The impulses from the nerve net cause contractions of musclelike cells in the two cell layers.
For example, themovement of the tentacles when a cnidarian captures prey is theresult of contractions of the musclelike cells.Diversity of CnidariansThere are four classes of cnidarians: Hydrozoa, Scyphozoa,Cubozoa, and Anthozoa.314Chapter 26READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How do cnidarians digest food?NameDateClassSection26.2Cnidarians, continuedWhat are the hydrozoans?The class Hydrozoa includes two groups—the hydroids, suchas the hydras, and the siphonophores, including the Portugueseman-of-war. All hydrozoans have open gastrovascular cavitieswith no internal divisions.
Most hydroids are marine animals.They form branching polyp colonies by budding. They arefound attached to pilings, seashells, and other surfaces. Thesiphonophores include floating colonies that drift about on theocean’s surface.Consider the Portuguese man-of-war. It looks so much like asingle organism that it is hard to see how it is actually a closelyassociated group of animals. One individual forms a large, gasfilled float. Other polyps hanging from the float have functions,such as feeding and reproduction. The polyps all functiontogether for the survival of the colony.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are the scyphozoans?Jellyfishes are beautiful and fragile.
The medusa form of thejellyfishes is the dominant stage. Like other cnidarians, scyphozoanshave musclelike cells in their outer cell layer that can contract.When these cells contract together, the bell contracts, which movesthe animal through the water. The gastrovascular cavity of scyphozoans has four internal divisions.Jellyfishes can be found in arctic to tropical waters.
They havebeen seen at depths of more than 3000 m. Swimmers should avoidjellyfishes because of their painful stings.3. What is the dominantform of jellyfishes?________________________________________________________________________What are the anthozoans?Anthozoans are cnidarians that exhibit only the polyp form.All anthozoans have many incomplete divisions in their gastrovascular cavities.Sea anemones are anthozoans that live as individual animals.They are thought to live for centuries.
They can be found intropical, temperate, and arctic seas. Some tropical sea anemonesmay be more than a meter in diameter.Corals are anthozoans that live in colonies. They are found inwarm ocean waters. Corals form protective shelters around theirsoft bodies. The shelter is made of calcium carbonate. Colonies ofmany coral species build beautiful coral reefs that provide foodand shelter for many marine species.
When a coral polyp dies, itsshelter is left behind, adding to the coral reef’s structure. The living portion of a coral reef is a thin, fragile layer that grows on topof the shelters left behind by previous generations. Coral reefsREADING ESSENTIALS4. Analyze Which anthozoans live in colonies andwhich live as individuals?________________________________________________________________________Chapter 26315NameDateClassSection26.2Cnidarians, continued5. What organism helpscorals survive?________________________________________________________________________form slowly.
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