Reading Essentials for Biology Glencoe (794133), страница 72
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It took thousands of years to form the reefs foundtoday. Corals that form reefs are known as hard corals. Othercorals, known as soft corals, do not build such structures.A coral polyp extends its tentacles to feed. Corals thrive innutrient-poor water because they have a symbiotic relationshipwith protists. Remember that symbiosis is a relationship in whichboth organisms benefit. The protists are called zooxanthellae(zoh oh zan THEH lee). Using photosynthesis, the protists produce oxygen and food that the corals use, while using carbondioxide and waste materials provided by the corals.
The protistsare primarily responsible for the bright colors found in coralreefs. The zooxanthellae are able to swim, and sometimes leavethe corals. When they do, the corals often die.Origins of Sponges and CnidariansHydrozoa2700 speciesScyphozoa200 speciesAnthozoa6200 speciesPorifera5000 speciesProtistaSpecies numbers are approximate and subject to change pending discoveries or extinctions.316Chapter 26READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Sponges and cnidarians evolved from a common ancestor earlyin geologic time.
The earliest sponge fossils date from the Precambrian Period, about 650 million years ago. The earliest cnidarian fossils are about 630 million years old. Scientists infer thatsponges and cnidarians evolved from protists because, today, flagellated protists resemble collar cells of sponges.
The larval formof cnidarians also resembles protists.NameDateClassSection26.2Cnidarians, continued◗ After You ReadMini Glossarygastrovascular (gas troh VAS kyuh lur) cavity: incnidarians, a large cavity in which digestiontakes placemedusa (mih DEW suh): a cnidarian body formthat is umbrella-shaped with tentacles thathang downnematocyst (nih MA tuh sihst): in cnidarians, acapsule that contains a coiled, threadliketube that may be sticky, barbed, or containpoisons; used in capturing preynerve net: simple netlike nervous system incnidarians that conducts nerve impulsesfrom all parts of the cnidarian’s bodypolyp (PAH lup): a cnidarian body form that istubelike with a mouth surrounded bytentacles1. Read the terms and definitions in the Mini Glossary above.
Highlight two terms that dealwith cnidarian body forms. Then use each term in a sentence that illustrates a fact aboutcnidarians. Write your sentences on the lines provided.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.2.
Match the concepts in Column 1 to the examples in Column 2.Column 1Column 2_____1. All cnidarians display two basicbody forms.a. The bell contracts_____2. Cnidarians sting their prey.c. Oxygen diffuses directly into cells_____3. A cnidarian body is only two layersof cells.d. Medusa and polyp_____4. The nerve net conducts impulses._____5.
Coral polyps live with zooxanthellae.b. Nematocystse. SymbiosisVisit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about cnidarians.READING ESSENTIALSChapter 26317NameDateClassSection26.3 Flatworms◗ Before You ReadSC.F.2.4.1 The student understands the mechanisms of asexual and sexual reproductionand knows the different genetic advantages and disadvantages of asexual and sexualreproduction.
SC.F.1.4.2 The student knows that body structures are uniquely designedand adapted for their function.From this point on, most of the animals you will learn about are bilaterally symmetrical. An organism with bilateral symmetry can be divided down its length into similar right and left halves. As areview, write on the lines below some of the other physical characteristics of bilateral symmetry.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCreate a Quiz After youhave read this section, createa quiz based on what youhave learned.
After you havewritten the quiz questions, besure to answer them.What type of nervous systemdo flatworms have?GangliaEyespotsNerve cordMuscle cellsPlanarian318Chapter 26Flatworms belong to the phylum Platyhelminthes(pla tee HEL min theez). Flatworms are the least complex worms.The name flatworm accurately describes their appearance.
Theyrange in size from 1 mm up to several meters. There are approximately 14 500 species of flatworms. They are found in marineand freshwater environments as well as moist habitats on land.Tapeworms and flukes are the most well known of the flatworms. Both are parasites and cause disease in humans and animals. Parasitic flatworms are discussed at the end of this section.Planarians are free-living flatworms.
That means they do notrequire another organism in order to survive. Planarians are themost commonly studied flatworms.A planarian is bilaterally symmetrical.Most of its nervous system is located in thehead. The nervous system helps the planarian respond to stimuli in its environment. Some flatworms have a nerve net;others have the beginning of a central nervous system. The illustration at left showsthe nervous system of a planarian.The nervous system includes two nervecords along each side of the body.
At thehead, eyespots can detect the presence orREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is a flatworm?NameDateClassSection26.3Flatworms, continuedabsence of light. Sensory cells detect chemicals and movement inwater. On each nerve cord, near the head, is a small swelling calleda ganglion (plural, ganglia).
The ganglion receives messages fromthe eyespots and sensory cells. The ganglion then communicateswith the rest of the body along the nerve cords. Messages fromthe nerve cords trigger responses in a planarian’s muscle cells.How do planarians reproduce?1. What do eyespots do?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Planarians are hermaphrodites, meaning they produce both eggsand sperm. During reproduction, individual planarians exchangesperm, which travel along special tubes to reach the eggs.
Fertilization happens inside the animals’ bodies. The resulting zygotesare released into the water. The zygotes are in capsules and thenhatch into tiny planarians.Planarians also can reproduce asexually. If a planarian is damaged, it has the ability to regenerate, or regrow, new body parts.Regeneration is the replacement or regrowth of missing bodyparts. Missing body parts are replaced through cell division. If aplanarian is cut in half horizontally, the section containing thehead will grow a new tail, and the tail section will grow a new head.Since a planarian that is cut into two pieces may grow into two neworganisms, scientists consider this a form of asexual reproduction.What do planarians eat?A planarian feeds on dead or slow-moving organisms.
It extendsa tubelike, muscular organ out of its mouth as shown in the illustration on page 320. The organ is called a pharynx (FAHR inx).Food particles are sucked into the digestive tract, where they arebroken up. Cells lining the digestive tract obtain food by endocytosis. Remember that endocytosis is a process in which a cell surrounds materials with a portion of the cell’s plasma membraneand then releases the contents inside the cell. Food is thendigested in individual cells.How do parasitic flatworms obtain nutrients?Parasitic flatworms live inside the bodies of their hosts. A parasite is an organism that lives on or in another organism. Parasitesdepend upon the host organism for nutrients. Parasitic flatwormshave mouthparts with hooks that keep them firmly attachedinside their hosts. Parasitic flatworms such as tapeworms are surrounded by nutrients.
They do not need to move to find theirfood. Parasitic flatworms do not have complex nervous or muscular tissue.READING ESSENTIALS2. Infer Why do parasitesnot require complex nervous or muscular tissue?________________________________________________________________________Chapter 26319NameDateClassSection26.3Flatworms, continuedA PlanarianEyespotsA HeadHeadB EyespotsFlatworms have aclearly defined head.The head senses andresponds to changesin the environment.Eyespots are sensitiveto light and enable theanimal to respond to theamount of light present.Sensory cellsC Sensory cellsLocated on each side of the head,sensory cells can detect food, chemicals,and movements in the environment.F PharynxThe pharynx is a musculartube that can extend outthrough a planarian’smouth.
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