Reading Essentials for Biology Glencoe (794133), страница 76
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This causestiny setae to protrude, anchoringthe worm in the soil. Then theearthworm’s circular musclescontract, the setae are withdrawn,and the worm moves forward.D NephridiaNephridia are excretory structuresthat eliminate metabolic wastesfrom nearly every segment.334Chapter 27How do segmentedworms reproduce?Some segmented worms,including earthworms andleeches, are hermaphrodites. Hermaphroditesproduce both sperm andeggs. When worms mate,two worms exchangesperm. Each worm formsa capsule for the eggs andthe sperm.
The sperm fertilize the eggs in this capsule. The capsule slips offthe worm, and the capsulestays behind in the soil.Two to three weeks later,young worms emerge fromthe eggs.Bristleworms and otherclosely related species haveseparate sexes. They reproduce sexually. Eggs andsperm are usually releasedinto the seawater wherethey live.
Fertilizationtakes place in the water.Bristleworm larvae hatchin the sea. They becomepart of the plankton. Whena larva begins to developsegments, the worm willsettle to the bottom ofthe ocean.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.27.2NameDateClassSection27.2Segmented Worms, continuedDiversity of Segmented WormsThere are three classes of segmented worms in the phylumAnnelida. Earthworms belong to the class Oligochaeta. Bristleworms and their relatives belong to the class Polychaeta.
Leechesbelong to the class Hirudinea.What are earthworms?Earthworms are probably the best-known Annelids becausethey can be seen easily by most people. Earthworms belong to theclass Oligochaeta. Earthworms have anterior and posterior sections, but lack distinct heads. They have only a few setae on eachof their segments. As you can see in the illustration, earthwormshave a brain, a pharynx, an esophagus, blood vessels, and nephridia.Notice that a single segment contains muscle layers, blood vessels, the esophagus, nephridia, and the ventral nerve cord.Earthworms improve garden soil. As they eat, they create newspaces for air and water to flow through the soil.
As soil passesthrough the worm’s digestive tract, nutrients are extracted. Theundigested materials pass out of the worm’s digestive tract. Thesewastes, called castings, help fertilize the soil.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are bristleworms and their relatives?Bristleworms and their relatives, including fanworms, lugworms, plumed worms, and sea mice, belong to the class Polychaeta. Most live in the oceans. Most body segments of theseworms have many setae. Their class name Polychaete means“many bristles.”Most body segments of Polychaetes have pairs of appendages,similar to tiny limbs.
The appendages are called parapodia.Polychaetes use parapodia for swimming or crawling over coralsand the bottom of the sea. The parapodia also help in gasexchange. Polychaetes have heads with well-developed senseorgans, including eyes.What are leeches?4. Why are earthwormsthe best known of theAnnelids?________________________________________________________________________23456789101112131415161BrainPharynxEsophagusBloodvesselNephridiaMuscle layersBlood vesselEsophagusNephridiaVentral nervecordLeeches belong to the class Hirudinea. Leeches are segmentedworms with flattened bodies.
Usually they do not have setae.Most leeches live in freshwater streams or in rivers. Many speciesof leeches are parasites that suck blood or other body fluids fromanimals called hosts. Host animals include ducks, turtles, fishes,and humans. Leeches have front and rear suckers than enablethem to attach themselves to their hosts.READING ESSENTIALSChapter 27335NameDateClassSection27.2Segmented Worms, continued5. Why isn’t the bite of aleech painful to its host?________________________________________________________________________The bite of a leech is not painful. The saliva of the leech contains chemicals that act as an anesthetic, a pain killer.
Otherchemicals stop the host’s blood from clotting. A leech can ingesttwo to fives times its own weight in a single meal. Once a leechhas finished eating, it lets go and drops off its host. The leechmay not need to eat again for several months.Origins of Mollusks and Segmented WormsFossil records show that there were great numbers of mollusks500 million years ago. Fossils of gastropods, bivalves, andcephalopods, the three classes of mollusks, have been found inPrecambrian deposits. Some species of mollusks, such as thechambered nautilus, have not changed much from their ancestorsthat lived long ago. Because mollusks lived so long ago, scientistsuse fossil mollusks to help determine the ages of rocks.Bivalves10 000 speciesAnnelids15 000 speciesCephalopods600 speciesSpecies numbers are approximate and subject to change pending discoveries or extinctions.336Chapter 27READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Gastropods40 000 speciesNameDateClassSection27.2Segmented Worms, continuedAnnelids, segmented worms, probably evolved from the sea.They may have originated from the larvae of ancestral flatworms.The fossil records for segmented worms are much more limitedthan for mollusks because segmented worms have almost no hardbody parts.
Little remains from their soft bodies in the fossilrecords. Tubes constructed by Polychaetes are the most commonfossils for this phylum. Some of these tubes appear in the fossilrecord as early as 540 million years ago.6. Why is the fossil record ofsegmented worms morelimited than the fossilrecord of mollusks?________________________________________________________________________◗ After You ReadMini Glossarygizzard: part of the worm’s digestive tract thatconsists of a muscular sac and hard particlesthat help grind soil and food before theypass into the worm’s intestinesetae (SEE tee): tiny bristles on each segment of asegmented worm that help the worm move1.
Read the terms and definitions in the Mini Glossary above. Then, using both terms, describehow a segmented worm moves and digests its food.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________2.
Complete the chart below to identify the various types of segmented worms discussed in thissection. Identify two characteristics of the Annelid in each class.Type of AnnelidClassCharacteristicsOligochaeta1._____________________________________________________________2._____________________________________________________________Polychaeta1._____________________________________________________________2._____________________________________________________________Hirudinea1._____________________________________________________________2._____________________________________________________________Visit the Glencoe Science Web site at science.glencoe.comto find your biology book and learn more about Annelids.READING ESSENTIALSChapter 27337NameDateClassSection28.1 Characteristics of ArthropodsSC.F.1.4.2 The student knows that body structures are uniquely designed and adapted for their function.
Also coversSC.F.1.4.7◗ Before You ReadHave you seen a drawing or picture of a knight in armor? What is the purpose of armor? On the linesbelow list the advantages and disadvantages of wearing a heavy suit of armor. As you read this section about arthropods, notice the similarities between a knight’s armor and a feature of arthropods.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to Learn________________________A typical arthropod is a segmented, coelomate invertebrate animal.
A coelom is a fluid-filled cavity completely surrounded bymesoderm. Arthropods have bilateral symmetry, an exoskeleton,and jointed structures called appendages. An exoskeleton is ahard, thick outer covering. An appendage (uh PEN dihj) is anystructure, such as a leg or an antenna that grows out of the bodyof an animal. In arthropods, appendages are adapted for a varietyof purposes. There are appendages for walking, sensing, feeding,and mating.Arthropods are the earliest known invertebrates to have jointedappendages.
Joints allow more flexibility in animals that have hard,rigid exoskeletons. Joints allow powerful movements of appendages.Joints allow appendages to be used in many different ways. Forexample, the second pair of appendages in spiders is used for mating. In scorpions, the second pair is used for seizing prey.________________________How does an exoskeleton provide protection?LocateInformation Underline everyheading in the reading thatasks a question. Then, highlight the answers to thosequestions as you find them.1.
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